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Extensive experimental data from high-energy nucleus-nucleus collisions were recorded using the PHENIX detector at the Relativistic Heavy Ion Collider (RHIC). The comprehensive set of measurements from the first three years of RHIC operation includes charged particle multiplicities, transverse energy, yield ratios and spectra of identified hadrons in a wide range of transverse momenta (pT), elliptic flow, two-particle correlations, nonstatistical fluctuations, and suppression of particle production at high pT. The results are examined with an emphasis on implications for the formation of a new state of dense matter. We find that the state of matter created at RHIC cannot be described in terms of ordinary color neutral hadrons.

The PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC) has measured electrons from heavy flavor (charm and bottom) decays for 0.3 < p_T < 9 GeV/c at midrapidity (|y| < 0.35) in Au+Au collisions at sqrt(s_NN) = 200 GeV. The nuclear modification factor R_AA relative to p+p collisions shows a strong suppression in central Au+Au collisions, indicating substantial energy loss of heavy quarks in the medium produced at RHIC. A large azimuthal anisotropy, v_2, with respect to the reaction plane is observed for 0.5 < p_T < 5 GeV/c indicating non-zero heavy flavor elliptic flow. Both R_AA and v_2 show a p_T dependence different from those of neutral pions. A comparison to transport models which simultaneously describe R_AA(p_T) and v_2(p_T) suggests that the viscosity to entropy density ratio is close to the conjectured quantum lower bound, i.e., near a perfect fluid.

The PHENIX experiment at the BNL Relativistic Heavy Ion Collider (RHIC) has measured J/psi production for rapidities -2.2<y<2.2 in Au+Au collisions at square root sNN=200 GeV. The J/psi invariant yield and nuclear modification factor RAA as a function of centrality, transverse momentum, and rapidity are reported. A suppression of J/psi relative to binary collision scaling of proton-proton reaction yields is observed. Models which describe the lower energy J/psi data at the CERN Super Proton Synchrotron invoking only J/psi destruction based on the local medium density predict a significantly larger suppression at RHIC and more suppression at midrapidity than at forward rapidity. Both trends are contradicted by our data.

Transverse momentum spectra of charged hadrons with p T < 8 GeV=c and neutral pions with p T < 10 GeV=c have been measured at midrapidity by the PHENIX experiment at BNL RHIC in d Au collisions at s NN p 200 GeV.The measured yields are compared to those in p p collisions at the same s NN p scaled up by the number of underlying nucleon-nucleon collisions in d Au.The yield ratio does not show the suppression observed in central Au Au collisions at RHIC.Instead, there is a small enhancement in the yield of high momentum particles.

The production of low mass e+e- pairs for m_{e+e-} < 300 MeV/c^2 and 1 < p_T <5 GeV/c is measured in p+p and Au+Au collisions at sqrt(s_NN)=200 GeV. Enhanced yield above hadronic sources is observed. Treating the excess as internal conversions, the invariant yield of direct photons is deduced. In central Au+Au collisions, the excess of direct photon yield over p+p is exponential in transverse momentum, with inverse slope T = 221 +/- 19 (stat) +/- 19 (syst) MeV. Hydrodynamical models with initial temperatures ranging from 300--600 MeV at times of ~ 0.6 - 0.15 fm/c after the collision are in qualitative agreement with the data. Lattice QCD predicts a phase transition to quark gluon plasma at ~ 170 MeV.

Differential measurements of elliptic flow (v2) for Au+Au and Cu+Cu collisions at √sNN=200 GeV are used to test and validate predictions from perfect fluid hydrodynamics for scaling of v2 with eccentricity, system size, and transverse kinetic energy (KET). For KET≡mT−m up to ∼1 GeV the scaling is compatible with hydrodynamic expansion of a thermalized fluid. For large values of KET mesons and baryons scale separately. Quark number scaling reveals a universal scaling of v2 for both mesons and baryons over the full KET range for Au+Au. For Au+Au and Cu+Cu the scaling is more pronounced in terms of KET, rather than transverse momentum.Received 16 August 2006DOI:https://doi.org/10.1103/PhysRevLett.98.162301©2007 American Physical Society

Flow coefficients v_n for n = 2, 3, 4, characterizing the anisotropic collective flow in Au+Au collisions at sqrt(s_NN) = 200 GeV, are measured relative to event planes \Psi_n determined at large rapidity. We report v_n as a function of transverse momentum and collision centrality, and study the correlations among the event planes of different order n. The v_n are well described by hydrodynamic models which employ a Glauber Monte Carlo initial state geometry with fluctuations, providing additional constraining power on the interplay between initial conditions and the effects of viscosity as the system evolves. This new constraint improves precision of the extracted viscosity to entropy density ratio eta/s.

Azimuthal angle (Δϕ) correlations are presented for a broad range of transverse momentum (0.4<pT<10 GeV/c) and centrality (0–92%) selections for charged hadrons from dijets in Au+Au collisions at √sNN=200 GeV. With increasing pT, the away-side Δϕ distribution evolves from a broad and relatively flat shape to a concave shape, then to a convex shape. Comparisons with p+p data suggest that the away-side distribution can be divided into a partially suppressed "head" region centered at Δϕ~π, and an enhanced "shoulder" region centered at Δϕ~π±1.1. The pT spectrum for the associated hadrons in the head region softens toward central collisions. The spectral slope for the shoulder region is independent of centrality and trigger pT. The properties of the near-side distributions are also modified relative to those in p+p collisions, reflected by the broadening of the jet shape in Δϕ and Δη, and an enhancement of the per-trigger yield. However, these modifications seem to be limited to pT≲4 GeV/c, above which both the hadron pair shape and per-trigger yield become similar to p+p collisions. These observations suggest that both the away- and near-side distributions contain a jet fragmentation component which dominates for pT≳5 GeV/c and a medium-induced component which is important for pT≲4 GeV/c. We also quantify the role of jets at intermediate and low pT through the yield of jet-induced pairs in comparison with binary scaled p+p pair yield. The yield of jet-induced pairs is suppressed at high pair proxy energy (sum of the pT magnitudes of the two hadrons) and is enhanced at low pair proxy energy. The former is consistent with jet quenching; the latter is consistent with the enhancement of soft hadron pairs due to transport of lost energy to lower pT.31 MoreReceived 30 January 2008DOI:https://doi.org/10.1103/PhysRevC.78.014901©2008 American Physical Society

PHENIX has measured the e^+e^- pair continuum in sqrt(s_NN)=200 GeV Au+Au and p+p collisions over a wide range of mass and transverse momenta. The e^+e^- yield is compared to the expectations from hadronic sources, based on PHENIX measurements. In the intermediate mass region, between the masses of the phi and the J/psi meson, the yield is consistent with expectations from correlated c^bar-c production, though other mechanisms are not ruled out. In the low mass region (below the phi) the p+p inclusive mass spectrum is well described by known contributions from light meson decays. In contrast, the Au+Au minimum bias inclusive mass spectrum in this region shows an enhancement by a factor of 4.7+/-0.4(stat)+/-1.5(syst)+/-0.9(model) At low mass (m_ee<0.3 GeV/c^2) and high p_T (1<p_T<5 GeV/c) an enhanced e^+e^- pair yield is observed that is consistent with production of virtual direct photons. This excess is used to infer the yield of real direct photons. In central Au+Au collisions, the excess of the direct photon yield over the p+p is exponential in p_T, with inverse slope T=221+/-19(stat)+/-19(syst) MeV. Hydrodynamical models with initial temperatures ranging from T_init ~=300--600 MeV at times of 0.6--0.15 fm/c after the collision are in qualitative agreement with the direct photon data in Au+Au. For low p_T<1 GeV/c the low mass region shows a further significant enhancement that increases with centrality and has an inverse slope of T ~=100 MeV. Theoretical models under predict the low mass, low p_T enhancement.

For Au + Au collisions at 200 GeV we measure neutral pion production with good statistics for transverse momentum, p_T, up to 20 GeV/c. A fivefold suppression is found, which is essentially constant for 5 &lt; p_T &lt; 20 GeV/c. Experimental uncertainties are small enough to constrain any model-dependent parameterization for the transport coefficient of the medium, e.g. \mean(q^hat) in the parton quenching model. The spectral shape is similar for all collision classes, and the suppression does not saturate in Au+Au collisions; instead, it increases proportional to the number of participating nucleons, as N_part^2/3.

The PHENIX collaboration at the Relativistic Heavy Ion Collider (RHIC) reports measurements of azimuthal dihadron correlations near midrapidity in $d$$+$Au collisions at $\sqrt{s_{_{NN}}}$=200 GeV. These measurements complement recent analyses by experiments at the Large Hadron Collider (LHC) involving central $p$$+$Pb collisions at $\sqrt{s_{_{NN}}}$=5.02 TeV, which have indicated strong anisotropic long-range correlations in angular distributions of hadron pairs. The origin of these anisotropies is currently unknown. Various competing explanations include parton saturation and hydrodynamic flow. We observe qualitatively similar, but larger, anisotropies in $d$$+$Au collisions compared to those seen in $p$$+$Pb collisions at the LHC. The larger extracted $v_2$ values in $d$$+$Au collisions at RHIC are consistent with expectations from hydrodynamic calculations owing to the larger expected initial-state eccentricity compared with that from $p$$+$Pb collisions. When both are divided by an estimate of the initial-state eccentricity the scaled anisotropies follow a common trend with multiplicity that may extend to heavy ion data at RHIC and the LHC, where the anisotropies are widely thought to arise from hydrodynamic flow.

At the LHC energy of , under various beam and background conditions, luminosities, and Roman Pot positions, TOTEM has measured the differential cross-section for proton-proton elastic scattering as a function of the four-momentum transfer squared t. The results of the different analyses are in excellent agreement demonstrating no sizeable dependence on the beam conditions. Due to the very close approach of the Roman Pot detectors to the beam center (≈5σbeam) in a dedicated run with β* = 90 m, |t|-values down to 5·10−3 GeV2 were reached. The exponential slope of the differential elastic cross-section in this newly explored |t|-region remained unchanged and thus an exponential fit with only one constant B = (19.9 ± 0.3) GeV−2 over the large |t|-range from 0.005 to 0.2 GeV2 describes the differential distribution well. The high precision of the measurement and the large fit range lead to an error on the slope parameter B which is remarkably small compared to previous experiments. It allows a precise extrapolation over the non-visible cross-section (only 9%) to t = 0. With the luminosity from CMS, the elastic cross-section was determined to be (25.4 ± 1.1) mb, and using in addition the optical theorem, the total pp cross-section was derived to be (98.6 ± 2.2) mb. For model comparisons the t-distributions are tabulated including the large |t|-range of the previous measurement (TOTEM Collaboration (Antchev G. et al), EPL, 95 (2011) 41001).

Transverse momentum (p^e_T) spectra of electrons from semileptonic weak decays of heavy flavor mesons in the range of 0.3 < p^e_T < 9.0 GeV/c have been measured at mid-rapidity (|eta| < 0.35) by the PHENIX experiment at the Relativistic Heavy Ion Collider in p+p and Au+Au collisions at sqrt(s_NN)=200 GeV. The nuclear modification factor R_AA with respect to p+p collisions indicates substantial energy loss of heavy quarks in the produced medium. In addition, the azimuthal anisotropy parameter v_2 has been measured for 0.3 < p^e_T < 5.0 GeV/c in Au+Au collisions. Comparisons of R_AA and v_2 are made to various model calculations.

The PHENIX experiment at the Relativistic Heavy Ion Collider has measured the invariant differential cross section for production of ${K}_{S}^{0}$, $\ensuremath{\omega}$, ${\ensuremath{\eta}}^{\ensuremath{'}}$, and $\ensuremath{\phi}$ mesons in $p+p$ collisions at $\sqrt{s}=200\text{ }\text{ }\mathrm{GeV}$. Measurements of $\ensuremath{\omega}$ and $\ensuremath{\phi}$ production in different decay channels give consistent results. New results for the $\ensuremath{\omega}$ are in agreement with previously published data and extend the measured ${p}_{T}$ coverage. The spectral shapes of all hadron transverse momentum distributions measured by PHENIX are well described by a Tsallis distribution functional form with only two parameters, $n$ and $T$, determining the high-${p}_{T}$ and characterizing the low-${p}_{T}$ regions of the spectra, respectively. The values of these parameters are very similar for all analyzed meson spectra, but with a lower parameter $T$ extracted for protons. The integrated invariant cross sections calculated from the fitted distributions are found to be consistent with existing measurements and with statistical model predictions.

The TOTEM collaboration has measured the proton-proton total cross section at √s=8 TeV using a luminosity-independent method. In LHC fills with dedicated beam optics, the Roman pots have been inserted very close to the beam allowing the detection of ~90% of the nuclear elastic scattering events. Simultaneously the inelastic scattering rate has been measured by the T1 and T2 telescopes. By applying the optical theorem, the total proton-proton cross section of (101.7±2.9) mb has been determined, well in agreement with the extrapolation from lower energies. This method also allows one to derive the luminosity-independent elastic and inelastic cross sections: σ(el)=(27.1±1.4) mb; σ(inel)=(74.7±1.7) mb.

Experimental studies of the collisions of heavy nuclei at relativistic energies have established the properties of the quark–gluon plasma (QGP), a state of hot, dense nuclear matter in which quarks and gluons are not bound into hadrons1–4. In this state, matter behaves as a nearly inviscid fluid5 that efficiently translates initial spatial anisotropies into correlated momentum anisotropies among the particles produced, creating a common velocity field pattern known as collective flow. In recent years, comparable momentum anisotropies have been measured in small-system proton–proton (p+p) and proton–nucleus (p+A) collisions, despite expectations that the volume and lifetime of the medium produced would be too small to form a QGP. Here we report on the observation of elliptic and triangular flow patterns of charged particles produced in proton–gold (p+Au), deuteron–gold (d+Au) and helium–gold (3He+Au) collisions at a nucleon–nucleon centre-of-mass energy $$\sqrt {s_{{\mathrm{NN}}}}$$ = 200 GeV. The unique combination of three distinct initial geometries and two flow patterns provides unprecedented model discrimination. Hydrodynamical models, which include the formation of a short-lived QGP droplet, provide the best simultaneous description of these measurements. A quark–gluon plasma is produced in proton–gold, deuteron–gold and helium–gold collisions. Observing elliptic and triangular flow in this nearly inviscid fluid from these different initial geometries provides a unique benchmark for hydrodynamic models.

The second Fourier component v_2 of the azimuthal anisotropy with respect to the reaction plane was measured for direct photons at midrapidity and transverse momentum (p_T) of 1--13 GeV/c in Au+Au collisions at sqr(s_NN)=200 GeV. Previous measurements of this quantity for hadrons with p_T < 6 GeV/c indicate that the medium behaves like a nearly perfect fluid, while for p_T > 6 GeV/c a reduced anisotropy is interpreted in terms of a path-length dependence for parton energy loss. In this measurement with the PHENIX detector at the Relativistic Heavy Ion Collider we find that for p_T > 4 GeV/c the anisotropy for direct photons is consistent with zero, as expected if the dominant source of direct photons is initial hard scattering. However, in the p_T < 4 GeV/c region dominated by thermal photons, we find a substantial direct photon v_2 comparable to that of hadrons, whereas model calculations for thermal photons in this kinematic region significantly underpredict the observed v_2.

The TOTEM experiment at the LHC has performed the first luminosity-independent determination of the total proton-proton cross-section at . This technique is based on the optical theorem and requires simultaneous measurements of the inelastic rate – accomplished with the forward charged-particle telescopes T1 and T2 in the range 3.1 < |η| < 6.5 – and of the elastic rate by detecting the outcoming protons with Roman Pot detectors. The data presented here were collected in a dedicated run in 2011 with special beam optics (β* = 90 m) and Roman Pots approaching the beam close enough to register elastic events with squared four-momentum transfers |t| as low as 5·10−3 GeV2. The luminosity-independent results for the elastic, inelastic and total cross-sections are σel = (25.1 ± 1.1) mb, σinel = (72.9 ± 1.5) mb and σtot = (98.0 ± 2.5) mb, respectively. At the same time this method yields the integrated luminosity, in agreement with measurements by CMS. TOTEM has also determined the total cross-section in two complementary ways, both using the CMS luminosity measurement as an input. The first method sums the elastic and inelastic cross-sections and thus does not depend on the ρ parameter. The second applies the optical theorem to the elastic-scattering measurements only and therefore is free of the T1 and T2 measurement uncertainties. The methods, having very different systematic dependences, give results in excellent agreement. Moreover, the ρ-independent measurement makes a first estimate for the ρ parameter at possible: |ρ| = 0.145 ± 0.091.

The TOTEM experiment has made a precise measurement of the elastic proton–proton differential cross-section at the centre-of-mass energy s=8TeV based on a high-statistics data sample obtained with the β⁎=90m optics. Both the statistical and systematic uncertainties remain below 1%, except for the t-independent contribution from the overall normalisation. This unprecedented precision allows to exclude a purely exponential differential cross-section in the range of four-momentum transfer squared 0.027<|t|<0.2GeV2 with a significance greater than 7σ. Two extended parametrisations, with quadratic and cubic polynomials in the exponent, are shown to be well compatible with the data. Using them for the differential cross-section extrapolation to t=0, and further applying the optical theorem, yields total cross-section estimates of (101.5±2.1)mb and (101.9±2.1)mb, respectively, in agreement with previous TOTEM measurements.

The $\Lambda$ ($\bar{\Lambda}$) hyperon polarization along the beam direction has been measured for the first time in Au+Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV. The polarization dependence on the hyperons' emission angle relative to the second-order event plane exhibits a sine modulation, indicating a quadrupole pattern of the vorticity component along the beam direction. The polarization is found to increase in more peripheral collisions, and shows no strong transverse momentum ($p_T$) dependence at $p_T>1$ GeV/$c$. The magnitude of the signal is about five times smaller than those predicted by hydrodynamic and multiphase transport models; the observed phase of the emission angle dependence is also opposite to these model predictions. In contrast, blast-wave model calculations reproduce the modulation phase measured in the data and capture the centrality and transverse momentum dependence of the signal once the model is required to reproduce the azimuthal dependence of the Gaussian source radii measured via the Hanbury-Brown and Twiss intensity interferometry technique.

Non-monotonic variation with collision energy ($\sqrt{s_{\rm NN}}$) of the moments of the net-baryon number distribution in heavy-ion collisions, related to the correlation length and the susceptibilities of the system, is suggested as a signature for the Quantum Chromodynamics (QCD) critical point. We report the first evidence of a non-monotonic variation in kurtosis times variance of the net-proton number (proxy for net-baryon number) distribution as a function of \rootsnn with 3.1$\sigma$ significance, for head-on (central) gold-on-gold (Au+Au) collisions measured using the STAR detector at RHIC. Data in non-central Au+Au collisions and models of heavy-ion collisions without a critical point show a monotonic variation as a function of $\sqrt{s_{\rm NN}}$.

The momentum distribution of electrons from decays of heavy flavor (charm and bottom) for midrapidity absolute value of y < 0.35 in p + p collisions at square root of s = 200 GeV has been measured by the PHENIX experiment at the BNL Relativistic Heavy Ion Collider over the transverse momentum range 0.3 < pT < 9 GeV/c. Two independent methods have been used to determine the heavy-flavor yields, and the results are in good agreement with each other. A fixed-order-plus-next-to-leading-log perturbative QCD calculation agrees with the data within the theoretical and experimental uncertainties, with the data/theory ratio of 1.71+/-0.02stat+/-0.18sys for 0.3 < pT < 9 GeV/c. The total charm production cross section at this energy has also been deduced to be sigma cc = 567+/-57stat+/-193sys microb.

Transverse momentum distributions and yields for ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$, ${K}^{\ifmmode\pm\else\textpm\fi{}}$, $p$, and $\mathrm{p\ifmmode \bar{}\else \={}\fi{}}$ in $p+p$ collisions at $\sqrt{s}$ $=$ 200 and 62.4 GeV at midrapidity are measured by the PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC). These data provide important baseline spectra for comparisons with identified particle spectra in heavy ion collisions at RHIC. We present the inverse slope parameter ${T}_{\mathrm{inv}}$, mean transverse momentum $\ensuremath{\langle}{p}_{T}\ensuremath{\rangle}$, and yield per unit rapidity $\mathit{dN}/\mathit{dy}$ at each energy, and compare them to other measurements at different $\sqrt{s}$ in $p+p$ and $p+\mathrm{p\ifmmode \bar{}\else \={}\fi{}}$ collisions. We also present the scaling properties such as ${m}_{T}$ scaling and ${x}_{T}$ scaling on the ${p}_{T}$ spectra between different energies. To discuss the mechanism of the particle production in $p+p$ collisions, the measured spectra are compared to next-to-leading-order or next-to-leading-logarithmic perturbative quantum chromodynamics calculations.

The PHENIX experiment presents results from the Relativistic Heavy Ion Collider 2005 run with polarized proton collisions at √s=200 GeV, for inclusive π0 production at midrapidity. Unpolarized cross section results are given for transverse momenta pT=0.5 to 20 GeV/c, extending the range of published data to both lower and higher pT. The cross section is described well for pT<1 GeV/c by an exponential in pT, and, for pT>2 GeV/c, by perturbative QCD. Double helicity asymmetries ALL are presented based on a factor of 5 improvement in uncertainties as compared to previously published results, due to both an improved beam polarization of 50%, and to higher integrated luminosity. These measurements are sensitive to the gluon polarization in the proton. Using one representative model of gluon polarization it is demonstrated that the gluon spin contribution to the proton spin is significantly constrained.Received 27 April 2007DOI:https://doi.org/10.1103/PhysRevD.76.051106©2007 American Physical Society

Heavy quarkonia are observed to be suppressed in relativistic heavy ion collisions relative to their production in p+p collisions scaled by the number of binary collisions. In order to determine if this suppression is related to color screening of these states in the produced medium, one needs to account for other nuclear modifications including those in cold nuclear matter. In this paper, we present new measurements from the PHENIX 2007 data set of J/psi yields at forward rapidity (1.2<|y|<2.2) in Au+Au collisions at sqrt(s_NN)=200 GeV. The data confirm the earlier finding that the suppression of J/psi at forward rapidity is stronger than at midrapidity, while also extending the measurement to finer bins in collision centrality and higher transverse momentum (pT). We compare the experimental data to the most recent theoretical calculations that incorporate a variety of physics mechanisms including gluon saturation, gluon shadowing, initial-state parton energy loss, cold nuclear matter breakup, color screening, and charm recombination. We find J/psi suppression beyond cold-nuclear-matter effects. However, the current level of disagreement between models and d+Au data precludes using these models to quantify the hot-nuclear-matter suppression.

J/psi production in p+p collisions at square root s=200 GeV has been measured by the PHENIX experiment at the BNL Relativistic Heavy Ion Collider over a rapidity range of -2.2<y<2.2 and a transverse momentum range of 0<pT<9 GeV/c. The size of the present data set allows a detailed measurement of both the pT and the rapidity distributions and is sufficient to constrain production models. The total cross section times the branching ratio is Bll sigma pp J/psi=178+/-3stat+/-53sys+/-18norm nb.

The dependence of transverse momentum spectra of neutral pions and $\ensuremath{\eta}$ mesons with ${p}_{T}&lt;16\text{ }\text{ }\mathrm{GeV}/c$ and ${p}_{T}&lt;12\text{ }\text{ }\mathrm{GeV}/c$, respectively, on the centrality of the collision has been measured at midrapidity by the PHENIX experiment at the BNL Relativistic Heavy Ion Collider (RHIC) in $d+\mathrm{Au}$ collisions at $\sqrt{{s}_{NN}}=200\text{ }\text{ }\mathrm{GeV}$. The measured yields are compared to those in $p+p$ collisions at the same $\sqrt{{s}_{NN}}$ scaled by the number of underlying nucleon-nucleon collisions in $d+\mathrm{Au}$. At all centralities, the yield ratios show no suppression, in contrast to the strong suppression seen for central $\mathrm{Au}+\mathrm{Au}$ collisions at RHIC. Only a weak ${p}_{T}$ and centrality dependence can be observed.

We present azimuthal angular correlations between charged hadrons and energy deposited in calorimeter towers in central d+Au and minimum bias p+p collisions at sqrt[s_{NN}]=200 GeV. The charged hadron is measured at midrapidity |η|<0.35, and the energy is measured at large rapidity (-3.7<η<-3.1, Au-going direction). An enhanced near-side angular correlation across |Δη|>2.75 is observed in d+Au collisions. Using the event plane method applied to the Au-going energy distribution, we extract the anisotropy strength v_{2} for inclusive charged hadrons at midrapidity up to p_{T}=4.5 GeV/c. We also present the measurement of v_{2} for identified π^{±} and (anti)protons in central d+Au collisions, and observe a mass-ordering pattern similar to that seen in heavy-ion collisions. These results are compared with viscous hydrodynamic calculations and measurements from p+Pb at sqrt[s_{NN}]=5.02 TeV. The magnitude of the mass ordering in d+Au is found to be smaller than that in p+Pb collisions, which may indicate smaller radial flow in lower energy d+Au collisions.

We present the first measurement of elliptic (v(2)) and triangular (v(3)) flow in high-multiplicity (3)He+Au collisions at √(s(NN))=200 GeV. Two-particle correlations, where the particles have a large separation in pseudorapidity, are compared in (3)He+Au and in p+p collisions and indicate that collective effects dominate the second and third Fourier components for the correlations observed in the (3)He+Au system. The collective behavior is quantified in terms of elliptic v(2) and triangular v(3) anisotropy coefficients measured with respect to their corresponding event planes. The v(2) values are comparable to those previously measured in d+Au collisions at the same nucleon-nucleon center-of-mass energy. Comparisons with various theoretical predictions are made, including to models where the hot spots created by the impact of the three (3)He nucleons on the Au nucleus expand hydrodynamically to generate the triangular flow. The agreement of these models with data may indicate the formation of low-viscosity quark-gluon plasma even in these small collision systems.

Proton-proton elastic scattering has been measured by the TOTEM experiment at the CERN Large Hadron Collider at in dedicated runs with the Roman Pot detectors placed as close as seven times the transverse beam size (σbeam) from the outgoing beams. After careful study of the accelerator optics and the detector alignment, |t|, the square of four-momentum transferred in the elastic scattering process, has been determined with an uncertainty of . In this letter, first results of the differential cross-section are presented covering a |t|-range from 0.36 to 2.5 GeV2. The differential cross-section in the range 0.36 < |t| < 0.47 GeV2 is described by an exponential with a slope parameter B = (23.6 ± 0.5stat ± 0.4syst) GeV−2, followed by a significant diffractive minimum at |t| = (0.53 ± 0.01stat ± 0.01syst) GeV2. For |t|-values larger than ∼1.5 GeV2, the cross-section exhibits a power law behaviour with an exponent of −7.8 ± 0.3stat ± 0.1syst. When compared to predictions based on the different available models, the data show a strong discriminative power despite the small t-range covered.

Cross sections for midrapidity production of direct photons in p+p collisions at the Relativistic Heavy Ion Collider (RHIC) are reported for transverse momenta of 3 < pT < 16 GeV/c. Next-to-leading order perturbative QCD (pQCD) describes the data well for pT >5 GeV/c, where the uncertainties of the measurement and theory are comparable. We also report on the effect of requiring the photons to be isolated from parton jet energy. The observed fraction of isolated photons is well described by pQCD for pT >7 GeV/c.

The PHENIX experiment at RHIC has measured the centrality dependence of the direct photon yield from Au+Au collisions at √sNN=200 GeV down to pT=0.4 GeV/c. Photons are detected via photon conversions to e+e− pairs and an improved technique is applied that minimizes the systematic uncertainties that usually limit direct photon measurements, in particular at low pT. We find an excess of direct photons above the Ncoll-scaled yield measured in p+p collisions. This excess yield is well described by an exponential distribution with an inverse slope of about 240MeV/c in the pT range 0.6–2.0 GeV/c. While the shape of the pT distribution is independent of centrality within the experimental uncertainties, the yield increases rapidly with increasing centrality, scaling approximately with Nαpart, where α=1.38±0.03(stat)±0.07(syst).3 MoreReceived 23 May 2014Revised 16 March 2015DOI:https://doi.org/10.1103/PhysRevC.91.064904©2015 American Physical Society

The transverse momentum (p_T) spectra and ratios of identified charged hadrons (\pi^+/-, K^+/-, p, p^bar) produced in sqrt(s_NN)=200 GeV Au+Au and d+Au collisions are reported in five different centrality classes for each collision species. The measurements of pions and protons are reported up to p_T=6 GeV/c (5 GeV/c), and the measurements of kaons are reported up to p_T=4 GeV/c (3.5 GeV/c) in Au+Au (d+Au) collisions. In the intermediate p_T region, between 2--5 GeV/c, a significant enhancement of baryon to meson ratios compared to those measured in p+p collisions is observed. This enhancement is present in both Au+Au and d+Au collisions, and increases as the collisions become more central. We compare a class of peripheral Au+Au collisions with a class of central d+Au collisions which have a comparable number of participating nucleons and binary nucleon-nucleon collisions. The p_T dependent particle ratios for these classes display a remarkable similarity, which is then discussed.

The PHENIX experiment has measured the production of neutral pions in Au+Au collisions at sqrt(s_NN)=200 GeV. The new data offer a fourfold increase in recorded luminosity, providing higher precision and a larger reach in transverse momentum, p_T, to 20 GeV/c. The production ratio of eta/pi^0 is 0.46+/-0.01(stat)+/-0.05(syst), constant with p_T and collision centrality. The observed ratio is consistent with earlier measurements, as well as with the p+p and d+Au values. The production of pi^0 is suppressed by a factor of 5, as in earlier findings. However, with the improved statistical precision a small but significant rise of the nuclear modification factor, R_AA, vs p_T, with a slope of 0.0106+/-^(0.0034)_(0.0029)[GeV/c]^-1, is discernible in central collisions. A phenomenological extraction of the average fractional parton energy loss shows a decrease with increasing p_T. To study the path length dependence of suppression, the pi^0 yield was measured at different angles with respect to the event plane; a strong azimuthal dependence of the pi^0 R_AA is observed. The data are compared to theoretical models of parton energy loss as a function of the path length, L, in the medium. Models based on pQCD are insufficient to describe the data, while a hybrid model utilizing pQCD for the hard interactions and AdS/CFT for the soft interactions is consistent with the data.

Back-to-back hadron pair yields in d+Au and p+p collisions at sqrt(s_NN)=200 GeV were measured with the PHENIX detector at the Relativistic Heavy Ion Collider. Rapidity separated hadron pairs were detected with the trigger hadron at pseudorapidity |eta|<0.35 and the associated hadron at forward rapidity (deuteron direction, 3.0<eta<3.8). Pairs were also detected with both hadrons measured at forward rapidity; in this case the yield of back-to-back hadron pairs in d+Au collisions with small impact parameters is observed to be suppressed by a factor of 10 relative to p+p collisions. The kinematics of these pairs is expected to probe partons in the Au nucleus with low fraction x of the nucleon momenta, where the gluon densities rise sharply. The observed suppression as a function of nuclear thickness, p_T, and eta points to cold nuclear matter effects arising at high parton densities.

The TOTEM experiment at the CERN LHC has measured elastic proton–proton scattering at the centre-of-mass energy $$\sqrt{s}=8\,$$ TeV and four-momentum transfers squared, |t|, from $$6\times 10^{-4}$$ to 0.2 GeV $$^{2}$$ . Near the lower end of the t-interval the differential cross-section is sensitive to the interference between the hadronic and the electromagnetic scattering amplitudes. This article presents the elastic cross-section measurement and the constraints it imposes on the functional forms of the modulus and phase of the hadronic elastic amplitude. The data exclude the traditional Simplified West and Yennie interference formula that requires a constant phase and a purely exponential modulus of the hadronic amplitude. For parametrisations of the hadronic modulus with second- or third-order polynomials in the exponent, the data are compatible with hadronic phase functions giving either central or peripheral behaviour in the impact parameter picture of elastic scattering. In both cases, the $$\rho $$ -parameter is found to be $$0.12 \pm 0.03$$ . The results for the total hadronic cross-section are $$\sigma _\mathrm{tot} = (102.9 \pm 2.3)$$ mb and $$(103.0 \pm 2.3)$$ mb for central and peripheral phase formulations, respectively. Both are consistent with previous TOTEM measurements.

The TOTEM collaboration has measured the proton–proton total cross section at $$\sqrt{s}=13~\hbox {TeV}$$ with a luminosity-independent method. Using dedicated $$\beta ^{*}=90~\hbox {m}$$ beam optics, the Roman Pots were inserted very close to the beam. The inelastic scattering rate has been measured by the T1 and T2 telescopes during the same LHC fill. After applying the optical theorem the total proton–proton cross section is $$\sigma _\mathrm{tot}=(110.6~\pm ~3.4$$ ) mb, well in agreement with the extrapolation from lower energies. This method also allows one to derive the luminosity-independent elastic and inelastic cross sections: $$\sigma _\mathrm{el}=(31.0~\pm ~1.7)~\hbox {mb}$$ and $$\sigma _\mathrm{inel}=(79.5~\pm ~1.8)~\hbox {mb}$$ .

Abstract The TOTEM experiment at the LHC has performed the first measurement at $$\sqrt{s} = 13\,\mathrm{TeV}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msqrt><mml:mi>s</mml:mi></mml:msqrt><mml:mo>=</mml:mo><mml:mn>13</mml:mn><mml:mspace /><mml:mi>TeV</mml:mi></mml:mrow></mml:math> of the $$\rho $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>ρ</mml:mi></mml:math> parameter, the real to imaginary ratio of the nuclear elastic scattering amplitude at $$t=0$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>t</mml:mi><mml:mo>=</mml:mo><mml:mn>0</mml:mn></mml:mrow></mml:math> , obtaining the following results: $$\rho = 0.09 \pm 0.01$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>ρ</mml:mi><mml:mo>=</mml:mo><mml:mn>0.09</mml:mn><mml:mo>±</mml:mo><mml:mn>0.01</mml:mn></mml:mrow></mml:math> and $$\rho = 0.10 \pm 0.01$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>ρ</mml:mi><mml:mo>=</mml:mo><mml:mn>0.10</mml:mn><mml:mo>±</mml:mo><mml:mn>0.01</mml:mn></mml:mrow></mml:math> , depending on different physics assumptions and mathematical modelling. The unprecedented precision of the $$\rho $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>ρ</mml:mi></mml:math> measurement, combined with the TOTEM total cross-section measurements in an energy range larger than $$10\,\mathrm{TeV}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>10</mml:mn><mml:mspace /><mml:mi>TeV</mml:mi></mml:mrow></mml:math> (from 2.76 to $$13\,\mathrm{TeV}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>13</mml:mn><mml:mspace /><mml:mi>TeV</mml:mi></mml:mrow></mml:math> ), has implied the exclusion of all the models classified and published by COMPETE. The $$\rho $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>ρ</mml:mi></mml:math> results obtained by TOTEM are compatible with the predictions, from other theoretical models both in the Regge-like framework and in the QCD framework, of a crossing-odd colourless 3-gluon compound state exchange in the t -channel of the proton–proton elastic scattering. On the contrary, if shown that the crossing-odd 3-gluon compound state t -channel exchange is not of importance for the description of elastic scattering, the $$\rho $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>ρ</mml:mi></mml:math> value determined by TOTEM would represent a first evidence of a slowing down of the total cross-section growth at higher energies. The very low-| t | reach allowed also to determine the absolute normalisation using the Coulomb amplitude for the first time at the LHC and obtain a new total proton–proton cross-section measurement $$\sigma _{\mathrm{tot}} = (110.3 \pm 3.5)\,\mathrm{mb}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>σ</mml:mi><mml:mi>tot</mml:mi></mml:msub><mml:mo>=</mml:mo><mml:mrow><mml:mo>(</mml:mo><mml:mn>110.3</mml:mn><mml:mo>±</mml:mo><mml:mn>3.5</mml:mn><mml:mo>)</mml:mo></mml:mrow><mml:mspace /><mml:mi>mb</mml:mi></mml:mrow></mml:math> , completely independent from the previous TOTEM determination. Combining the two TOTEM results yields $$\sigma _{\mathrm{tot}} = (110.5 \pm 2.4)\,\mathrm{mb}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>σ</mml:mi><mml:mi>tot</mml:mi></mml:msub><mml:mo>=</mml:mo><mml:mrow><mml:mo>(</mml:mo><mml:mn>110.5</mml:mn><mml:mo>±</mml:mo><mml:mn>2.4</mml:mn><mml:mo>)</mml:mo></mml:mrow><mml:mspace /><mml:mi>mb</mml:mi></mml:mrow></mml:math> .

The PHENIX experiment at the BNL Relativistic Heavy Ion Collider has measured second- and third-order Fourier coefficients of the azimuthal distributions of direct photons emitted at midrapidity in Au+Au collisions at √sNN=200 GeV for various collision centralities. Combining two different analysis techniques, results were obtained in the transverse momentum range of 0.4<pT<4.0 GeV/c. At low pT the second-order coefficients, v2, are similar to the ones observed in hadrons. Third-order coefficients, v3, are nonzero and almost independent of centrality. These new results on v2 and v3, combined with previously published results on yields, are compared to model calculations that provide yields and asymmetries in the same framework. Those models are challenged to explain simultaneously the observed large yield and large azimuthal anisotropies.2 MoreReceived 5 October 2015DOI:https://doi.org/10.1103/PhysRevC.94.064901©2016 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasPhoton, lepton & quark productionRelativistic heavy-ion collisionsNuclear Physics

According to the CPT theorem, which states that the combined operation of charge conjugation, parity transformation and time reversal must be conserved, particles and their antiparticles should have the same mass and lifetime but opposite charge and magnetic moment. Here, we test CPT symmetry in a nucleus containing a strange quark, more specifically in the hypertriton. This hypernucleus is the lightest one yet discovered and consists of a proton, a neutron, and a $\Lambda$ hyperon. With data recorded by the STAR detector{\cite{TPC,HFT,TOF}} at the Relativistic Heavy Ion Collider, we measure the $\Lambda$ hyperon binding energy $B_{\Lambda}$ for the hypertriton, and find that it differs from the widely used value{\cite{B_1973}} and from predictions{\cite{2019_weak, 1995_weak, 2002_weak, 2014_weak}}, where the hypertriton is treated as a weakly bound system. Our results place stringent constraints on the hyperon-nucleon interaction{\cite{Hammer2002, STAR-antiH3L}}, and have implications for understanding neutron star interiors, where strange matter may be present{\cite{Chatterjee2016}}. A precise comparison of the masses of the hypertriton and the antihypertriton allows us to test CPT symmetry in a nucleus with strangeness for the first time, and we observe no deviation from the expected exact symmetry.

The Breit-Wheeler process which produces matter and anti-matter from photon collisions is investigated experimentally through the observation of 6085 exclusive electron-positron pairs in ultra-peripheral Au+Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV. The measurements reveal a large fourth-order angular modulation of $\cos{4\Delta\phi}=(16.8\pm2.5)\%$ and smooth invariant mass distribution absent of vector mesons ($\phi$, $\omega$ and $\rho$) at the experimental limit of $\le 0.2\%$ of the observed yields. The differential cross section as a function of $e^+e^-$ pair transverse momentum $P_\perp$ peaks at low value with $\sqrt{ \langle P_\perp^2 \rangle } = 38.1\pm0.9$ MeV and displays a significant centrality dependence. These features are consistent with QED calculations for the collision of linearly polarized photons quantized from the extremely strong electromagnetic fields generated by the highly charged Au nuclei at ultra-relativistic speed. The experimental results have implications for vacuum birefringence and for mapping the magnetic field which is important for emergent QCD phenomena.

We report a systematic measurement of cumulants, $C_{n}$, for net-proton, proton and antiproton multiplicity distributions, and correlation functions, $\kappa_n$, for proton and antiproton multiplicity distributions up to the fourth order in Au+Au collisions at $\sqrt{s_{\mathrm {NN}}}$ = 7.7, 11.5, 14.5, 19.6, 27, 39, 54.4, 62.4 and 200 GeV. The $C_{n}$ and $\kappa_n$ are presented as a function of collision energy, centrality and kinematic acceptance in rapidity, $y$, and transverse momentum, $p_{T}$. The data were taken during the first phase of the Beam Energy Scan (BES) program (2010 -- 2017) at the BNL Relativistic Heavy Ion Collider (RHIC) facility. The measurements are carried out at midrapidity ($|y| <$ 0.5) and transverse momentum 0.4 $<$ $p_{\rm T}$ $<$ 2.0 GeV/$c$, using the STAR detector at RHIC. We observe a non-monotonic energy dependence ($\sqrt{s_{\mathrm {NN}}}$ = 7.7 -- 62.4 GeV) of the net-proton $C_{4}$/$C_{2}$ with the significance of 3.1$\sigma$ for the 0-5\% central Au+Au collisions. This is consistent with the expectations of critical fluctuations in a QCD-inspired model. Thermal and transport model calculations show a monotonic variation with $\sqrt{s_{\mathrm {NN}}}$. For the multiparticle correlation functions, we observe significant negative values for a two-particle correlation function, $\kappa_2$, of protons and antiprotons, which are mainly due to the effects of baryon number conservation. Furthermore, it is found that the four-particle correlation function, $\kappa_4$, of protons plays a role in determining the energy dependence of proton $C_4/C_1$ below 19.6 GeV, which cannot be understood by the effect of baryon number conservation.

quarks (φ, Ξ -), indicating that D 0 mesons kinetically decouple from the system earlier than light hadrons.The calculated D 0 nuclear modification factors re-affirm that charm quarks suffer large amount of energy loss in the medium, similar to those of light quarks for pT > 4 GeV/c in central 0-10% Au+Au collisions.At low pT , the nuclear modification factors show a characteristic structure qualitatively consistent with the expectation from model predictions that charm quarks gain sizable collective motion during the medium evolution.The improved measurements are expected to offer new constraints to model calculations and help gain further insights into the hot and dense medium created in these collisions.

We report the first measurement of rapidity-odd directed flow ($v_{1}$) for $D^{0}$ and $\overline{D^{0}}$ mesons at mid-rapidity ($|y| < 0.8$) in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200\,GeV using the STAR detector at the Relativistic Heavy Ion Collider. In 10--80\% Au+Au collisions, the slope of the $v_{1}$ rapidity dependence ($dv_{1}/dy$), averaged over $D^{0}$ and $\overline{D^{0}}$ mesons, is -0.080 $\pm$ 0.017 (stat.) $\pm$ 0.016 (syst.) for transverse momentum $p_{\rm T}$ above 1.5~GeV/$c$. The absolute value of $D^0$-meson $dv_1/dy$ is about 25 times larger than that for charged kaons, with 3.4$\sigma$ significance. These data give a unique insight into the initial tilt of the produced matter, and offer constraints on the geometric and transport parameters of the hot QCD medium created in relativistic heavy-ion collisions.

Global polarization of $\Xi$ and $\Omega$ hyperons has been measured for the first time in Au+Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV. The measurements of the $\Xi^-$ and $\bar{\Xi}^+$ hyperon polarization have been performed by two independent methods, via analysis of the angular distribution of the daughter particles in the parity violating weak decay $\Xi\rightarrow\Lambda+\pi$, as well as by measuring the polarization of the daughter $\Lambda$-hyperon, polarized via polarization transfer from its parent. The polarization, obtained by combining the results from the two methods and averaged over $\Xi^-$ and $\bar{\Xi}^+$, is measured to be $\langle P_\Xi \rangle = 0.47\pm0.10~({\rm stat.})\pm0.23~({\rm syst.})\,\%$ for the collision centrality 20\%-80\%. The $\langle P_\Xi \rangle$ is found to be slightly larger than the inclusive $\Lambda$ polarization and in reasonable agreement with a multi-phase transport model (AMPT). The $\langle P_\Xi \rangle$ is found to follow the centrality dependence of the vorticity predicted in the model, increasing toward more peripheral collisions. The global polarization of $\Omega$, $\langle P_\Omega \rangle = 1.11\pm0.87~({\rm stat.})\pm1.97~({\rm syst.})\,\%$ was obtained by measuring the polarization of daughter $\Lambda$ in the decay $\Omega \rightarrow \Lambda + K$, assuming the polarization transfer factor $C_{\Omega\Lambda}=1$.

Global hyperon polarization, $\overline{P}_\mathrm{H}$, in Au+Au collisions over a large range of collision energy, $\sqrt{s_\mathrm{NN}}$, was recently measured and successfully reproduced by hydrodynamic and transport models with intense fluid vorticity of the quark-gluon plasma. While na\"{i}ve extrapolation of data trends suggests a large $\overline{P}_\mathrm{H}$ as the collision energy is reduced, the behavior of $\overline{P}_\mathrm{H}$ at small $\sqrt{s_\mathrm{NN}}<7.7$ GeV is unknown. Operating the STAR experiment in fixed-target mode, we measured the polarization of $\Lambda$ hyperons along the direction of global angular momentum in Au+Au collisions at $\sqrt{s_\mathrm{NN}}=3$ GeV. The observation of substantial polarization of $4.91\pm0.81(\rm stat.)\pm0.15(\rm syst.)$% in these collisions may require a reexamination of the viscosity of any fluid created in the collision, of the thermalization timescale of rotational modes, and of hadronic mechanisms to produce global polarization.

We describe an analysis comparing the $p\overline{p}$ elastic cross section as measured by the D0 Collaboration at a center-of-mass energy of 1.96 TeV to that in $pp$ collisions as measured by the TOTEM Collaboration at 2.76, 7, 8, and 13 TeV using a model-independent approach. The TOTEM cross sections, extrapolated to a center-of-mass energy of $\sqrt{s}=1.96\text{ }\text{ }\mathrm{TeV}$, are compared with the D0 measurement in the region of the diffractive minimum and the second maximum of the $pp$ cross section. The two data sets disagree at the $3.4\ensuremath{\sigma}$ level and thus provide evidence for the $t$-channel exchange of a colorless, $C$-odd gluonic compound, also known as the odderon. We combine these results with a TOTEM analysis of the same $C$-odd exchange based on the total cross section and the ratio of the real to imaginary parts of the forward elastic strong interaction scattering amplitude in $pp$ scattering for which the significance is between $3.4\ensuremath{\sigma}$ and $4.6\ensuremath{\sigma}$. The combined significance is larger than $5\ensuremath{\sigma}$ and is interpreted as the first observation of the exchange of a colorless, $C$-odd gluonic compound.

We report on the measurements of directed flow $v_1$ and elliptic flow $v_2$ for hadrons ($\pi^{\pm}$, $K^{\pm}$, $K_{S}^0$, $p$, $\phi$, $\Lambda$ and $\Xi^{-}$) from Au+Au collisions at $\sqrt{s_{NN}}$ = 3\,GeV and $v_{2}$ for ($\pi^{\pm}$, $K^{\pm}$, $p$ and $\overline{p}$) at 27 and 54.4\,GeV with the STAR experiment. While at the two higher energy midcentral collisions the number-of-constituent-quark (NCQ) scaling holds, at 3\,GeV the $v_{2}$ at midrapidity is negative for all hadrons and the NCQ scaling is absent. In addition, the $v_1$ slopes at midrapidity for almost all observed hadrons are found to be positive, implying dominant repulsive baryonic interactions. The features of negative $v_2$ and positive $v_1$ slope at 3\,GeV can be reproduced with a baryonic mean-field in transport model calculations. These results imply that the medium in such collisions is likely characterized by baryonic interactions.

We report cumulants of the proton multiplicity distribution from dedicated fixed-target Au+Au collisions at sqrt[s_{NN}]=3.0 GeV, measured by the STAR experiment in the kinematic acceptance of rapidity (y) and transverse momentum (p_{T}) within -0.5<y<0 and 0.4<p_{T}<2.0 GeV/c. In the most central 0%-5% collisions, a proton cumulant ratio is measured to be C_{4}/C_{2}=-0.85±0.09 (stat)±0.82 (syst), which is 2σ below the Poisson baseline with respect to both the statistical and systematic uncertainties. The hadronic transport UrQMD model reproduces our C_{4}/C_{2} in the measured acceptance. Compared to higher energy results and the transport model calculations, the suppression in C_{4}/C_{2} is consistent with fluctuations driven by baryon number conservation and indicates an energy regime dominated by hadronic interactions. These data imply that the QCD critical region, if created in heavy-ion collisions, could only exist at energies higher than 3 GeV.

We report the beam energy and collision centrality dependence of fifth and sixth order cumulants (C_{5}, C_{6}) and factorial cumulants (κ_{5}, κ_{6}) of net-proton and proton number distributions, from center-of-mass energy (sqrt[s_{NN}]) 3 GeV to 200 GeV Au+Au collisions at RHIC. Cumulant ratios of net-proton (taken as proxy for net-baryon) distributions generally follow the hierarchy expected from QCD thermodynamics, except for the case of collisions at 3 GeV. The measured values of C_{6}/C_{2} for 0%-40% centrality collisions show progressively negative trend with decreasing energy, while it is positive for the lowest energy studied. These observed negative signs are consistent with QCD calculations (for baryon chemical potential, μ_{B}≤110 MeV) which contains the crossover transition range. In addition, for energies above 7.7 GeV, the measured proton κ_{n}, within uncertainties, does not support the two-component (Poisson+binomial) shape of proton number distributions that would be expected from a first-order phase transition. Taken in combination, the hyperorder proton number fluctuations suggest that the structure of QCD matter at high baryon density, μ_{B}∼750 MeV at sqrt[s_{NN}]=3 GeV is starkly different from those at vanishing μ_{B}∼24 MeV at sqrt[s_{NN}]=200 GeV and higher collision energies.

Femtoscopy is a unique tool to investigate the space-time geometry of the matter created in ultra-relativistic collisions. If the probability density distribution of hadron emission is parametrized, then the dependence of its parameters on particle momentum, collision energy, and collision geometry can be given. In recent years, several measurements came to light that indicated the adequacy of assuming a Lévy-stable shape for the mentioned distribution. In parallel, several new phenomenological developments appeared, aiding the interpretation of the experimental results, or providing tools for the measurements. In this paper we review and discuss some of these advances, phenomenological and experimental.

J/psi production in d + Au and p + p collisions at square root of S(NN) = 200 GeV has been measured by the PHENIX experiment at rapidities -2.2 < y < +2.4. The cross sections and nuclear dependence of J/psi production versus rapidity, transverse momentum, and centrality are obtained and compared to lower energy p + A results and to theoretical models. The observed nuclear dependence in d + Au collisions is found to be modest, suggesting that the absorption in the final state is weak and the shadowing of the gluon distributions is small and consistent with Dokshitzer-Gribov-Lipatov-Altarelli-Parisi-based parametrizations that fit deep-inelastic scattering and Drell-Yan data at lower energies.

Differential elliptic flow (v2) for ϕ mesons and (anti)deuterons (¯d)d is measured for Au+Au collisions at √sNN=200 GeV. The v2 for ϕ mesons follows the trend of lighter π± and K± mesons, suggesting that ordinary hadrons interacting with standard hadronic cross sections are not the primary driver for elliptic flow development. The v2 values for (¯d)d suggest that elliptic flow is additive for composite particles. This further validation of the universal scaling of v2 per constituent quark for baryons and mesons suggests that partonic collectivity dominates the transverse expansion dynamics.Received 14 March 2007DOI:https://doi.org/10.1103/PhysRevLett.99.052301©2007 American Physical Society

Yields for J/psi production in Cu+Cu collisions at sqrt s_NN=200 GeV have been measured over the rapidity range |y|<2.2 and compared with results in p+p and Au+Au collisions at the same energy. The Cu+Cu data offer greatly improved precision over existing Au+Au data for J/psi production in collisions with small to intermediate numbers of participants, in the range where the quark-gluon plasma transition threshold is predicted to lie. Cold nuclear matter estimates based on ad hoc fits to d+Au data describe the Cu+Cu data up to N_part approximately 50, corresponding to a Bjorken energy density of at least 1.5 GeV/fm(3).

We present the first measurement of photoproduction of J/psi and of two-photon production of high-mass e+e- pairs in electromagnetic (or ultra-peripheral) nucleus-nucleus interactions, using Au+Au data at sqrt(s_NN) = 200 GeV. The events are tagged with forward neutrons emitted following Coulomb excitation of one or both Au^{star} nuclei. The event sample consists of 28 events with m_{e+e-} > 2 GeV/c^2 with zero like-sign background. The measured cross sections at midrapidity of d\sigma / dy (J/psi + Xn, y=0) = 76 +/- 33 (stat) +/- 11 (syst) micro b and d^2\sigma/dm dy (e^+e^- + Xn, y=0) = 86 +/- 23 (stat) +/- 16 (syst) micro b/(GeV/c^2) for m_{e+e-} \in [2.0,2.8] GeV/c^2 are consistent with various theoretical predictions.

The PHENIX experiment has measured the suppression of semi-inclusive single high-transverse-momentum π0's in Au+Au collisions at √sNN=200 GeV. The present understanding of this suppression is in terms of energy loss of the parent (fragmenting) parton in a dense color-charge medium. We have performed a quantitative comparison between various parton energy-loss models and our experimental data. The statistical point-to-point uncorrelated as well as correlated systematic uncertainties are taken into account in the comparison. We detail this methodology and the resulting constraint on the model parameters, such as the initial color-charge density dNg/dy, the medium transport coefficient ⟨^q⟩, or the initial energy-loss parameter ε0. We find that high-transverse-momentum π0 suppression in Au+Au collisions has sufficient precision to constrain these model-dependent parameters at the ±20–25% (one standard deviation) level. These constraints include only the experimental uncertainties, and further studies are needed to compute the corresponding theoretical uncertainties.2 MoreReceived 10 January 2008DOI:https://doi.org/10.1103/PhysRevC.77.064907©2008 American Physical Society

We present a new analysis of J/ψ production yields in deuteron-gold collisions at √sNN=200 GeV using data taken from the PHENIX experiment in 2003 and previously published in S. S. Adler et al. [Phys. Rev. Lett 96, 012304 (2006)]. The high statistics proton-proton J/ψ data taken in 2005 are used to improve the baseline measurement and thus construct updated cold nuclear matter modification factors (RdAu). A suppression of J/ψ in cold nuclear matter is observed as one goes forward in rapidity (in the deuteron-going direction), corresponding to a region more sensitive to initial-state low-x gluons in the gold nucleus. The measured nuclear modification factors are compared to theoretical calculations of nuclear shadowing to which a J/ψ (or precursor) breakup cross section is added. Breakup cross sections of σbreakup=2.8+1.7−1.4 (2.2+1.6−1.5) mb are obtained by fitting these calculations to the data using two different models of nuclear shadowing. These breakup cross-section values are consistent within large uncertainties with the 4.2±0.5 mb determined at lower collision energies. Projecting this range of cold nuclear matter effects to copper-copper and gold-gold collisions reveals that the current constraints are not sufficient to firmly quantify the additional hot nuclear matter effect.6 MoreReceived 26 November 2007DOI:https://doi.org/10.1103/PhysRevC.77.024912©2008 American Physical Society

We have measured the azimuthal anisotropy of π⁰ production for 1<p(T)<18 GeV/c for Au+Au collisions at sqrt((s)NN)=200 GeV. The observed anisotropy shows a gradual decrease for 3≲p(T)≲7-10 GeV/c, but remains positive beyond 10 GeV/c. The magnitude of this anisotropy is underpredicted, up to at least ∼10 GeV/c, by current perturbative QCD (PQCD) energy-loss model calculations. An estimate of the increase in anisotropy expected from initial-geometry modification due to gluon saturation effects and fluctuations is insufficient to account for this discrepancy. Calculations that implement a path-length dependence steeper than what is implied by current PQCD energy-loss models show reasonable agreement with the data.

We present measurements of $J/\ensuremath{\psi}$ yields in $d+\mathrm{Au}$ collisions at $\sqrt{{s}_{NN}}=200\text{ }\text{ }\mathrm{GeV}$ recorded by the PHENIX experiment and compare them with yields in $p+p$ collisions at the same energy per nucleon-nucleon collision. The measurements cover a large kinematic range in $J/\ensuremath{\psi}$ rapidity ($\ensuremath{-}2.2&lt;y&lt;2.4$) with high statistical precision and are compared with two theoretical models: one with nuclear shadowing combined with final state breakup and one with coherent gluon saturation effects. In order to remove model dependent systematic uncertainties we also compare the data to a simple geometric model. The forward rapidity data are inconsistent with nuclear modifications that are linear or exponential in the density weighted longitudinal thickness, such as those from the final state breakup of the bound state.

PHENIX has measured the centrality dependence of midrapidity pion, kaon, and proton transverse momentum distributions in d + Au and p + p collisions at √ s NN = 200 GeV.The p + p data provide a reference for nuclear effects in d + Au and previously measured Au + Au collisions.Hadron production is enhanced in d + Au, relative to independent nucleon-nucleon scattering, as was observed in lower

The PHENIX experiment presents results from the RHIC 2006 run with polarized $p+p$ collisions at $\sqrt{s}=62.4\text{ }\text{ }\mathrm{GeV}$, for inclusive ${\ensuremath{\pi}}^{0}$ production at midrapidity. Unpolarized cross section results are measured for transverse momenta ${p}_{T}=0.5$ to $7\text{ }\text{ }\mathrm{GeV}/c$. Next-to-leading order perturbative quantum chromodynamics calculations are compared with the data, and while the calculations are consistent with the measurements, next-to-leading logarithmic corrections improve the agreement. Double helicity asymmetries ${A}_{LL}$ are presented for ${p}_{T}=1$ to $4\text{ }\text{ }\mathrm{GeV}/c$ and probe the higher range of Bjorken $x$ of the gluon (${x}_{g}$) with better statistical precision than our previous measurements at $\sqrt{s}=200\text{ }\text{ }\mathrm{GeV}$. These measurements are sensitive to the gluon polarization in the proton for $0.06&lt;{x}_{g}&lt;0.4$.

Differential measurements of the elliptic (v(2)) and hexadecapole (v(4)) Fourier flow coefficients are reported for charged hadrons as a function of transverse momentum (p(T)) and collision centrality or number of participant nucleons (N(part)) for Au+Au collisions at sq.rt(s(NN))=200 GeV. The v(2,4) measurements at pseudorapidity |η|≤0.35, obtained with four separate reaction-plane detectors positioned in the range 1.0<|η|<3.9, show good agreement, indicating the absence of significant Δη-dependent nonflow correlations. Sizable values for v(4)(p(T)) are observed with a ratio v(4)(p(T),N(part))/v(2)(2)(p(T),N(part))≈0.8 for 50≲N(part)≲200, which is compatible with the combined effects of a finite viscosity and initial eccentricity fluctuations. For N(part)≳200 this ratio increases up to 1.7 in the most central collisions.

Inclusive transverse momentum spectra of \ensuremath{\eta} mesons in the range ${p}_{T}\ensuremath{\approx}2$--12 GeV/$c$ have been measured at midrapidity ($|\ensuremath{\eta}|&lt;0.35$) by the PHENIX experiment at RHIC in $p+p,d$+Au, and Au+Au collisions at $\sqrt{{s}_{\mathit{NN}}}=200$ GeV. The \ensuremath{\eta} mesons are reconstructed through their $\ensuremath{\eta}\ensuremath{\rightarrow}\ensuremath{\gamma} \ensuremath{\gamma}$ channel for the three colliding systems as well as through the $\ensuremath{\eta}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ decay mode in $p+p$ and $d$+Au collisions. The nuclear modification factor in $d$+Au collisions, ${R}_{d\mathrm{Au}}({p}_{T})\ensuremath{\approx}1.0$--1.1, suggests at most only modest ${p}_{T}$ broadening (``Cronin enhancement''). In central Au+Au reactions, the \ensuremath{\eta} yields are significantly suppressed, with ${R}_{\mathrm{AuAu}}({p}_{T})\ensuremath{\approx}0.2$. The ratio of \ensuremath{\eta} to ${\ensuremath{\pi}}^{0}$ yields is approximately constant as a function of ${p}_{T}$ for the three colliding systems in agreement with the high-${p}_{T}$ world average of ${R}_{\ensuremath{\eta}/{\ensuremath{\pi}}^{0}}\ensuremath{\approx}0.5$ in hadron-hadron, hadron-nucleus, and nucleus-nucleus collisions for a wide range of center-of-mass energies ($\sqrt{{s}_{\mathit{NN}}}\ensuremath{\approx}3$--1800 GeV) as well as, for high scaled momentum ${x}_{p}$, in ${e}^{+}{e}^{\ensuremath{-}}$ annihilations at $\sqrt{s}=91.2$ GeV. These results are consistent with a scenario where high-${p}_{T}$ \ensuremath{\eta} production in nuclear collisions at the Relativistic Heavy Ion Collider is largely unaffected by initial-state effects but where light-quark mesons (${\ensuremath{\pi}}^{0},\ensuremath{\eta}$) are equally suppressed due to final-state interactions of the parent partons in the dense medium produced in Au+Au reactions.

The PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC) has performed systematic measurements of phi meson production in the K+K- decay channel at midrapidity in p+p, d+Au, Cu+Cu and Au+Au collisions at sqrt(S_NN)=200 GeV. Results are presented on the phi invariant yield and the nuclear modification factor R_AA for Au+Au and Cu+Cu, and R_dA for d+Au collisions, studied as a function of transverse momentum (1<p_T<7 GeV/c) and centrality. In central and mid-central Au+Au collisions, the R_AA of phi exhibits a suppression relative to expectations from binary scaled p+p results. The amount of suppression is smaller than that of the neutral pion and the eta meson in the intermediate p_T range (2--5 GeV/c); whereas at higher p_T the phi, pi^0, and eta show similar suppression. The baryon (protons and anti-protons) excess observed in central Au+Au collisions at intermediate p_T is not observed for the phi meson despite the similar mass of the proton and the phi. This suggests that the excess is linked to the number of constituent quarks rather than the hadron mass. The difference gradually disappears with decreasing centrality and for peripheral collisions the R_AA values for both particles are consistent with binary scaling. Cu+Cu collisions show the same yield and suppression as Au+Au collisions for the same number of N_part. The R_dA of phi shows no evidence for cold nuclear effects within uncertainties.

We report the measurement of direct photons at midrapidity in Au+Au collisions at sqrt{s_NN} = 200 GeV. The direct photon signal was extracted for the transverse-momentum range of 4 GeV/c < p_T < 22 GeV/c, using a statistical method to subtract decay photons from the inclusive-photon sample. The direct-photon nuclear-modification factor R_AA was calculated as a function of p_T for different Au+Au collision centralities using the measured p+p direct-photon spectrum and compared to theoretical predictions. R_AA was found to be consistent with unity for all centralities over the entire measured p_T range. Theoretical models that account for modifications of initial-direct-photon production due to modified-parton-distribution functions in Au and the different isospin composition of the nuclei, predict a modest change of R_AA from unity and are consistent with the data. Models with compensating effects of the quark-gluon plasma on high-energy photons, such as suppression of jet-fragmentation photons and induced-photon bremsstrahlung from partons traversing the medium, are also consistent with this measurement.

We present results for three charmonia states (psi^prime, chi_c and J/psi) in d+Au collisions at |y|<0.35 and sqrt(s_NN)=200 GeV. We find that the modification of the psi^prime yield relative to that of the J/psi scales approximately with charged-particle multiplicity at midrapidity across p+A, d+Au, and A+A results from the Super Proton Synchrotron and the Relativistic Heavy Ion Collider. In large impact-parameter collisions we observe a similar suppression for the psi^prime and J/psi, while in small impact-parameter collisions the more weakly bound psi^prime is more strongly suppressed. Owing to the short time spent traversing the Au nucleus, the larger psi^prime suppression in central events is not explained by an increase of the nuclear absorption due to meson formation time effects.

We present inclusive charged hadron elliptic flow v_2 measured over the pseudorapidity range |\eta| < 0.35 in Au+Au collisions at sqrt(s_NN) = 200 GeV. Results for v_2 are presented over a broad range of transverse momentum (p_T = 0.2-8.0 GeV/c) and centrality (0-60%). In order to study non-flow effects that are not correlated with the reaction plane, as well as the fluctuations of v_2, we compare two different analysis methods: (1) event plane method from two independent sub-detectors at forward (|\eta| = 3.1-3.9) and beam (|\eta| > 6.5) pseudorapidities and (2) two-particle cumulant method extracted using correlations between particles detected at midrapidity. The two event-plane results are consistent within systematic uncertainties over the measured p_T and in centrality 0-40%. There is at most 20% difference of the v_2 between the two event plane methods in peripheral (40-60%) collisions. The comparisons between the two-particle cumulant results and the standard event plane measurements are discussed.

We report on charmonium measurements [J/ψ (1S), ψ′ (2S), and χc (1P)] in p+p collisions at √s=200 GeV. We find that the fraction of J/ψ coming from the feed-down decay of ψ′ and χc in the midrapidity region (|y|<0.35) is 9.6±2.4% and 32±9%, respectively. We also present the pT and rapidity dependencies of the J/ψ yield measured via dielectron decay at midrapidity (|y|<0.35) and via dimuon decay at forward rapidity (1.2<|y|<2.2). The statistical precision greatly exceeds that reported in our previous publication [Phys. Rev. Lett. 98, 232002 (2007)]. The new results are compared with other experiments and discussed in the context of current charmonium production models.20 MoreReceived 9 May 2011DOI:https://doi.org/10.1103/PhysRevD.85.092004© 2012 American Physical Society

PHENIX has measured the electron–positron pair mass spectrum from 0 to 8GeV/c2 in p+p collisions at s=200GeV. The contributions from light meson decays to e+e− pairs have been determined based on measurements of hadron production cross sections by PHENIX. Within the systematic uncertainty of ∼20% they account for all e+e− pairs in the mass region below ∼1GeV/c2. The e+e− pair yield remaining after subtracting these contributions is dominated by semileptonic decays of charmed hadrons correlated through flavor conservation. Using the spectral shape predicted by PYTHIA, we estimate the charm production cross section to be 544±39(stat)±142(syst)±200(model)μb, which is consistent with QCD calculations and measurements of single leptons by PHENIX.

The PHENIX experiment has measured electrons and positrons at midrapidity from the decays of hadrons containing charm and bottom quarks produced in d+Au and p+p collisions at sqrt[S(NN)]=200 GeV in the transverse-momentum range 0.85 ≤ p(T)(e) ≤ 8.5 GeV/c. In central d+Au collisions, the nuclear modification factor R(dA) at 1.5<p(T)<5 GeV/c displays evidence of enhancement of these electrons, relative to those produced in p+p collisions, and shows that the mass-dependent Cronin enhancement observed at the Relativistic Heavy Ion Collider extends to the heavy D meson family. A comparison with the neutral-pion data suggests that the difference in cold-nuclear-matter effects on light- and heavy-flavor mesons could contribute to the observed differences between the π(0) and heavy-flavor-electron nuclear modification factors R(AA).

Measurements of the anisotropy parameter ${v}_{2}$ of identified hadrons (pions, kaons, and protons) as a function of centrality, transverse momentum ${p}_{T}$, and transverse kinetic energy KE${}_{T}$ at midrapidity ($|\ensuremath{\eta}|&lt;0.35$) in Au + Au collisions at $\sqrt{{s}_{NN}}=200$ GeV are presented. Pions and protons are identified up to ${p}_{T}=$ 6 GeV$/c$, and kaons up to ${p}_{T}=$ 4 GeV$/c$, by combining information from time-of-flight and aerogel \ifmmode \check{C}\else \v{C}\fi{}erenkov detectors in the PHENIX Experiment. The scaling of ${v}_{2}$ with the number of valence quarks (${n}_{q}$) has been studied in different centrality bins as a function of transverse momentum and transverse kinetic energy. A deviation from previously observed quark-number scaling is observed at large values of KE${}_{T}/{n}_{q}$ in noncentral Au + Au collisions ($20--60%$), but this scaling remains valid in central collisions ($0--10%$).

Measurements of transverse-single-spin asymmetries (${A}_{N}$) in $p+p$ collisions at $\sqrt{s}=62.4$ and 200 GeV with the PHENIX detector at the Relativistic Heavy Ion Collider are presented. At midrapidity, ${A}_{N}$ is measured for neutral pion and eta mesons reconstructed from diphoton decay, and, at forward rapidities, neutral pions are measured using both diphotons and electromagnetic clusters. The neutral-pion measurement of ${A}_{N}$ at midrapidity is consistent with zero with uncertainties a factor of 20 smaller than previous publications, which will lead to improved constraints on the gluon Sivers function. At higher rapidities, where the valence quark distributions are probed, the data exhibit sizable asymmetries. In comparison with previous measurements in this kinematic region, the new data extend the kinematic coverage in $\sqrt{s}$ and ${p}_{T}$, and it is found that the asymmetries depend only weakly on $\sqrt{s}$. The origin of the forward ${A}_{N}$ is presently not understood quantitatively. The extended reach to higher ${p}_{T}$ probes the transition between transverse momentum dependent effects at low ${p}_{T}$ and multiparton dynamics at high ${p}_{T}$.

The TOTEM experiment at the LHC has measured the inelastic proton-proton cross-section at in a β* = 90 m run with low inelastic pile-up. The measurement was based on events with at least one charged particle in the T2 telescope acceptance of 5.3 < |η| < 6.5 in pseudorapidity. Combined with data from the T1 telescope, covering 3.1 < |η| < 4.7, the cross-section for inelastic events with at least one |η| ⩽ 6.5 final-state particle was determined to be (70.5 ± 2.9) mb. This cross-section includes all central diffractive events of which maximally 0.25 mb is estimated to escape the detection of the telescopes. Based on models for low mass diffraction, the total inelastic cross-section was deduced to be (73.7 ± 3.4) mb. An upper limit of 6.31 mb at 95% confidence level on the cross-section for events with diffractive masses below 3.4 GeV was obtained from the difference between the overall inelastic cross-section obtained by TOTEM using elastic scattering and the cross-section for inelastic events with at least one |η| ⩽ 6.5 final-state particle.

We present measured J/psi production rates in d+Au collisions at sqrt(s_NN) = 200 GeV over a broad range of transverse momentum (p_T=0-14 GeV/c) and rapidity (-2.2<y<2.2). We construct the nuclear-modification factor R_dAu for these kinematics and as a function of collision centrality (related to impact parameter for the R_dAu collision). We find that the modification is largest for collisions with small impact parameters, and observe a suppression (R_dAu<1) for p_T<4 GeV/c at positive rapidities. At negative rapidity we observe a suppression for p_T<2 GeV/c then an enhancement (R_dAu>1) for p_T>2 GeV/c. The observed enhancement at negative rapidity has implications for the observed modification in heavy-ion collisions at high p_T.

Measurements of the midrapidity transverse energy distribution, $d\Et/d\eta$, are presented for $p$$+$$p$, $d$$+$Au, and Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV and additionally for Au$+$Au collisions at $\sqrt{s_{_{NN}}}=62.4$ and 130 GeV. The $d\Et/d\eta$ distributions are first compared with the number of nucleon participants $N_{\rm part}$, number of binary collisions $N_{\rm coll}$, and number of constituent-quark participants $N_{qp}$ calculated from a Glauber model based on the nuclear geometry. For Au$+$Au, $\mean{d\Et/d\eta}/N_{\rm part}$ increases with $N_{\rm part}$, while $\mean{d\Et/d\eta}/N_{qp}$ is approximately constant for all three energies. This indicates that the two component ansatz, $dE_{T}/d\eta \propto (1-x) N_{\rm part}/2 + x N_{\rm coll}$, which has been used to represent $E_T$ distributions, is simply a proxy for $N_{qp}$, and that the $N_{\rm coll}$ term does not represent a hard-scattering component in $E_T$ distributions. The $dE_{T}/d\eta$ distributions of Au$+$Au and $d$$+$Au are then calculated from the measured $p$$+$$p$ $E_T$ distribution using two models that both reproduce the Au$+$Au data. However, while the number-of-constituent-quark-participant model agrees well with the $d$$+$Au data, the additive-quark model does not.

High-energy proton- and deuteron-nucleus collisions provide an excellent tool for studying a wide array of physics effects, including modifications of parton distribution functions in nuclei, gluon saturation, and color neutralization and hadronization in a nuclear environment, among others. All of these effects are expected to have a significant dependence on the size of the nuclear target and the impact parameter of the collision, also known as the collision centrality. In this article, we detail a method for determining centrality classes in p(d)+A collisions via cuts on the multiplicity at backward rapidity (i.e., the nucleus-going direction) and for determining systematic uncertainties in this procedure. For d+Au collisions at sqrt(s_NN) = 200 GeV we find that the connection to geometry is confirmed by measuring the fraction of events in which a neutron from the deuteron does not interact with the nucleus. As an application, we consider the nuclear modification factors R_{p(d)+A}, for which there is a potential bias in the measured centrality dependent yields due to auto-correlations between the process of interest and the backward rapidity multiplicity. We determine the bias correction factor within this framework. This method is further tested using the HIJING Monte Carlo generator. We find that for d+Au collisions at sqrt(s_NN)=200 GeV, these bias corrections are small and vary by less than 5% (10%) up to p_T = 10 (20) GeV. In contrast, for p+Pb collisions at sqrt(s_NN) = 5.02 TeV we find these bias factors are an order of magnitude larger and strongly p_T dependent, likely due to the larger effect of multi-parton interactions.

Results are presented from data recorded in 2009 by the PHENIX experiment at the Relativistic Heavy Ion Collider for the double-longitudinal spin asymmetry, $A_{LL}$, for $\pi^0$ and $\eta$ production in $\sqrt{s} = 200$ GeV polarized $p$$+$$p$ collisions. Comparison of the $\pi^0$ results with different theory expectations based on fits of other published data showed a preference for small positive values of gluon polarization, $\Delta G$, in the proton in the probed Bjorken $x$ range. The effect of adding the new 2009 \pz data to a recent global analysis of polarized scattering data is also shown, resulting in a best fit value $\Delta G^{[0.05,0.2]}_{\mbox{DSSV}} = 0.06^{+0.11}_{-0.15}$ in the range $0.05<x<0.2$, with the uncertainty at $\Delta \chi^2 = 9$ when considering only statistical experimental uncertainties. Shifting the PHENIX data points by their systematic uncertainty leads to a variation of the best-fit value of $\Delta G^{[0.05,0.2]}_{\mbox{DSSV}}$ between $0.02$ and $0.12$, demonstrating the need for full treatment of the experimental systematic uncertainties in future global analyses.

Abstract The TOTEM collaboration has measured the elastic proton-proton differential cross section $$\mathrm{d}\sigma /\mathrm{d}t$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>d</mml:mi><mml:mi>σ</mml:mi><mml:mo>/</mml:mo><mml:mi>d</mml:mi><mml:mi>t</mml:mi></mml:mrow></mml:math> at $$\sqrt{s}=13$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msqrt><mml:mi>s</mml:mi></mml:msqrt><mml:mo>=</mml:mo><mml:mn>13</mml:mn></mml:mrow></mml:math> TeV LHC energy using dedicated $$\beta ^{*}=90$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mi>β</mml:mi><mml:mrow><mml:mrow /><mml:mo>∗</mml:mo></mml:mrow></mml:msup><mml:mo>=</mml:mo><mml:mn>90</mml:mn></mml:mrow></mml:math> m beam optics. The Roman Pot detectors were inserted to 10 $$\sigma $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>σ</mml:mi></mml:math> distance from the LHC beam, which allowed the measurement of the range [0.04 GeV $$^{2}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow /><mml:mn>2</mml:mn></mml:msup></mml:math> ; 4 GeV $$^{2}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow /><mml:mn>2</mml:mn></mml:msup></mml:math> $$]$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>]</mml:mo></mml:math> in four-momentum transfer squared | t |. The efficient data acquisition allowed to collect about 10 $$^{9}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow /><mml:mn>9</mml:mn></mml:msup></mml:math> elastic events to precisely measure the differential cross-section including the diffractive minimum (dip), the subsequent maximum (bump) and the large-| t | tail. The average nuclear slope has been found to be $$B=(20.40 \pm 0.002^{\mathrm{stat}} \pm 0.01^{\mathrm{syst}})~$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>B</mml:mi><mml:mo>=</mml:mo><mml:mo>(</mml:mo><mml:mn>20.40</mml:mn><mml:mo>±</mml:mo><mml:mn>0</mml:mn><mml:mo>.</mml:mo><mml:msup><mml:mn>002</mml:mn><mml:mi>stat</mml:mi></mml:msup><mml:mo>±</mml:mo><mml:mn>0</mml:mn><mml:mo>.</mml:mo><mml:msup><mml:mn>01</mml:mn><mml:mi>syst</mml:mi></mml:msup><mml:mo>)</mml:mo><mml:mspace /></mml:mrow></mml:math> GeV $$^{-2}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow /><mml:mrow><mml:mo>-</mml:mo><mml:mn>2</mml:mn></mml:mrow></mml:msup></mml:math> in the | t |-range 0.04–0.2 GeV $$^{2}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow /><mml:mn>2</mml:mn></mml:msup></mml:math> . The dip position is $$|t_{\mathrm{dip}}|=(0.47 \pm 0.004^{\mathrm{stat}} \pm 0.01^{\mathrm{syst}})~$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mrow><mml:mo>|</mml:mo></mml:mrow><mml:msub><mml:mi>t</mml:mi><mml:mi>dip</mml:mi></mml:msub><mml:mrow><mml:mo>|</mml:mo><mml:mo>=</mml:mo></mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mn>0.47</mml:mn><mml:mo>±</mml:mo><mml:mn>0</mml:mn><mml:mo>.</mml:mo><mml:msup><mml:mn>004</mml:mn><mml:mi>stat</mml:mi></mml:msup><mml:mo>±</mml:mo><mml:mn>0</mml:mn><mml:mo>.</mml:mo><mml:msup><mml:mn>01</mml:mn><mml:mi>syst</mml:mi></mml:msup><mml:mo>)</mml:mo></mml:mrow><mml:mspace /></mml:mrow></mml:math> GeV $$^{2}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow /><mml:mn>2</mml:mn></mml:msup></mml:math> . The differential cross section ratio at the bump vs. at the dip $$R=1.77\pm 0.01^{\mathrm{stat}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>R</mml:mi><mml:mo>=</mml:mo><mml:mn>1.77</mml:mn><mml:mo>±</mml:mo><mml:mn>0</mml:mn><mml:mo>.</mml:mo><mml:msup><mml:mn>01</mml:mn><mml:mi>stat</mml:mi></mml:msup></mml:mrow></mml:math> has been measured with high precision. The series of TOTEM elastic pp measurements show that the dip is a permanent feature of the pp differential cross-section at the TeV scale.

We report on the first measurement of the charmed baryon Λ ± c production at midrapidity (|y| < 1) in Au+Au collisions at √ s NN = 200 GeV collected by the STAR experiment at the Relativistic Heavy

The Beam Energy Scan (BES) program at the Relativistic Heavy Ion Collider (RHIC) was extended to energies below $\sqrt{\textit{s}_{NN}}$ = 7.7 GeV in 2015 by successful implementation of the fixed-target mode of operation in the STAR (Solenoidal Track At RHIC) experiment. In the fixed-target mode, ions circulate in one ring of the collider and interact with a stationary target at the entrance of the STAR Time Projection Chamber. The first results for Au+Au collisions at $\sqrt{\textit{s}_{NN}}$ = 4.5 GeV are presented, including directed and elliptic flow of identified hadrons, and radii from pion femtoscopy. The proton flow and pion femtoscopy results agree quantitatively with earlier measurements by Alternating Gradient Synchrotron experiments at similar energies. This validates running the STAR experiment in the fixed-target configuration. Pion directed and elliptic flow are presented for the first time at this beam energy. Pion and proton elliptic flow show behavior which hints at constituent quark scaling, but large error bars preclude reliable conclusions. The ongoing second phase of BES (BES-II) will provide fixed-target data sets with 100 times more events at each of several energies down to $\sqrt{\textit{s}_{NN}}$ = 3.0 GeV.

In high-energy heavy-ion collisions, partonic collectivity is evidenced by the constituent quark number scaling of elliptic flow anisotropy for identified hadrons. A breaking of this scaling and dominance of baryonic interactions is found for identified hadron collective flow measurements in sNN = 3 GeV Au+Au collisions. In this paper, we report measurements of the first- and second-order azimuthal anisotropic parameters, v1 and v2, of light nuclei (d, t, 3He, 4He) produced in sNN = 3 GeV Au+Au collisions at the STAR experiment. An atomic mass number scaling is found in the measured v1 slopes of light nuclei at mid-rapidity. For the measured v2 magnitude, a strong rapidity dependence is observed. Unlike v2 at higher collision energies, the v2 values at mid-rapidity for all light nuclei are negative and no scaling is observed with the atomic mass number. Calculations by the Jet AA Microscopic Transport Model (JAM), with baryonic mean-field plus nucleon coalescence, are in good agreement with our observations, implying baryonic interactions dominate the collective dynamics in 3 GeV Au+Au collisions at RHIC.

The chiral magnetic effect (CME) refers to charge separation along a strong magnetic field due to imbalanced chirality of quarks in local parity and charge-parity violating domains in quantum chromodynamics. The experimental measurement of the charge separation is made difficult by the presence of a major background from elliptic azimuthal anisotropy. This background and the CME signal have different sensitivities to the spectator and participant planes, and could thus be determined by measurements with respect to these planes. We report such measurements in Au+Au collisions at a nucleon-nucleon center-of-mass energy of 200 GeV at the Relativistic Heavy-Ion Collider. It is found that the charge separation, with the flow background removed, is consistent with zero in peripheral (large impact parameter) collisions. Some indication of finite CME signals is seen in midcentral (intermediate impact parameter) collisions. Significant residual background effects may, however, still be present.

We measure triangular flow relative to the reaction plane at 3 GeV center-of-mass energy in Au+Au collisions at the BNL Relativistic Heavy Ion Collider. A significant v3 signal for protons is observed, which increases for higher rapidity, higher transverse momentum, and more peripheral collisions. The triangular flow is essentially rapidity-odd with a slope at midrapidity, dv3/dy|(y=0), opposite in sign compared to the slope for directed flow. No significant v3 signal is observed for charged pions and kaons. Comparisons with models suggest that a mean field potential is required to describe these results, and that the triangular shape of the participant nucleons is the result of stopping and nuclear geometry.7 MoreReceived 21 September 2023Accepted 6 March 2024DOI:https://doi.org/10.1103/PhysRevC.109.044914©2024 American Physical SocietyPhysics Subject Headings (PhySH)Equations of state of nuclear matterResearch AreasNuclear structure & decaysNuclear matterEquations of state of nuclear matterResearch AreasCollective flowEquations of state of nuclear matterRelativistic heavy-ion collisionsTransport in heavy-ion collisionsNuclear Physics

width of the away-side peak is shown to be a convolution of j T with the fragmentation variable, z, and the partonic transverse momentum, k T .The hzi is determined through a combined analysis of the measured 0 inclusive and associated spectra using jet-fragmentation functions measured in e e ÿ collisions.The final extracted values of k T are then determined to also be independent of the trigger particle transverse momentum, over the

The momentum distribution of electrons from semileptonic decays of charm and bottom quarks for midrapidity |y|<0.35 in p+p collisions at square root of s=200 GeV is measured by the PHENIX experiment at the Relativistic Heavy Ion Collider over the transverse momentum range 2<pT<7 GeV/c. The ratio of the yield of electrons from bottom to that from charm is presented. The ratio is determined using partial D/D-->e(+/-)K(-/+)X (K unidentified) reconstruction. It is found that the yield of electrons from bottom becomes significant above 4 GeV/c in pT. A fixed-order-plus-next-to-leading-log perturbative quantum chromodynamics calculation agrees with the data within the theoretical and experimental uncertainties. The extracted total bottom production cross section at this energy is sigma(bb)=3.2(-1.1)(+1.2)(stat)(-1.3)(+1.4)(syst)mub.

The double helicity asymmetry in neutral pion production for pT=1 to 12 GeV/c was measured with the PHENIX experiment to access the gluon-spin contribution, DeltaG, to the proton spin. Measured asymmetries are consistent with zero, and at a theory scale of micro2=4 GeV2 a next to leading order QCD analysis gives DeltaG[0.02,0.3]=0.2, with a constraint of -0.7<DeltaG[0.02,0.3]<0.5 at Deltachi2=9 (approximately 3sigma) for the sampled gluon momentum fraction (x) range, 0.02 to 0.3. The results are obtained using predictions for the measured asymmetries generated from four representative fits to polarized deep inelastic scattering data. We also consider the dependence of the DeltaG constraint on the choice of the theoretical scale, a dominant uncertainty in these predictions.

Neutral pion transverse momentum (p(T)) spectra at midrapidity (|y| less than or approximately 0.35) were measured in Cu+Cu collisions at sqrt[s(NN)]=22.4, 62.4, and 200 GeV. Relative to pi_(0) yields in p+p collisions scaled by the number of inelastic nucleon-nucleon collisions (N(coll) the pi_(0) yields for p(T) more than or approximately 2 GeV/c in central Cu+Cu collisions are suppressed at 62.4 and 200 GeV whereas an enhancement is observed at 22.4 GeV. A comparison with a jet-quenching model suggests that final state parton energy loss dominates in central Cu+Cu collisions at 62.4 and 200 GeV, while the enhancement at 22.4 GeV is consistent with nuclear modifications in the initial state alone.

We report the measurement of the transverse momentum dependence of inclusive J/psi polarization in p+p collisions at sqrt(s)=200 GeV performed by the PHENIX Experiment at RHIC. The polarization is studied in the helicity, Gottfried-Jackson, and Collins-Soper frames for p_T < 5 GeV/c and |y| < 0.35. The J/psi polarization in the helicity and Gottfried-Jackson frames is consistent with zero for all transverse momenta, with a slight (1.8 sigma) trend towards longitudinal polarization for transverse momenta above 2 GeV/c. No conclusion is allowed due to the limited acceptance in the Collins-Soper frame and the uncertainties of the current data. The results are compared to observations for other collision systems and center of mass energies and to different quarkonia production models.

A comprehensive survey of event-by-event fluctuations of charged hadron multiplicity in relativistic heavy ions is presented. The survey covers Au+Au collisions at sqrt(s_NN) = 62.4 and 200 GeV, and Cu+Cu collisions sqrt(s_NN) = 22.5, 62.4, and 200 GeV. Fluctuations are measured as a function of collision centrality, transverse momentum range, and charge sign. After correcting for non-dynamical fluctuations due to fluctuations in the collision geometry within a centrality bin, the remaining dynamical fluctuations expressed as the variance normalized by the mean tend to decrease with increasing centrality. The dynamical fluctuations are consistent with or below the expectation from a superposition of participant nucleon-nucleon collisions based upon p+p data, indicating that this dataset does not exhibit evidence of critical behavior in terms of the compressibility of the system. An analysis of Negative Binomial Distribution fits to the multiplicity distributions demonstrates that the heavy ion data exhibit weak clustering properties.

Azimuthal angle ($\ensuremath{\Delta}\ensuremath{\phi}$) correlations are presented for charged hadrons from dijets for $0.4&lt;{p}_{T}&lt;10$ GeV/$c$ in Au+Au collisions at $\sqrt{{s}_{\mathrm{NN}}}=200$ GeV. With increasing ${p}_{T}$, the away-side distribution evolves from a broad and relatively flat shape to a concave shape, then to a convex shape. Comparisons to $p+p$ data suggest that the away-side can be divided into a partially suppressed ``head'' region centered at $\ensuremath{\Delta}\ensuremath{\phi}~\ensuremath{\pi}$ and an enhanced ``shoulder'' region centered at $\ensuremath{\Delta}\ensuremath{\phi}~\ensuremath{\pi}\ifmmode\pm\else\textpm\fi{}1.1$. The ${p}_{T}$ spectrum for the head region softens toward central collisions, consistent with the onset of jet quenching. The spectral slope for the shoulder region is independent of centrality and trigger ${p}_{T}$, which offers constraints on energy transport mechanisms and suggests that it contains the medium response to energetic jets.

We report the observation at the Relativistic Heavy Ion Collider (RHIC) of suppression of back-to-back correlations in the direct photon+jet channel in Au+Au relative to p+p collisions. Two-particle correlations of direct photon triggers with associated hadrons are obtained by statistical subtraction of the decay photon-hadron background. The initial momentum of the away-side parton is tightly constrained, because the parton-photon pair exactly balance in momentum at leading order in perturbative quantum chromodynamics (pQCD), making such correlations a powerful probe of the in-medium parton energy loss. The away-side nuclear suppression factor, I_AA, in central Au+Au collisions, is 0.32 +/- 0.12(stat) +/- 0.09(syst) for hadrons of 3 < p_T < 5 in coincidence with photons of 5 < p_T < 15 GeV/c. The suppression is comparable to that observed for high-p_T single hadrons and dihadrons. The direct photon associated yields in p+p collisions scale approximately with the momentum balance, z_T = p_T^hadron/p_T^photon, as expected for a measure of the away-side parton fragmentation function. We compare to Au+Au collisions for which the momentum balance dependence of the nuclear modification should be sensitive to the path-length dependence of parton energy loss.

Hard-scattered parton probes produced in collisions of large nuclei indicate large partonic energy loss, possibly with collective produced-medium response to the lost energy. We present measurements of π^{0} trigger particles at transverse momenta p{T}{t}=4-12 GeV/c and associated charged hadrons (p{T}{a}=0.5-7 GeV/c) vs relative azimuthal angle Δϕ in Au+Au and p+p collisions at sqrt[s{NN}]=200 GeV. The Au+Au distribution at low p{T}{a}, whose shape has been interpreted as a medium effect, is modified for p{T}{t}<7 GeV/c. At higher p{T}{t}, the data are consistent with unmodified or very weakly modified shapes, even for the lowest measured p{T}{a}, which quantitatively challenges some medium response models. The associated yield of hadrons opposing the trigger particle in Au+Au relative to p+p (I{AA}) is suppressed at high p{T} (I{AA}≈0.35-0.5), but less than for inclusive suppression (R{AA}≈0.2).

Large parity-violating longitudinal single-spin asymmetries A(L)(e+) = -0.86(-0.14) (+0.30) and A(L)(e-) = 0.88(-0.71) (+0.12) are observed for inclusive high transverse momentum electrons and positrons in polarized p+p collisions at a center-of-mass energy of sqrt[s] = 500 GeV with the PHENIX detector at RHIC. These e± come mainly from the decay of W± and Z0 bosons, and their asymmetries directly demonstrate parity violation in the couplings of the W± to the light quarks. The observed electron and positron yields were used to estimate W± boson production cross sections for the e± channels of σ(pp → W+ X) × BR(W+ → e+ ν(e)) = 144.1 ± 21.2(stat)(-10.3) (+3.4) (syst) ± 21.6(norm) pb, and σ(pp → W- X) × BR(W- → e- ν[over ¯](e)) = 31.7 ± 12.1(stat)(-8.2) (+10.1) (syst) ± 4.8(norm) pb.

and reflect the initial geometry.Good agreement is observed between the measured

Recently, multiparticle-correlation measurements of relativistic p/d/^{3}He+Au, p+Pb, and even p+p collisions show surprising collective signatures. Here, we present beam-energy-scan measurements of two-, four-, and six-particle angular correlations in d+Au collisions at sqrt[s_{NN}]=200, 62.4, 39, and 19.6 GeV. We also present measurements of two- and four-particle angular correlations in p+Au collisions at sqrt[s_{NN}]=200 GeV. We find the four-particle cumulant to be real valued for d+Au collisions at all four energies. We also find that the four-particle cumulant in p+Au has the opposite sign as that in d+Au. Further, we find that the six-particle cumulant agrees with the four-particle cumulant in d+Au collisions at 200 GeV, indicating that nonflow effects are subdominant. These observations provide strong evidence that the correlations originate from the initial geometric configuration, which is then translated into the momentum distribution for all particles, commonly referred to as collectivity.

The PHENIX experiment has measured open heavy-flavor production via semileptonic decay muons over the transverse momentum range 1 < pT < 6 GeV/c at forward and backward rapidity (1.4 < |y| < 2.0) in d+Au and p+p collisions at ?sNN = 200 GeV. In central d+Au collisions an enhancement (suppression) of heavy-flavor muon production is observed at backward (forward) rapidity relative to the yield in p+p collisions scaled by the number of binary collisions. Modification of the gluon density distribution in the Au nucleus contributes in terms of anti-shadowing enhancement and shadowing suppression; however, the enhancement seen at backward rapidity exceeds expectations from this effect alone. These results, implying an important role for additional cold nuclear matter effects, serves as a key baseline for heavy-quark measurements in A+A collisions and in constraining the magnitude of charmonia breakup effects at the Relativistic Heavy Ion Collider and the Large Hadron Collider.

Jet production rates are measured in p+p and d+Au collisions at sqrt[s_{NN}]=200 GeV recorded in 2008 with the PHENIX detector at the Relativistic Heavy Ion Collider. Jets are reconstructed using the R=0.3 anti-k_{t} algorithm from energy deposits in the electromagnetic calorimeter and charged tracks in multiwire proportional chambers, and the jet transverse momentum (p_{T}) spectra are corrected for the detector response. Spectra are reported for jets with 12<p_{T}<50 GeV/c, within a pseudorapidity acceptance of |η|<0.3. The nuclear-modification factor (R_{dAu}) values for 0%-100% d+Au events are found to be consistent with unity, constraining the role of initial state effects on jet production. However, the centrality-selected R_{dAu} values and central-to-peripheral ratios (R_{CP}) show large, p_{T}-dependent deviations from unity, challenging the conventional models that relate hard-process rates and soft-particle production in collisions involving nuclei.

The standard model (SM) of particle physics is spectacularly successful, yet the measured value of the muon anomalous magnetic moment ${(g\ensuremath{-}2)}_{\ensuremath{\mu}}$ deviates from SM calculations by $3.6\ensuremath{\sigma}.$ Several theoretical models attribute this to the existence of a ``dark photon,'' an additional U(1) gauge boson, which is weakly coupled to ordinary photons. The PHENIX experiment at the Relativistic Heavy Ion Collider has searched for a dark photon, $U$, in ${\ensuremath{\pi}}^{0},\ensuremath{\eta}\ensuremath{\rightarrow}\ensuremath{\gamma}{e}^{+}{e}^{\ensuremath{-}}$ decays and obtained upper limits of $\mathcal{O}(2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6})$ on $U\ensuremath{-}\ensuremath{\gamma}$ mixing at 90% C.L. for the mass range $30&lt;{m}_{U}&lt;90$ $\mathrm{MeV}/{c}^{2}$. Combined with other experimental limits, the remaining region in the $U\ensuremath{-}\ensuremath{\gamma}$ mixing parameter space that can explain the ${(g\ensuremath{-}2)}_{\ensuremath{\mu}}$ deviation from its SM value is nearly completely excluded at the 90% confidence level, with only a small region of $29&lt;{m}_{U}&lt;32$ $\mathrm{MeV}/{c}^{2}$ remaining.

We present measurements of ${e}^{+}{e}^{\ensuremath{-}}$ production at midrapidity in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{{}_{NN}}}=200$ GeV. The invariant yield is studied within the PHENIX detector acceptance over a wide range of mass (${m}_{ee}&lt;5 \mathrm{GeV}/{c}^{2}$) and pair transverse momentum (${p}_{T}&lt;5 \mathrm{GeV}/c$) for minimum bias and for five centrality classes. The ${e}^{+}{e}^{\ensuremath{-}}$ yield is compared to the expectations from known sources. In the low-mass region (${m}_{ee}=0.30--0.76 \mathrm{GeV}/{c}^{2}$) there is an enhancement that increases with centrality and is distributed over the entire pair ${p}_{T}$ range measured. It is significantly smaller than previously reported by the PHENIX experiment and amounts to $2.3\ifmmode\pm\else\textpm\fi{}0.4(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.4(\mathrm{syst})\ifmmode\pm\else\textpm\fi{}0.2(\mathrm{model})$ or to $1.7\ifmmode\pm\else\textpm\fi{}0.3(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.3(\mathrm{syst})\ifmmode\pm\else\textpm\fi{}0.2(\mathrm{model})$ for minimum bias collisions when the open heavy-flavor contribution is calculated with pythia or mc@nlo, respectively. The inclusive mass and ${p}_{T}$ distributions, as well as the centrality dependence, are well reproduced by model calculations where the enhancement mainly originates from the melting of the $\ensuremath{\rho}$ meson resonance as the system approaches chiral symmetry restoration. In the intermediate-mass region (${m}_{ee}=1.2--2.8 \mathrm{GeV}/{c}^{2}$), the data hint at a significant contribution in addition to the yield from the semileptonic decays of heavy-flavor mesons.

PHENIX measurements are presented for the cross section and double-helicity asymmetry ($A_{LL}$) in inclusive $\pi^0$ production at midrapidity from $p$$+$$p$ collisions at $\sqrt{s}=510$~GeV from data taken in 2012 and 2013 at the Relativistic Heavy Ion Collider. The next-to-leading-order perturbative-quantum-chromodynamics theory calculation is in excellent agreement with the presented cross section results. The calculation utilized parton-to-pion fragmentation functions from the recent DSS14 global analysis, which prefer a smaller gluon-to-pion fragmentation function. The $\pi^{0}A_{LL}$ results follow an increasingly positive asymmetry trend with $p_T$ and $\sqrt{s}$ with respect to the predictions and are in excellent agreement with the latest global analysis results. This analysis incorporated earlier results on $\pi^0$ and jet $A_{LL}$, and suggested a positive contribution of gluon polarization to the spin of the proton $\Delta G$ for the gluon momentum fraction range $x>0.05$. The data presented here extend to a currently unexplored region, down to $x\sim0.01$, and thus provide additional constraints on the value of $\Delta G$. The results confirm the evidence for nonzero $\Delta G$ using a different production channel in a complementary kinematic region.