C. Henderson

This paper describes the conclusions that can be drawn from the data taken thus far with the PHOBOS detector at RHIC. In the most central Au + Au collisions at the highest beam energy, evidence is found for the formation of a very high energy density system whose description in terms of simple hadronic degrees of freedom is inappropriate. Furthermore, the constituents of this novel system are found to undergo a significant level of interaction. The properties of particle production at RHIC energies are shown to follow a number of simple scaling behaviors, some of which continue trends found at lower energies or in simpler systems. As a function of centrality, the total number of charged particles scales with the number of participating nucleons. When comparing Au + Au at different centralities, the dependence of the yield on the number of participants at higher pT (∼4GeV/c) is very similar to that at low transverse momentum. The measured values of charged particle pseudorapidity density and elliptic flow were found to be independent of energy over a broad range of pseudorapidities when effectively viewed in the rest frame of one of the colliding nuclei, a property we describe as “extended longitudinal scaling”. Finally, the centrality and energy dependences of several observables were found to factorize to a surprising degree.

Pseudorapidity distributions of charged particles emitted in $Au+Au$, $Cu+Cu$, $d+Au$, and $p+p$ collisions over a wide energy range have been measured using the PHOBOS detector at RHIC. The centrality dependence of both the charged particle distributions and the multiplicity at midrapidity were measured. Pseudorapidity distributions of charged particles emitted with $|\eta|<5.4$, which account for between 95% and 99% of the total charged-particle emission associated with collision participants, are presented for different collision centralities. Both the midrapidity density, $dN_{ch}/d\eta$, and the total charged-particle multiplicity, $N_{ch}$, are found to factorize into a product of independent functions of collision energy, $\sqrt{s_{_{NN}}}$, and centrality given in terms of the number of nucleons participating in the collision, $N_{part}$. The total charged particle multiplicity, observed in these experiments and those at lower energies, assumes a linear dependence of $(\ln s_{_{NN}})^2$ over the full range of collision energy of $\sqrt{s_{_{NN}}}$=2.7-200 GeV.

We present measurements of the pseudorapidity distribution of primary charged particles produced in Au+Au collisions at three energies, √sNN=19.6, 130, and 200 GeV, for a range of collision centralities. The distribution narrows for more central collisions and excess particles are produced at high pseudorapidity in peripheral collisions. For a given centrality, however, the distributions are found to scale with energy according to the "limiting fragmentation" hypothesis. The universal fragmentation region described by this scaling grows in pseudorapidity with increasing collision energy, extending well away from the beam rapidity and covering more than half of the pseudorapidity range over which particles are produced. This approach to a universal limiting curve appears to be a dominant feature of the pseudorapidity distribution and therefore of the total particle production in these collisions.Received 5 October 2002DOI:https://doi.org/10.1103/PhysRevLett.91.052303©2003 American Physical Society

We present the first measurement of pseudorapidity densities of primary charged particles near midrapidity in Au+Au collisions at sqrt[s(NN)] = 56 and 130 GeV. For the most central collisions, we find the charged-particle pseudorapidity density to be dN/deta|(|eta|<1) = 408+/-12(stat)+/-30(syst) at 56 GeV and 555+/-12(stat)+/-35(syst) at 130 GeV, values that are higher than any previously observed in nuclear collisions. Compared to proton-antiproton collisions, our data show an increase in the pseudorapidity density per participant by more than 40% at the higher energy.

We have measured transverse momentum distributions of charged hadrons produced in d+Au collisions at sqrt[s(NN)]=200 GeV. The spectra were obtained for transverse momenta 0.25<p(T)<6.0 GeV/c, in a pseudorapidity range of 0.2<eta<1.4 in the deuteron direction. The evolution of the spectra with collision centrality is presented in comparison to p+pmacr; collisions at the same collision energy. With increasing centrality, the yield at high transverse momenta increases more rapidly than the overall particle density, leading to a strong modification of the spectral shape. This change in spectral shape is qualitatively different from observations in Au+Au collisions at the same energy. The results provide important information for discriminating between different models for the suppression of high-p(T) hadrons observed in Au+Au collisions.

A measurement of two-particle correlations with a high transverse momentum trigger particle (p trig T > 2.5 GeV/c) is presented for Au+Au collisions at √ s N N =200 GeV over the uniquely broad longitudinal acceptance of the PHOBOS detector (-4 < ∆η < 2).A broadening of the away-side azimuthal correlation compared to elementary collisions is observed at all ∆η.As in p+p collisions, the near-side is characterized by a peak of correlated partners at small angle relative to the trigger particle.However, in central Au+Au collisions an additional correlation extended in ∆η and known as the 'ridge' is found to reach at least |∆η| ≈ 4. The ridge yield is largely independent of ∆η over the measured range, and it decreases towards more peripheral collisions.For the chosen p trig T cut, the ridge yield is consistent with zero for events with less than roughly 100 participating nucleons.

This Rapid Communication describes the measurement of elliptic flow for charged particles in Au+Au collisions at √sNN=200 GeV using the PHOBOS detector at the Relativistic Heavy Ion Collider. The measured azimuthal anisotropy is presented over a wide range of pseudorapidity for three broad collision centrality classes for the first time at this energy. Two distinct methods of extracting the flow signal were used to reduce systematic uncertainties. The elliptic flow falls sharply with increasing |η| at 200 GeV for all the centralities studied, as observed for minimum-bias collisions at √sNN=130 GeV.Received 12 July 2004DOI:https://doi.org/10.1103/PhysRevC.72.051901©2005 American Physical Society

The charged-particle pseudorapidity density dNch/deta has been measured for Au+Au collisions at sqrt(sNN)=130 GeV at RHIC, using the PHOBOS apparatus. The total number of charged particles produced for the 3% most central Au+Au collisions for |eta|<=5.4 is found to be 4200+-470. The evolution of dNch/deta with centrality is discussed, and compared to model calculations and to data from proton-induced collisions. The data show an enhancement in charged-particle production at mid-rapidity, while in the fragmentation regions, the results are consistent with expectations from pp and pA scattering.

The PHOBOS experiment has measured the charged particle multiplicity at midrapidity in Au+Au collisions at sNN=200 GeV as a function of the collision centrality. Results on dNch/dη||η|<1 divided by the number of participating nucleon pairs 〈Npart〉/2 are presented as a function of 〈Npart〉. As was found from similar data at sNN=130 GeV, the data can be equally well described by parton saturation models and two-component fits, which include contributions that scale as Npart and the number of binary collisions Ncoll. We compare the data at the two energies by means of the ratio R200/130 of the charged particle multiplicity for the two different energies as a function of 〈Npart〉. For events with 〈Npart〉>100, we find that this ratio is consistent with a constant value of 1.14±0.01(stat)±0.05(syst). Received 10 January 2002DOI:https://doi.org/10.1103/PhysRevC.65.061901©2002 American Physical Society

We present transverse momentum distributions of charged hadrons produced in Au+Au collisions at sNN=200 GeV. The spectra were measured for transverse momenta pT from 0.25 to 4.5 GeV/c in a pseudorapidity range of 0.2<η<1.4. The evolution of the spectra is studied as a function of collision centrality, from 65 to 344 participating nucleons. The results are compared to data from proton–antiproton collisions and Au+Au collisions at lower RHIC energies. We find a significant change of the spectral shape between proton–antiproton and semi-peripheral Au+Au collisions. Comparing semi-peripheral to central Au+Au collisions, we find that the yields at high pT exhibit approximate scaling with the number of participating nucleons, rather than scaling with the number of binary collisions.

This paper describes the measurement of collective flow for charged particles in Au+Au collisions at sqrt[s(NN)]=130 GeV using the PHOBOS detector at the Relativistic Heavy Ion Collider (RHIC). The measured azimuthal hit anisotropy is presented over a wide range of pseudorapidity (-5.0<eta<5.3) for the first time at this energy. The result, averaged over momenta and particle species, is observed to reach 7% for peripheral collisions at midrapidity, falling off with centrality and increasing |eta|. These results call into question the common assumption of longitudinal boost invariance over a large region of rapidity in RHIC collisions.

This paper describes the measurement of the energy dependence of elliptic flow for charged particles in Au+Au collisions using the PHOBOS detector at the Relativistic Heavy Ion Collider (RHIC). Data taken at collision energies of $\sqrt{s_{_{NN}}} =$ 19.6, 62.4, 130 and 200 GeV are shown over a wide range in pseudorapidity. These results, when plotted as a function of $\eta'=|\eta|-y_{beam}$, scale with approximate linearity throughout $\eta'$, implying no sharp changes in the dynamics of particle production as a function of pseudorapidity or increasing beam energy.

The centrality dependence of the midrapidity charged particle multiplicity in $\mathrm{Au}+\mathrm{Au}$ heavy-ion collisions at $\sqrt{{s}_{NN}}=19.6$ and $200\phantom{\rule{0.3em}{0ex}}\mathrm{GeV}$ is presented. Within a simple model, the fraction of hard (scaling with number of binary collisions) to soft (scaling with number of participant pairs) interactions is consistent with a value of $x=0.13\ifmmode\pm\else\textpm\fi{}0.01(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.05(\mathrm{syst})$ at both energies. The experimental results at both energies, scaled by inelastic $p(\overline{p})+p$ collision data, agree within systematic errors. The ratio of the data was found not to depend on centrality over the studied range and yields a simple linear scale factor of ${R}_{200∕19.6}=2.03\ifmmode\pm\else\textpm\fi{}0.02(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.05(\mathrm{syst})$.

We present results on two-particle angular correlations in Cu+Cu and Au+Au collisions at a center of mass energy per nucleon pair of 200 GeV over a broad range of pseudorapidity ($\eta$) and azimuthal angle ($\phi$) as a function of collision centrality. The PHOBOS detector at RHIC has a uniquely-large angular coverage for inclusive charged particles, which allows for the study of correlations on both long- and short-range scales. A complex two-dimensional correlation structure in $\Delta \eta$ and $\Delta \phi$ emerges, which is interpreted in the context of a cluster model. The effective cluster size and decay width are extracted from the two-particle pseudorapidity correlation functions. The effective cluster size found in semi-central Cu+Cu and Au+Au collisions is comparable to that found in proton-proton collisions but a non-trivial decrease of the size with increasing centrality is observed. Moreover, a comparison between results from Cu+Cu and Au+Au collisions shows an interesting scaling of the effective cluster size with the measured fraction of total cross section (which is related to the ratio of the impact parameter to the nuclear radius, $b/2R$), suggesting a geometric origin. Further analysis for pairs from restricted azimuthal regions shows that the effective cluster size at $\Delta\phi \sim 180^{\circ}$ drops more rapidly toward central collisions than the size at $\Delta\phi \sim 0^{\circ}$. The effect of limited $\eta$ acceptance on the cluster parameters is also addressed, and a correction is applied to present cluster parameters for full $\eta$ coverage, leading to much larger effective cluster sizes and widths than previously noted in the literature. These results should provide insight into the hot and dense medium created in heavy ion collisions.

We present the first measurement of the pseudorapidity density of primary charged particles in Au+Au collisions at root square[s(NN)] = 200 GeV. For the 6% most central collisions, we obtain dN(ch)/d(eta)/(/eta/<1) = 650+/-35(syst). Compared to collisions at root square[s(NN)] = 130 GeV, the highest energy studied previously, an increase by a factor of 1.14+/-0.05 at 90% confidence level, is found. The energy dependence of the pseudorapidity density is discussed in comparison with data from proton-induced collisions and theoretical predictions.

This manuscript contains a detailed description of the PHOBOS experiment as it is configured for the Year 2001 running period. It is capable of detecting charged particles over the full solid angle using a multiplicity detector and measuring identified charged particles near mid-rapidity in two spectrometer arms with opposite magnetic fields. Both of these components utilize silicon pad detectors for charged particle detection. The minimization of material between the collision vertex and the first layers of silicon detectors allows for the detection of charged particles with very low transverse momenta, which is a unique feature of the PHOBOS experiment. Additional detectors include a time-of-flight wall which extends the particle identification range for one spectrometer arm, as well as sets of scintillator paddle and Cherenkov detector arrays for event triggering and centrality selection.

The measured pseudorapidity distribution of primary charged particles in minimum-bias d+Au collisions at sqrt[s(NN)]=200 GeV is presented for the first time. This distribution falls off less rapidly in the gold direction as compared to the deuteron direction. The average value of the charged particle pseudorapidity density at midrapidity is <dN(ch)/d eta>|eta|< or =0.6)=9.4+/-0.7(syst) and the integrated primary charged particle multiplicity in the measured region is 82+/-6(syst). Estimates of the total charged particle production, based on extrapolations outside the measured pseudorapidity region, are also presented. The pseudorapidity distribution, normalized to the number of participants in d+Au collisions, is compared to those of Au+Au and p+(-)p systems at the same energy. The d+Au distribution is also compared to the predictions of the parton saturation model, as well as microscopic models.

The measured pseudorapidity distributions of primary charged particles over a wide pseudorapidity range of |η|≤5.4 and integrated charged particle multiplicities in d+Au collisions at √sNN=200GeV are presented as a function of collision centrality. The longitudinal features of d+Au collisions at √sNN=200GeV are found to be very similar to those seen in p+A collisions at lower energies. The total multiplicity of charged particles is found to scale with the total number of participants according to NdAuch=12⟨Npart⟩Nppch, and the energy dependence of the density of charged particles produced in the fragmentation region exhibits extended longitudinal scaling.Received 30 September 2004DOI:https://doi.org/10.1103/PhysRevC.72.031901©2005 American Physical Society

The charged-particle pseudorapidity density for Au+Au collisions at √sNN=62.4 GeV has been measured over a wide range of impact parameters and compared to results obtained at other energies. As a function of collision energy, the pseudorapidity distribution grows systematically both in height and width. The midrapidity density is found to grow approximately logarithmically between BNL Alternating Gradient Synchrotron (AGS) energies and the top BNL Relativistic Heavy Ion Collider (RHIC) energy. There is also an approximate factorization of the centrality and energy dependence of the midrapidity yields. The new results at √sNN=62.4 GeV confirm the previously observed phenomenon of “extended longitudinal scaling” in the pseudorapidity distributions when viewed in the rest frame of one of the colliding nuclei. It is also found that the evolution of the shape of the distribution with centrality is energy independent, when viewed in this reference frame. As a function of centrality, the total charged particle multiplicity scales linearly with the number of participant pairs as it was observed at other energies.Received 26 September 2005DOI:https://doi.org/10.1103/PhysRevC.74.021901©2006 American Physical Society

This paper presents results on event-by-event elliptic flow fluctuations in Au+Au collisions at sqrt(s_NN)=200Gev, where the contribution from non-flow correlations has been subtracted. An analysis method is introduced to measure non-flow correlations, relying on the assumption that non-flow correlations are most prominent at short ranges (Delta eta < 2). Assuming that non-flow correlations are of the order that is observed in p+p collisions for long range correlations (Delta eta > 2), relative elliptic flow fluctuations of approximately 30-40% are observed. These results are consistent with predictions based on spatial fluctuations of the participating nucleons in the initial nuclear overlap region. It is found that the long range non-flow correlations in Au+Au collisions would have to be more than an order of magnitude stronger compared to the p+p data to lead to the observed azimuthal anisotropy fluctuations with no intrinsic elliptic flow fluctuations.

This Letter presents the first measurement of event-by-event fluctuations of the elliptic flow parameter v(2) in Au+Au collisions at square root(s(NN))=200 GeV as a function of collision centrality. The relative nonstatistical fluctuations of the v(2) parameter are found to be approximately 40%. The results, including contributions from event-by-event elliptic flow fluctuations and from azimuthal correlations that are unrelated to the reaction plane (nonflow correlations), establish an upper limit on the magnitude of underlying elliptic flow fluctuations. This limit is consistent with predictions based on spatial fluctuations of the participating nucleons in the initial nuclear overlap region. These results provide important constraints on models of the initial state and hydrodynamic evolution of relativistic heavy ion collisions.

We present a measurement of the pseudorapidity density of primary charged particles near mid-rapidity in Au+Au collisions at sqrt(s_NN) = 130 GeV as a function of the number of participating nucleons. These results are compared to models in an attempt to discriminate between competing scenarios of particle production in heavy ion collisions.

We report on measurements of directed flow as a function of pseudorapidity in Au+Au collisions at energies of √ s N N = 19.6,62.4, 130 and 200 GeV as measured by the PHOBOS detector at the Relativistic Heavy Ion Collider (RHIC).These results are particularly valuable because of the extensive, continuous pseudorapidity coverage of the PHOBOS detector.There is no significant indication of structure near midrapidity and the data surprisingly exhibit extended longitudinal scaling similar to that seen for elliptic flow and charged particle pseudorapidity density.

We have measured transverse momentum distributions of charged hadrons produced in Au+Au collisions at sqrt(s_NN) = 62.4 GeV. The spectra are presented for transverse momenta 0.25 < p_T< 4.5 GeV/c, in a pseudo-rapidity range of 0.2 < eta < 1.4. The nuclear modification factor R_AA is calculated relative to p+p data at the same collision energy as a function of collision centrality. For p_T > 2 GeV/c, R_AA is found to be significantly larger than in Au+Au collisions at sqrt(s_NN) =130 and 200 GeV. In contrast, we find that the evolution of the invariant yields per participant pair from peripheral to central collisions is approximately energy independent over this range of collision energies. This observation challenges models of high p_T hadron suppression in terms of parton energy loss.

Forward-backward correlations of charged-particle multiplicities in symmetric bins in pseudorapidity are studied to gain insight into the underlying correlation structure of particle production in Au+Au collisions. The PHOBOS detector is used to measure integrated multiplicities in bins centered at \ensuremath{\eta}, defined within $|\ensuremath{\eta}|&lt;3$, and covering intervals $\ensuremath{\Delta}\ensuremath{\eta}$. The variance ${\ensuremath{\sigma}}_{C}^{2}$ of a suitably defined forward-backward asymmetry variable $C$ is calculated as a function of $\ensuremath{\eta},\ensuremath{\Delta}\ensuremath{\eta}$, and centrality. It is found to be sensitive to short-range correlations, and the concept of ``clustering'' is used to interpret comparisons to phenomenological models.

We present results on two-particle angular correlations in proton-proton collisions at center-of-mass energies of 200 and 410 GeV. The PHOBOS experiment at the BNL Relativistic Heavy Ion Collider has a uniquely large coverage for charged particles, giving the opportunity to explore the correlations at both short- and long-range scales. At both energies, a complex two-dimensional correlation structure in Δη and Δϕ is observed. In the context of an independent cluster model of short-range correlations, the cluster size and its decay width are extracted from the two-particle pseudorapidity correlation function and compared with previous measurements in proton-proton and proton-antiproton collisions, as well as PYTHIA and HIJING predictions.3 MoreReceived 6 April 2007DOI:https://doi.org/10.1103/PhysRevC.75.054913©2007 American Physical Society

We present the first measurements of the pseudorapidity distribution of primary charged particles in Cu þ Cu collisions as a function of collision centrality and energy, ffiffiffiffiffiffiffiffi s NN p ¼ 22:4, 62.4, and 200 GeV, over a wide range of pseudorapidity, using the PHOBOS detector.A comparison of Cu þ Cu and Au þ Au results shows that the total number of produced charged particles and the rough shape (height and width) of the pseudorapidity distributions are determined by the number of nucleon participants.More detailed studies reveal that a more precise matching of the shape of the Cu þ Cu and Au þ Au pseudorapidity distributions over the full range of pseudorapidity occurs for the same N part =2A rather than the same N part .In other words, it is the collision geometry rather than just the number of nucleon participants that drives the detailed shape of the pseudorapidity distribution and its centrality dependence at RHIC energies.

We have measured the ratios of antiparticles to particles for charged pions, kaons, and protons near mid-rapidity in central Au+Au collisions at sqrt[s(NN)] = 130 GeV. We observe <pi(-)>/<pi(+)> = 1.00+/-0.01(stat)+/-0.02(syst), <K->/<K+> = 0.91+/-0.07(stat)+/-0.06(syst), and / = 0.60+/-0.04(stat)+/-0.06(syst). The <K->/<K+> and / ratios give a consistent estimate of the baryo-chemical potential mu(B) of 45 MeV, a factor of 5-6 smaller than in central Pb+Pb collisions at sqrt[s(NN)] = 17.2 GeV.

We present transverse momentum distributions of charged hadrons produced in Cu + Cu collisions at square root of SNN = 62.4 and 200 GeV. The spectra are measured for transverse momenta of 0.25 < pT < 5.0 GeV/c at square root of SNN = 62.4 GeV and 0.25 < pT < 7.0 GeV/c at square root of SNN = 200 GeV, in a pseudorapidity range of 0.2 < eta < 1.4. The nuclear modification factor R(AA) is calculated relative to p + p data at both collision energies as a function of collision centrality. At a given collision energy and fractional cross section, R(AA) is observed to be systematically larger in Cu + Cu collisions compared to Au + Au. However, for the same number of participating nucleons, R(AA) is essentially the same in both systems over the measured range of pT, in spite of the significantly different geometries of the Cu + Cu and Au + Au systems.

The measured pseudorapidity distributions of primary charged particles are presented for d+Au and p+p collisions at over a wide pseudorapidity range of |η|⩽ 5.4. The results for d+Au collisions are presented for minimum-bias events and as a function of collision centrality. The measurements for p+p collisions are shown for minimum-bias events. The ratio of the charged particle multiplicity in d+Au and p+A collisions relative to that for inelastic p+p collisions is found to depend only on ⟨Npart⟩, and it is remarkably independent of collision energy and system mass. The deuteron and gold fragmentation regions in d+Au collisions are in good agreement with proton nucleus data at lower energies.

The PHOBOS experiment at the BNL Relativistic Heavy Ion Collider has measured the total multiplicity of primary charged particles as a function of collision centrality in Au+Au collisions at √sNN= 19.6, 130, and 200 GeV. An approximate independence of ⟨Nch⟩/⟨Npart/2⟩ on the number of participating nucleons is observed, reminiscent of "wounded nucleon" scaling (Nch∝Npart) observed in proton-nucleus collisions. Unlike p+A, the constant of proportionality does not seem to be set by the pp/¯pp data at the same energy. Rather, there seems to be a surprising correspondence with the total multiplicity measured in e+e− annihilations, as well as the rapidity shape measured over a large range. The energy dependence of the integrated multiplicity per participant pair shows that e+e− and A+A data agree over a large range of center-of-mass energies (√s>20 GeV), and pp/¯pp data can be brought to agree approximately with the e+e− data by correcting for the typical energy taken away by leading particles. This is suggestive of a mechanism for soft particle production that depends mainly on the amount of available energy. It is conjectured that the dominant distinction between A+A and p+p collisions is the multiple collisions per participant, which appears to be sufficient to substantially reduce the energy taken away by leading particles.Received 11 January 2005DOI:https://doi.org/10.1103/PhysRevC.74.021902©2006 American Physical Society

We have measured the transverse momentum distributions of charged hadrons in d+Au collisions at sqrt sNN = 200 GeV in the range of 0.5 < p_T < 4.0 GeV/c. The total range of pseudorapidity, eta, is 0.2 < eta < 1.4, where positive eta is in the deuteron direction. The data has been divided into three regions of pseudorapidity, covering 0.2 < eta < 0.6, 0.6 < \eta < 1.0, and 1.0 < eta < 1.4 and has been compared to charged hadron spectra from p+pbar collisions at the same energy. There is a significant change in the spectral shape as a function of pseudorapidity. As eta increases we see a decrease in the nuclear modification factor RdAu.

Argonne flow and Bose-Einstein correlations have been measured in Au-Au collisions at SNN=130 and 200 GeV using the PHOBOS detector at RHIC. The systematic dependencies of the flow signal on the transverse momentum, pseudorapidity, and centrality of the collision, as well as the beam energy are shown. In addition, results of a 3-dimensional analysis of two-pion correlations in the 200 GeV data are presented.

Two-particle correlations of identical charged pion pairs from Au+Au collisions at √sNN=62.4 and 200 GeV were measured by the PHOBOS experiment at BNL Relativistic Heavy Ion Collider (RHIC). Data for the 15% most central events were analyzed with Bertsch-Pratt and Yano-Koonin-Podgoretskii parametrizations using pairs with rapidities of 0.4<yππ<1.3 and transverse momenta 0.1<kT<1.4 GeV/c. The Bertsch-Pratt radii Ro and Rℓ decrease as a function of pair transverse momentum. Ro and Rs are independent of collision energy, while Rℓ shows a slight increase. The source rapidity yYKP scales roughly with the pair rapidity yππ, indicating strong dynamical correlations.Received 1 September 2004DOI:https://doi.org/10.1103/PhysRevC.73.031901©2006 American Physical Society

Transverse momentum spectra of pions, kaons, protons, and antiprotons from Au+Au collisions at √sNN = 62.4 GeV have been measured by the PHOBOS experiment at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory. The identification of particles relies on three different methods: low momentum particles stopping in the first detector layers; the specific energy loss (dE/dx) in the silicon spectrometer, and time-of-flight measurement. These methods cover the transverse momentum ranges 0.03–0.2, 0.2–1.0, and 0.5–3.0 GeV/c, respectively. Baryons are found to have substantially harder transverse momentum spectra than mesons. The pT region in which the proton to pion ratio reaches unity in central Au+Au collisions at √sNN = 62.4 GeV fits into a smooth trend as a function of collision energy. At low transverse mass, the spectra of various species exhibit a significant deviation from transverse mass scaling. The observed particle yields at very low pT are comparable to extrapolations from higher pT for kaons, protons and antiprotons. By comparing our results to Au+Au collisions at √sNN = 200 GeV, we conclude that the net proton yield at midrapidity is proportional to the number of participant nucleons in the collision.5 MoreReceived 27 September 2006DOI:https://doi.org/10.1103/PhysRevC.75.024910©2007 American Physical Society

The study of flow can provide information on the initial state dynamics and the degree of equilibration attained in heavy-ion collisions. This contribution presents results for both elliptic and directed flow as determined from data recorded by the PHOBOS experiment in Au+Au runs at RHIC at and 200 GeV. The PHOBOS detector provides a unique coverage in pseudorapidity for measuring flow at RHIC. The systematic dependence of flow on pseudorapidity, transverse momentum, centrality and energy is discussed.

We present results on charged particle production at very low transverse momenta in the 15% most central Au+Au collisions at sqrt(s_NN) = 200 GeV obtained with the PHOBOS detector at RHIC. The invariant yields were measured at mid-rapidity in the transverse momentum ranges from 30 to 50 MeV/c for charged pions, 90 to 130 MeV/c for charged kaons and 140 to 210 MeV/c for protons and antiprotons. No significant enhancement in low transverse momentum particle production is observed as compared to extrapolations of identified particle spectra measured at an intermediate pT range. The spectra tend to flatten at low pT, consistent with the expectations of transverse expansion of the system.

The ratios of the yields of primary charged antiparticles to particles have been obtained for pions, kaons, and protons near midrapidity for p+p collisions at sNN=200GeV. Ratios of 〈π−/π+〉=1.000±0.012 (stat.) ±0.019 (syst.), 〈K−/K+〉=0.93±0.05 (stat.) ±0.03 (syst.), and 〈p¯/p〉=0.85±0.04 (stat.) ±0.03 (syst.) have been measured. The reported values represent the ratio of the yields averaged over the rapidity range of 0.1<yπ<1.3 and 0<yK,p<0.8, and for transverse momenta of 0.1<pTπ,K<1.0GeV/c and 0.3<pTp<1.0GeV/c. Within the uncertainties, all three ratios are consistent with the values measured in d+Au collisions at the same energy. The data are compared to results from other collision systems and energies.Received 2 September 2004DOI:https://doi.org/10.1103/PhysRevC.71.021901©2005 American Physical Society

The ratios of charged antiparticles to particles have been obtained for pions, kaons and protons near midrapidity in central Au+Au collisions at sqrt(s_NN) = 200 GeV. Ratios of / = 1.025 +/- 0.006 (stat.) +/- 0.018 (syst.), / = 0.95 +/- 0.03 (stat.) +/- 0.03 (syst.) and / = 0.73 +/- 0.02 (stat.) +/- 0.03 (syst.) have been observed. The / and / ratios are consistent with a baryochemical potential mu_B of 27 MeV, roughly a factor of 2 smaller than in sqrt(s_NN) = 130 GeV collisions. The data are compared to results from lower energies and model calculations. Our accurate measurements of the particle ratios impose stringent constraints on current and future models dealing with baryon production and transport.

The centrality dependence of the midrapidity charged-particle multiplicity density ($|\ensuremath{\eta}|&lt;1$) is presented for $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ collisions at RHIC over a broad range of collision energies. The multiplicity measured in the $\mathrm{Cu}+\mathrm{Cu}$ system is found to be similar to that measured in the $\mathrm{Au}+\mathrm{Au}$ system, for an equivalent ${N}_{\mathrm{part}}$, with the observed factorization in energy and centrality still persistent in the smaller $\mathrm{Cu}+\mathrm{Cu}$ system. The extent of the similarities observed for bulk particle production is tested by a comparative analysis of the inclusive transverse momentum distributions for $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ collisions near midrapidity. It is found that, within the uncertainties of the data, the ratio of yields between the various energies for both $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ systems are similar and constant with centrality, both in the bulk yields and as a function of ${p}_{T}$, up to at least 4 GeV/$c$. The effects of multiple nucleon collisions that strongly increase with centrality and energy appear to only play a minor role in bulk and intermediate transverse momentum particle production.

The ratios of the yields of charged antiparticles to particles have been obtained for pions, kaons, and protons near midrapidity for $d+\mathrm{Au}$ collisions at $\sqrt{{s}_{NN}}=200\phantom{\rule{0.3em}{0ex}}\mathrm{GeV}$ as a function of centrality. The reported values represent the ratio of the yields averaged over the rapidity range of $0.1&lt;{y}_{\ensuremath{\pi}}&lt;1.3$ and $0&lt;{y}_{K,p}&lt;0.8$, where positive rapidity is in the deuteron direction, and for transverse momenta $0.1&lt;{p}_{T}^{\ensuremath{\pi},K}&lt;1\phantom{\rule{0.3em}{0ex}}\mathrm{GeV}∕c$ and $0.3&lt;{p}_{T}^{p}&lt;1\phantom{\rule{0.3em}{0ex}}\mathrm{GeV}∕c$. Within the uncertainties, a lack of centrality dependence is observed in all three ratios. The data are compared to results from other systems and model calculations.

This paper presents the analysis of the dynamic fluctuations in the inclusive charged particle multiplicity measured by PHOBOS for Au+Au collisions at √sNN = 200GeV within the pseudo-rapidity range of −3 < η < 3. First the definition of the fluctuations observables used in this analysis is presented, together with the discussion of their physics meaning. Then the procedure for the extraction of dynamic fluctuations is described. Some preliminary results are included to illustrate the correlation features of the fluctuation observable. New dynamic fluctuations results will be available in a later publication.

Antiparticle to particle ratios for identified protons, kaons, and pions at $\sqrt{{s}_{\mathit{NN}}}=62.4$ and 200 GeV in Cu+Cu collisions are presented as a function of centrality for the midrapidity region of $0.2&lt;\ensuremath{\eta}&lt;1.4$. No strong dependence on centrality is observed. For the $\ensuremath{\langle}\overline{p}\ensuremath{\rangle}/\ensuremath{\langle}p\ensuremath{\rangle}$ ratio at $\ensuremath{\langle}{p}_{T}\ensuremath{\rangle}\ensuremath{\approx}0.51$ GeV/$c$, we observe an average value of $0.50\ifmmode\pm\else\textpm\fi{}0.{003}_{(\mathrm{stat})}\ifmmode\pm\else\textpm\fi{}0.{04}_{(\mathrm{syst})}$ and $0.77\ifmmode\pm\else\textpm\fi{}0.{008}_{(\mathrm{stat})}\ifmmode\pm\else\textpm\fi{}0.{05}_{(\mathrm{syst})}$ for the 10% most central collisions of 62.4 and 200 GeV $\mathrm{Cu}+\mathrm{Cu}$, respectively. The values for all three particle species measured at $\sqrt{{s}_{\mathit{NN}}}=200$ GeV are in agreement within systematic uncertainties with that seen in both heavier and lighter systems measured at the same RHIC energy. This indicates that system size does not appear to play a strong role in determining the midrapidity chemical freeze-out properties affecting the antiparticle to particle ratios of the three most abundant particle species produced in these collisions.

In an experiment using a heavy liquid bubble chamber the Ξ hyperon lifetimes and α parameters were measured to be τΞo = (3.04 ± 0.230.26) × 10−10 sec, τΞ− = (1.73 ± 0.070.08) × 10−10 sec, αΞo = −0.84 ± 0.27 and αΞ− = −0.42 ± 0.11. These values agree with measurements which employed different techniques and are consistent with the predictions of the ΔI = 12 rule.

Particle production in Au+Au collisions has been measured in the PHOBOS experiment at RHIC for a range of collision energies. Three empirical observations have emerged from this dataset which require theoretical examination. First, there is clear evidence of limiting fragmentation. Namely, particle production in central Au+Au collisions, when expressed as $dN/dη'$ ($η' \equiv η-y_{beam}$), becomes energy independent at high energy for a broad region of $η'$ around $η'=0$. This energy-independent region grows with energy, allowing only a limited region (if any) of longitudinal boost-invariance. Second, there is a striking similarity between particle production in e+e- and Au+Au collisions (scaled by the number of participating nucleon pairs). Both the total number of produced particles and the longitudinal distribution of produced particles are approximately the same in e+e- and in scaled Au+Au. This observation was not predicted and has not been explained. Finally, particle production has been found to scale approximately with the number of participating nucleon pairs for $N_{part}&gt;65$. This scaling occurs both for the total multiplicity and for high $\pT$ particles (3 $

The yields of identified particles have been measured at RHIC for Au+Au collisions at SNN=200 GeV using the PHOBOS spectrometer. The ratios of antiparticle to particle yields near mid-rapidity are presented. The first measurements of the invariant yields of charged pions, kaons and protons at very low transverse momenta are also shown.

The PHOBOS experiment at RHIC has measured the multiplicity of primary charged particles as a function of centrality and pseudorapidity in Au+Au collisions at sqrt(s_NN) = 19.6, 130 and 200 GeV. Two kinds of universal behavior are observed in charged particle production in heavy ion collisions. The first is that forward particle production, over a range of energies, follows a universal limiting curve with a non-trivial centrality dependence. The second arises from comparisons with pp/pbar-p and e+e- data. N_tot/(N_part/2) in nuclear collisions at high energy scales with sqrt(s) in a similar way as N_tot in e+e- collisions and has a very weak centrality dependence. This feature may be related to a reduction in the leading particle effect due to the multiple collisions suffered per participant in heavy ion collisions.

This paper presents the first PHOBOS results on charged particle multiplicity fluctuations measured for Au+Au collisions at the highest RHIC energy within a wide pseudorapidity range of |η| < 3. The dependence on collision geometry is removed in the analysis by using the normalized difference between the number of particles in separate η bins. We compare our data to HIJING model predictions.

The High-Luminosity Large Hadron Collider is expected to deliver up to 3000 fb ^{-1} <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi /><mml:mrow><mml:mo>−</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:math> of proton-proton collisions at 14 TeV center-of-mass energy. We present prospects for selected heavy-ion, Standard Model and Higgs sector measurements with the CMS detector at the HL-LHC, and discuss potential sensitivity to several beyond-Standard Model new physics scenarios.

Spectator fragments resulting from relativistic heavy ion collisions, consisting of single protons and neutrons along with groups of stable nuclear fragments up to Nitrogen (Z=7), are measured in PHOBOS. These fragments are observed in Au+Au (sqrt(sNN)=19.6 GeV) and Cu+Cu (22.4 GeV) collisions at high pseudorapidity ($\eta$). The dominant multiply-charged fragment is the tightly bound Helium ($\alpha$), with Lithium, Beryllium, and Boron all clearly seen as a function of collision centrality and pseudorapidity. We observe that in Cu+Cu collisions, it becomes much more favorable for the $\alpha$ fragments to be released than Lithium. The yields of fragments approximately scale with the number of spectator nucleons, independent of the colliding ion. The shapes of the pseudorapidity distributions of fragments indicate that the average deflection of the fragments away from the beam direction increases for more central collisions. A detailed comparison of the shapes for $\alpha$ and Lithium fragments indicates that the centrality dependence of the deflections favors a scaling with the number of participants in the collision.

We present transverse momentum distributions of charged hadrons produced in Au+Au collisions at sqrt(s_NN) = 200 GeV. The evolution of the spectra for transverse momenta p_T from 0.25 to 5GeV/c is studied as a function of collision centrality over a range from 65 to 344 participating nucleons. We find a significant change of the spectral shape between proton-antiproton and peripheral Au+Au collisions. Comparing peripheral to central Au+Au collisions, we find that the yields at the highest p_T exhibit approximate scaling with the number of participating nucleons, rather than scaling with the number of binary collisions.

Transverse momentum spectra of pions, kaons and protons, as well as antiparticle to particle ratios near mid-rapidity from d+Au collisions at have been measured by the PHOBOS experiment at RHIC. The transverse momentum range of particle identification was extended to beyond 3 GeV/c using the TOF detector and a new trigger system. The pseudorapidity dependence of the nuclear modification factor for charged hadrons in d+Au collisions is presented.

Some of the latest results of the PHOBOS experiment from the Au+Au data are discussed. Those relevant to strangeness production are emphasized. These observations relate to the nature of the matter created when heavy ions collide at the highest achieved energy.

Collision centrality is a valuable parameter used in relativistic nuclear physics which relates to geometrical quantities such as the number of participating nucleons. PHOBOS utilizes a multiplicity measurement as a means to estimate fractional cross-section of a collision event-by-event. From this, the centrality of this collision can be deduced. The details of the centrality determination depend both on the collision system and collision energy. Presented here are the techniques developed over the course of the RHIC program that are used by PHOBOS to extract the centrality. Possible biases that have to be overcome before a final measurement can be interpreted are discussed.

Two-particle correlations of identical charged pion pairs from Au+Au collisions at sqrt(s_NN) = 200 GeV were measured by the PHOBOS experiment at RHIC. Data for the most central (0--15%) events were analyzed with Bertsch-Pratt (BP) and Yano-Koonin-Podgoretskii (YKP) parameterizations using pairs with rapidities of 0.4 < y < 1.3 and transverse momenta 0.1 < k_T < 1.4 GeV/c. The Bertsch-Pratt radii decrease as a function of pair transverse momentum. The pair rapidity Y_pipi roughly scales with the source rapidity Y_YKP, indicating strong dynamical correlations.

An overview of results for interactions of Au+Au ions at centre-of-mass energies of √sNN = 56, 130 and 200 GeV obtained by the PHOBOS collaboration at RHIC is given. Measurements of primary charged particle density near mid-rapidity indicate that particle production grows logarithmically with collision energy and faster than linearly with the number of interacting nucleons. Elliptic flow is found to be much stronger at RHIC than at SPS energy. The effect is strongest in peripheral events and decreases for more central collisions and emission angles |η| > 1. The measured anti-particle to particle ratios of production rates for pions, kaons and protons in central Au+Au interactions at √sNN = 130 GeV are compatible with the statistical model of particle production, showing an increasingly baryon-free region in mid-rapidity with the increase of collision energy.

Data collected in Run II of the Fermilab Tevatron are searched for indications of new electroweak-scale physics. Rather than focusing on particular new physics scenarios, CDF data are analyzed for discrepancies with the Standard Model prediction. A model-independent approach (Vista) considers gross features of the data, and is sensitive to new large cross-section physics. Further sensitivity to new physics is provided by two additional algorithms: a ``Bump-Hunter'' searches invariant mass distributions for ``bumps'' that could indicate resonant production of new particles; and the Sleuth procedure scans for data excesses in the high-pT tails. This combined global search for new physics in 2.0/fb of p-pbar collisions at sqrt(s)=1.96 TeV reveals no indication of physics beyond the Standard Model.

The Phobos experiment concluded its first year of operation at RHIC taking data in Au–Au nucleus collisions at snn=65GeV and 130GeV/nucleon pair. First preliminary results of the performances of our silicon detectors in the experiment are summarized. The Phobos experiment uses silicon pad detectors for both tracking and multiplicity measurements. The silicon sensors vary strongly in their pad geometry. In this paper, we compare the signal response, the signal uniformity and signal-to-noise performance as measured in the experiment for the different geometries. Additionally, we investigate effects of very high channel occupancy on the signal response.

Recent results from the PHOBOS experiment at RHIC are presented, both from Au+Au collisions from the 2001 run and p+p and d+Au collisions from 2003. The centrality dependence of the total charged particle multiplicity in p+p and d+Au show features, such as Npart-scaling and limiting fragmentation, similar to p+A collisions at lower energies. Multiparticle physics in Au+Au is found to be local in (pseudo)rapidity, both when observed by HBT correlations and by forward-backward pseudorapidity correlations. The shape of elliptic flow in Au+Au, measured over the full range of pseudorapidity, appears to have a very weak centrality dependence. Identified particle ratios in d+Au reactions show little difference between the shape of proton and anti-proton spectra, while the absolute yields show an approximate m_T scaling.

PHOBOS is one of the four heavy ion experiments currently running at the Relativistic Heavy Ion Collider (RHIC). In this paper we will present some of the current results from PHOBOS: the study of charged multiplicity distribution as function of pseudorapidity and centrality at <IMG SRC="/img/revistas/bjp/v34n3a/a32img01.gif" > or = 19.6, 130 and 200 GeV, a discussion of multiplicity scaling, and a measurement of transverse momentum spectra (pT) of charged hadrons produced in d+Au and Au+Au interactions at <IMG SRC="/img/revistas/bjp/v34n3a/a32img01.gif" > or = 200 GeV. The dependence of the Au+Au yields as function of centrality is found to be independent of pT. A strong suppression in the Au+Au hadron spectra relative to p<IMG SRC="/img/revistas/bjp/v34n3a/a32img05.gif"> at high pT is observed. The spectral shape for d+Au collisions is significantly different from the observations in Au+Au, the yield suppression is not observed in d+Au.

Elliptic flow is an interesting probe of the dynamical evolution of the dense system formed in the ultrarelativistic heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC).The elliptic flow dependences on transverse momentum, centrality, and pseudorapidity were measured using data collected by the PHOBOS detector, which offers a unique opportunity to study the azimuthal anisotropies of charged particles over a wide range of pseudorapidity.These measurements are presented, together with an overview of the analysis methods and a discussion of the results.Collisions of Au nuclei at the Relativistic Heavy Ion Collider (RHIC) are the most energetic nucleus-nucleus collisions ever achieved in a laboratory, allowing for the study of the properties of nuclear matter under extreme conditions.In order to characterize the medium created in the Au+Au collisions at RHIC, it is necessary to establish that the particles in the system undergo enough reinteractions to reach a state of thermal equilibrium.Only in such a state may the evolution of the medium be described in terms of thermodynamical quantities.Elliptic flow is one of the experimental observables than can help resolve this question.

Transverse momentum spectra of charged hadrons with p T < 6 GeV/c have been measured near mid-rapidity (0.2 < η < 1.4) by the PHOBOS experiment at RHIC in Au + Au and d + Au collisions at ${\sqrt{s_{_{NN}}}~= \rm {200~GeV}}$ . The spectra for different collision centralities are compared to ${p + \bar{p}}$ collisions at the same energy. The resulting nuclear modification factor for central Au + Au collisions shows evidence of strong suppression of charged hadrons in the high-p T region (>2 GeV/c). In contrast, the d + Au nuclear modification factor exhibits no suppression of the high-p T yields. These measurements suggest a large energy loss of the high-p T particles in the highly interacting medium created in the central Au + Au collisions. The lack of suppression in d + Au collisions suggests that it is unlikely that initial state effects can explain the suppression in the central Au + Au collisions. PACS: 25.75.-q

The early summer of 2000 saw the start of the RHIC experimental programme. The PHOBOS collaboration used data from the beginning of the running period to extract the first physics information about this exciting new regime. This first measurement consisted of pseudorapidity densities of primary charged particles near mid-rapidity in central Au + Au collisions at beam energies which gave centre-of-mass energies per pair of colliding nucleons of √snn = 56 and 130 GeV. The observed densities are higher than those observed previously in any collisions in the laboratory. In addition, the rate of increase in density between the two energies is significantly larger than that for nucleon-nucleon collisions at comparable beam energies. This paper will briefly describe the PHOBOS experiment (including the subset of the full detector installed for the early period of the RHIC beam), discuss the results of the first physics measurement and conclude with the prospects for the future, especially in the area of strangeness production.

The angular distribution of photons from n-p radiative capture of 63.4 MeV neutrons has been measured. Data taken at eight n-γ laboratory angles (45°–150°) were transformed to deuteron photodisintegration cross sections at the equivalent laboratory energy, Eγ = 33.9 MeV, and analysed in combination with independent measurements of the 0° and 180° cross sections. Legendre polynomial coefficients obtained from the analysis are consistent with values reported from global fits and deviate marginally from predictions based on the Bonn and Paris potentials.

PHOBOS is one of the four experiments at the Relativistic Heavy Ion Collider measuringp + p, d + Au, andAu + Au collisions over a broad range of energies. PHOBOS is a silicon-pad based detector with a 4π multiplicity detector and a high resolution mid-rapidity spectrometer, along with other detectors (time-of-flight walls, proton and zero degree calorimeters). PHOBOS is able to measure particles at low transverse momentum, spectra, flow, particle ratios, and multiplicity over a large region of phase space. A comparison of results forAu + Au andd + Au collisions at√SNN = 220GeV will be discussed.

Forward calorimetry in the PHOBOS detector has been used to study charged hadron production in d+Au, p+Au, and n+Au collisions at sNN=200GeV. The forward proton calorimeter detectors are described and a procedure for determining collision centrality with these detectors is detailed. The deposition of energy by deuteron spectator nucleons in the forward calorimeters is used to identify p+Au and n+Au collisions in the data. A weighted combination of the yield of p+Au and n+Au is constructed to build a reference for Au+Au collisions that better matches the isospin composition of the gold nucleus. The pT and centrality dependence of the yield of this improved reference system is found to match that of d+Au. The shape of the charged-particle transverse momentum distribution is observed to extrapolate smoothly from p+p¯ to central d+Au as a function of the charged-particle pseudorapidity density. The asymmetry of positively and negatively charged hadron production in p+Au is compared to that of n+Au. No significant asymmetry is observed at midrapidity. These studies augment recent results from experiments at the CERN Large Hadron Collider and BNL Relativistic Heavy Ion Collider facilities to give a more complete description of particle production in p+A and d+A collisions, essential for the understanding the medium produced in high-energy nucleus-nucleus collisions.11 MoreReceived 25 May 2015DOI:https://doi.org/10.1103/PhysRevC.92.034915©2015 American Physical Society

A survey is presented of results from some recent searches for exotic physics such as heavy resonances, dark matter, and long-lived particles by the CMS experiment at the LHC. Presented at LaThuile 2015 XXIXth Rencontres de Physique de la Vallee dAoste IL NUOVO CIMENTO Vol. ?, N. ? ? Recent exotics search results from CMS C. Henderson on behalf of the CMS Collaboration Department of Physics & Astronomy, University of Alabama Tuscaloosa, AL, USA Summary. — A survey is presented of results from some recent searches for exotic physics such as heavy resonances, dark matter, and long-lived particles by the CMS experiment at the LHC. PACS 12.60.-i – Models beyond the standard model.

The existence of Current Free Double Layers (CFDL) in high electronegativity plasmas has not been experimentally verified, and the validation of theoretical predictions of these structures is required. As part of this experimental verification a numerical simulation of electronegative plasma using OOPIC Pro and XOOPIC was developed to determine the required experimental apparatus and plasma parameters. Electronegative plasma in this context is defined as a high ratio of negative ions as the charge carriers versus electrons, with theory predicting a required ratio of 3 for CFDL formation. Monte Carlo Collision (MCC) features in OOPIC Pro were configured to produce negative SF6 ions from background gas after collisions with low-energy electrons using cross-section data for attachment. Positive ion formation was also modeled using the existing MCC code, along with secondary electron emission from surfaces. Steady-state conditions were achieved in the simulation allowing analysis of plasma electronegativity, positive ion velocity profiles, electron energy distribution functions, and other plasma parameters for various conditions. Introduction of conductors into the simulation at various potentials allowed analysis of the potential profile in a search for the expected CFDLs. The experimental parameters required to produce electronegative plasmas at a sufficiently high ratio in a physical machine, and achieve experimental verification of CFDLs, were determined.

The lineshape of a neutron spectrometer consisting of a single deuterated anthracene scintillation crystal has been investigated as a function of neutron energy from 7 - 30 MeV. The spectrometer has been used to study backscattering of 14 MeV neutrons by samples containing carbon, nitrogen, and oxygen. The results indicate that it should be possible to analyze bulk samples for these elements by means of this technique.

This article presents results on event-by-event elliptic flow fluctuations in Au + Au collisions at √s NN = 200 GeV, where the contribution from non-flow correlations has been subtracted. An analysis method is introduced to measure non-flow correlations, relying on the assumption that non-flow correlations are most prominent at short ranges (|Δη| 2), relative elliptic flow fluctuations of approximately 30-40% are observed. These results are consistent with predictions based on spatial fluctuations of the participating nucleons in the initial nuclear overlap region. It is found that the long-range non-flow correlations in Au + Au collisions would have to be more than an order of magnitude stronger compared to the p + p data to lead to the observed azimuthal anisotropy fluctuations with no intrinsic elliptic flow fluctuations.

A selection of experimental results from the PHOBOS Collaboration relevant for probing high-energy nuclear collisions with high transverse momentum particles is presented. The inclusive yields of charged particles and comparisons between nuclear and elementary collisions already reveal a large amount of parton energy loss in the hot and dense medium created in heavy ion collisions. Remarkable scaling and factorization features are observed, unifying the data taken at various collision energies, centralities and nuclear sizes. To further analyze the nature of the energy loss, a measurement of pseudorapidity (Δη) and azimuthal angle (Δφ) correlations between high transverse momentum charged hadrons (p T >2.5 GeV/c) and all associated charged particles is presented at both short-range (small Δη) and long-range (large Δη) over a continuous detector acceptance covering −4<Δη<2. Various near- and away-side features of the correlation structure are discussed as a function of centrality in Au + Au collisions at $\sqrt{s_{NN}}=200$ GeV. The results provide new information about the longitudinal (Δη) extent of the near-side ‘ridge’ structure, first observed by the STAR Collaboration over a narrower η range. In central Au + Au collisions the ridge structure extends to at least Δη=4, and its strength completely diminishes as collisions become more peripheral.

over a wide range of pseudorapidity, using the PHOBOS detector. Making a global comparison of Cu + Cu and Au + Au results, we nd that the total number of produced charged particles and the rough shape (height and width) of the pseudorapidity distributions are determined by the number of nucleon participants. More detailed studies reveal that a more precise matching of the shape of the Cu + Cu and Au + Au pseudorapidity distributions over the full range of pseudorapidity occurs for the same Npart/2A value rather than the same Npart value. In other words, it is the collision geometry rather than just the number of nucleon participants that drives the detailed shape of the pseudorapidity distribution and its centrality dependence at RHIC energies. PACS numbers: 25.75.-q, 25.75.Dw The advent of Cu + Cu collisions from the Relativistic Heavy Ion Collider (RHIC) at energies similar to those of the earlier Au + Au collisions presents a new opportunity to measure the system size dependence of important observables using dieren t collision geometries. The Cu + Cu results are expected to provide critical tests of the parametric dependence of the pseudorapidity density of charged particles, dNch=d , observed previously in Au + Au collisions [1{3]. They signican tly extend the range of measurements with respect to the number of participant nucleons, Npart, compared to Au + Au and also allow for a direct comparison at the same Npart. The observed dNch=d is a conceptually well-dened quantity that reects most eects that contribute to particle production in heavy-ion collisions. It is sensitive to the initial conditions of the system, i.e. parton shadowing, and also to the eects of rescattering and hadronic nal-state interactions. In short, the full distribution of dNch=d represents a time-integral of particle production throughout the entire heavy-ion collision.

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation Edmundo García, B. B. Back, M. D. Baker, M. Ballintijn, D. S. Barton, R. R. Betts, A. A. Bickley, R. Bindel, W. Busza, A. Carroll, Z. Chai, M. P. Decowski, E. García, T. Gburek, N. George, K. Gulbrandsen, C. Halliwell, J. Hamblen, M. Hauer, C. Henderson, D. J. Hofman, R. S. Hollis, R. Hołyński, B. Holzman, A. Iordanova, E. Johnson, J. L. Kane, N. Khan, P. Kulinich, C. M. Kuo, W. T. Lin, S. Manly, A. C. Mignerey, R. Nouicer, A. Olszewski, R. Pak, C. Reed, C. Roland, G. Roland, J. Sagerer, H. Seals, I. Sedykh, C. E. Smith, M. A. Stankiewicz, P. Steinberg, G. S. F. Stephans, A. Sukhanov, M. B. Tonjes, A. Trzupek, C. Vale, G. J. van Nieuwenhuizen, S. S. Vaurynovich, R. Verdier, G. I. Veres, E. Wenger, F. L. H. Wolfs, B. Wosiek, K. Woźniak, B. Wysłouch; Recent Results from Phobos. AIP Conf. Proc. 12 February 2007; 884 (1): 396–404. https://doi.org/10.1063/1.2710611 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAIP Publishing PortfolioAIP Conference Proceedings Search Advanced Search |Citation Search

Recent measurements of event-by-event elliptic flow in Au+Au collisions at root s(NN) = 200GeV exhibit large relative fluctuations of about 40%-50%. The data are well described by fluctuations in the shape of the initial collision region, as estimated event-by-event with the participant eccentricity using Glauber Monte Carlo. These results, combined with the demonstrated participant eccentricity scaling of the elliptic flow across nuclear species, constitute evidence of transverse granularity in the initial matter production in these collisions.

We discuss several observables measured by PHOBOS that show common scaling features in Cu+Cu and Au+Au collisions at RHIC energies. In particular, we examine the centrality and energy dependence of the charged particle multiplicity, as well as the centrality dependence of the elliptic flow at mid-rapidity. The discrepancy between Cu+Cu and Au+Au of the final state azimuthal asymmetry (elliptic flow), relative to the initial state geometry of the collision, can be resolved by accounting for fluctuations in the description of the initial geometry.

Recent results on identified particle production from the PHOBOS experiment at the relativistic heavy-ion collider (RHIC) are summarized. Transverse momentum spectra of pions, kaons, protons and antiprotons from Au+Au collisions at s NN = 62.4 GeV are presented close to mid-rapidity. Baryons have been found to have substantially harder transverse momentum spectra than mesons. The p/π + ratio reaches unity at high p T , which fits into a smooth trend together with measurements at lower and higher collision energies. At very low transverse momenta no significant excess of particle yield was found, compared to extrapolations from higher p T . The net proton yield at mid-rapidity appears to be proportional to the number of participant nucleons in Au+Au collisions. The PHOBOS acceptance and mass resolution for the 0 meson at low p T were studied in detail.

The distributions of charged particles from heavy ion collisions have been measured by the PHOBOS collaboration. Results from Au+Au collisions at various energies and over a broad range of centrality are presented and discussed. This data are compared to p+p, d+Au and Cu+Cu collisions at the corresponding energy and centrality.

The PHOBOS collaboration has carried out a systematic study of charged particle multiplicities in Cu+Cu and Au+Au collisions at the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory. A unique feature of the PHOBOS detector is its ability to measure charged particles over a very wide angular range from 0.5 to 179.5 deg. corresponding to |eta|<5.4. The general features of the charged particle multiplicity distributions as a function of pseudo-rapidity, collision energy and centrality, as well as system size, are discussed.

A brief overview of the current results and conclusions from the PHOBOS experiment at the Relativistic Heavy Ion Collider (RHIC) is given. No evidence is found for non-monotonic behavior of observables measured by PHOBOS in the RHIC energy region. Convincing evidence is found that we have created a state of matter with high energy-density, that is nearly net-baryon free and is strongly interacting. The data are found to exhibit "simple" scaling behaviors, which include extended longitudinal scaling and scaling with the number of participating nucleons. The Au+Au collision charged particle data also exhibit a remarkable factorization of collision energy and geometry.

In this talk I will review PHOBOS data on charged particle multiplicities, obtained in Au + Au and d + Au collisions at RHIC. The general features of the Au + Au pseudorapidity distributions results will be discussed and compared to those of [Formula: see text] collisions. The total charged particle multiplicity, scaled by the number of participant pairs, is observed to be about 40% higher in Au + Au collisions than in [Formula: see text] and d + Au systems, but, surprisingly at the same level of e + e - collisions. Limiting fragmentation scaling is seen to be obeyed in Au + Au collisions.

We have measured transverse momentum distributions of charged hadrons produced in d + Au collisions at s N N = 200 GeV. The spectra were obtained for transverse momenta 0.25 < p T < 6.0 GeV/c, in a pseudorapidity range of0.2 < η < 1.4 in the deuteron direction. The evolution of the spectra with collision centrality is presented in comparison to p + p collisions at the same collision energy. With increasing centrality, the yield at high transverse momenta increases more rapidly than the overall particle density, leading to a strong modification of the spectral shape. This change in spectral shape is qualitatively different from observations in Au + Au collisions at the same energy. The results provide important information for discriminating between different models for the suppression of high-p T hadrons observed in Au + Au collisions.

The PHOBOS experiment at RHIC has measured the multiplicity of primary charged particles as a function of centrality and pseudorapidity in Au+Au collisions at √SNN = 19.6, 130 and 200 GeV. Two kinds of universal behavior are observed in charged particle production in heavy ion collisions. The first is that forward particle production, over a range of energies, follows a universal limiting curve with a non-trivial centrality dependence. The second arises from comparisons with pp/pp and e+e− data. 〈Nch〉/〈Npart/2〉 in nuclear collisions at high energy scales with √s in a similar way as Nch in e+e− collisions and has a very weak centrality dependence. This feature may be related to a reduction in the leading particle effect due to the multiple collisions suffered per participant in heavy ion collisions.