W. Busza

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.

Heavy ion collisions quickly form a droplet of quark–gluon plasma (QGP) with a remarkably small viscosity. We give an accessible introduction to how to study this smallest and hottest droplet of liquid made on Earth and why it is so interesting. The physics of heavy ion collisions ranges from highly energetic quarks and gluons described by perturbative QCD to a bath of strongly interacting gluons at lower energy scales. These gluons quickly thermalize and form QGP, while the energetic partons traverse this plasma and end in a shower of particles called jets. Analyzing the final particles in various ways allows us to study the properties of QGP and the complex dynamics of multiscale processes in QCD that govern its formation and evolution, providing what is perhaps the simplest form of complex quantum matter that we know of. Much remains to be understood, and throughout the review big open questions are encountered.

This paper presents measurements of the elliptic flow of charged particles as a function of pseudorapidity and centrality from Cu-Cu collisions at 62.4 and 200 GeV using the PHOBOS detector at the Relativistic Heavy Ion Collider (RHIC).The elliptic flow in Cu-Cu collisions is found to be significant even for the most central events.For comparison with the Au-Au results, it is found that the detailed way in which the collision geometry (eccentricity) is estimated is of critical importance when scaling out system-size effects.A new form of eccentricity, called the participant eccentricity, is introduced which yields a scaled elliptic flow in the Cu-Cu system that has the same relative magnitude and qualitative features as that in the Au-Au system.

This writeup is a compilation of the predictions for the forthcoming Heavy Ion Program at the Large Hadron Collider, as presented at the CERN Theory Institute 'Heavy Ion Collisions at the LHC - Last Call for Predictions', held from May 14th to June 10th 2007.

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.

In this paper, we investigate various ways of defining the initial source eccentricity using the Monte Carlo Glauber (MCG) approach. In particular, we examine the participant eccentricity, which quantifies the eccentricity of the initial source shape by the major axes of the ellipse formed by the interaction points of the participating nucleons. We show that reasonable variation of the density parameters in the Glauber calculation, as well as variations in how matter production is modeled, do not significantly modify the already established behavior of the participant eccentricity as a function of collision centrality. Focusing on event-by-event fluctuations and correlations of the distributions of participating nucleons we demonstrate that, depending on the achieved event-plane resolution, fluctuations in the elliptic flow magnitude $v_2$ lead to most measurements being sensitive to the root-mean-square, rather than the mean of the $v_2$ distribution. Neglecting correlations among participants, we derive analytical expressions for the participant eccentricity cumulants as a function of the number of participating nucleons, $\Npart$,keeping non-negligible contributions up to $\ordof{1/\Npart^3}$. We find that the derived expressions yield the same results as obtained from mixed-event MCG calculations which remove the correlations stemming from the nuclear collision process. Most importantly, we conclude from the comparison with MCG calculations that the fourth order participant eccentricity cumulant does not approach the spatial anisotropy obtained assuming a smooth nuclear matter distribution. In particular, for the Cu+Cu system, these quantities deviate from each other by almost a factor of two over a wide range in centrality.

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.

Data are presented on the inclusive production of ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$, ${K}^{\ifmmode\pm\else\textpm\fi{}}$, $p$, and $\overline{p}$ for ${\ensuremath{\pi}}^{+}$, ${K}^{+}$, and protons incident on nuclear targets at 100 GeV. The results cover the kinematic range $30\ensuremath{\le}P\ensuremath{\le}88$ GeV/c for ${P}_{t}=0.3 \mathrm{and} 0.5$ GeV/c. The observed $A$ dependence of the invariant cross sections exhibits remarkable simplicity, which does not naturally follow from current models of particle production. The results show that the hypothesis of limiting fragmentation can be extended to include collisions with nuclei.

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 differential cross sections for single-${\ensuremath{\pi}}^{+}$ photoproduction from hydrogen have been measured over a range of momentum transfers from -2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ to -2 ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$, and photon energies from 5 to 16 GeV. The differential cross section increases by roughly a factor of 2 as the magnitude of the square of the momentum transfer decreases from 0.02 ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$. The cross section falls approximately as $\mathrm{exp}(\ensuremath{-}3|t|)$ at large momentum transfers, with a similar momentum-transfer dependence of the cross section at all photon energies studied.

Inclusive cross sections for protons emitted in 100 GeV proton-nucleus collisions are used to estimate the stopping power of nuclear matter for fast nucleons. The typical recoil momentum obtained for a nucleon struck by the center of a lead nucleus is 4–10 GeV/c, an order of magnitude greater than in p-p collisions, and an order of magnitude smaller than in a naive cascade model. Possible implications for high energy heavy ion collisions are discussed.

An experiment using the Fermilab Single Arm Spectrometer (SAS) facility and an associated nonmagnetic vertex detector studied the reactions a+p→c+X, where a and c were π±, K±, p, or p¯. Extensive measurements were made at 100 and 175 GeV/c beam momenta with the outgoing hadrons detected in the SAS covering a kinematic range 0.12<x<1.0 and pT<1.25 GeV/c. Additional data covering a more restricted range in x were also gathered at 70 GeV/c incident momentum. In this high-statistics experiment, the identification of both the incoming and outgoing charged hadrons were made with a total of eight Čerenkov counters. New and extensive single-particle inclusive data for charged-particle production in low-pT hadronic fragmentation are presented. The average associated charged-particle multiplicity and pseudorapidity distributions are also given.Received 13 January 1982DOI:https://doi.org/10.1103/PhysRevD.26.1497©1982 American Physical Society

The space-time evolution of particle production at high energies was investigated by measuring the charged multiplicity in ${\ensuremath{\pi}}^{\ensuremath{-}}$-nucleus collisions at 100 and 175 GeV/c. We find that (a) the forward multiplicity (in the $\ensuremath{\pi}p$ center-of-mass system) is independent of the target nucleus; (b) the backward multiplicity is approximately proportional to the nuclear thickness; and (c) the data are consistent with the assumption that the absorption cross section of the incident particle characterizes the multiplication process.

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.

Hadron-nucleus interactions have been studied in the 50 to 200 GeV/c momentum range for incident pions, kaons, protons, and antiprotons. Average charged multiplicities, dispersions of multiplicity distributions, absorption cross sections, and angular distributions are presented. The energy dependence of the target and projectile fragmentation regions is studied in detail.

The ratio of cross sections for inelastic muon scattering on xenon and deuterium nuclei was measured at very low Bjorken x (0.000 02${\mathit{x}}_{\mathrm{Bj}}$0.25). The data were taken at Fermilab experiment E-665 with a 490 GeV/c muon beam incident on liquid deuterium and gaseous xenon targets. Two largely independent analysis techniques gave statistically consistent results. The xenon-to-deterium per-nucleon cross-section ratio is constant at approximately 0.7 for ${\mathit{x}}_{\mathrm{Bj}}$ below 0.003.

We present a precise measurement of the left-right cross section asymmetry ($A_{LR}$) for $Z$ boson production by $\ee$ collisions. The measurement was performed at a center-of-mass energy of 91.26 GeV with the SLD detector at the SLAC Linear Collider (SLC). The luminosity-weighted average polarization of the SLC electron beam was (63.0$\pm$1.1)%. Using a sample of 49,392 $\z0$ decays, we measure $A_{LR}$ to be 0.1628$\pm$0.0071(stat.)$\pm$0.0028(syst.) which determines the effective weak mixing angle to be $\swein=0.2292\pm0.0009({\rm stat.})\pm0.0004({\rm syst.})$.}

Nuclear shadowing is observed in the per-nucleon cross sections of positive muons on carbon, calcium and lead as compared to deuterium. The data were taken by Fermilab experiment E665 using inelastically scattered muons of mean incident momentum 470 GeV/c. Cross-section ratios are presented in the kinematic region 0.0001<x Bj <0.56 and 0.1<Q 2<80 GeV2. The data are consistent with no significant ν orQ 2 dependence at fixedx Bj . Asx Bj decreases, the size of the shadowing effect, as well as itsA dependence, are found to approach the corresponding measurements in photoproduction.

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 have measured the absolute differential cross section for photoproduction of neutral rho mesons from complex nuclei at a photon energy of 8.8 BeV. Using a two-parameter optical model, we have deduced a value for the total rho-nucleon cross section of ${30}_{\ensuremath{-}4}^{+6}$ mb. Application of the vector-dominance model results in a value for the rho-photon coupling of $\frac{\ensuremath{\gamma}_{\ensuremath{\rho}}^{}{}_{}{}^{2}}{4\ensuremath{\pi}}=1.1\ifmmode\pm\else\textpm\fi{}0.2$.

Cross sections for the reactions γp→K+Λ and γp→K+Σ0 have been measured at squared four-momentum transfer (−t) from 0.005 to 2 GeV2, at photon energies 5, 8, 11, and 16 GeV. For −t>0.2 GeV2 each of the K+ cross sections is about ⅓ of the π+n photoproduction cross section, having nearly the same energy and momentum-transfer dependence. The K+ cross sections fall off at small |t|, however, in contrast to the sharp forward spike seen in π+n; this leads to a disagreement with an SU(3) prediction for −t<0.1 GeV2. The ratio of K+Σ0 to K+Λ cross sections is typically between 0.5 and 1.0.

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 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

Following a major shortage of 99Mo in the 2009–2010 period, concern grew that the aging reactor production facilities needed to be replaced. Most producers were using highly enriched 235U (HEU) as the target material. The Organisation for Economic Co-...Read More

We have performed a high-sensitivity search for massive long-lived particles produced at 2.5 mrad by 400-GeV/c protons on a beryllium target using time-of-flight, Cherenkov, and calorimetric techniques. A total of ${10}^{11}$ light particles (${\ensuremath{\pi}}^{\ensuremath{-}},{K}^{\ensuremath{-}},\overline{p}$) was sampled at 70 GeV/c. This experiment places a limit of 1.1\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}37}$ ${\mathrm{cm}}^{2}$/${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$ \ifmmode\cdot\else\textperiodcentered\fi{} nucleon on the invariant cross section for the production of stable particles in the mass range of 4 to 10 ${\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c}}^{2}$.

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.

Nuclear transparencies measured in exclusive incoherent ρ0 meson production from hydrogen, deuterium, carbon, calcium, and lead in muon-nucleus scattering are reported. The data were obtained with the E665 spectrometer using the Fermilab Tevatron muon beam with a mean beam energy of 470 GeV. Increases in the nuclear transparencies are observed as the virtuality of the photon increases, in qualitative agreement with the expectations of color transparency.Received 5 August 1994DOI:https://doi.org/10.1103/PhysRevLett.74.1525©1995 American Physical Society

We present measurements of ${\mathit{R}}_{\mathit{b}}$ using the SLD at the SLC. The analyses use 2D and 3D impact parameter tags and a displaced 3D vertex tag which all exploit the small size and stability of the ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ interaction point and the precision 3D CCD pixel vertex detector to achieve high bb\ifmmode\bar\else\textasciimacron\fi{}-tagging efficiencies and purities. The combined measurement yields ${\mathit{R}}_{\mathit{b}}$=0.229\ifmmode\pm\else\textpm\fi{}0.011 and is consistent with standard model predictions. \textcopyright{} 1996 The American Physical Society.

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.

Pseudorapidity distributions for proton-nucleus interactions are presented. The data cover twelve nuclei ranging from carbon to uranium and three incident proton momenta, 50, 100, and 200 GeV/c.Received 21 April 1977DOI:https://doi.org/10.1103/PhysRevLett.39.1499©1977 American Physical Society

This paper discusses speculative disaster scenarios inspired by hypothetical new fundamental processes that might occur in high-energy relativistic heavy-ion collisions. The authors estimate the parameters relevant to black-hole production and find that they are absurdly small. They show that other accelerator and (especially) cosmic-ray environments have already provided far more auspicious opportunities for transition to a new vacuum state, so that existing observations provide stringent bounds. The possibility of producing a dangerous strangelet is discussed in most detail. The authors argue that four separate requirements are necessary for this to occur: existence of large stable strangelets, metastability of intermediate size strangelets, negative charge for strangelets along the stability line, and production of intermediate size strangelets in the heavy ion environment. Both theoretical and experimental reasons why each of these appears unlikely are discussed. In particular, the authors know of no plausible suggestion for why the third or especially the fourth might be true. Given minimal physical assumptions, the continued existence of the Moon, in the form we know it, despite billions of years of cosmic-ray exposure, provides powerful empirical evidence against the possibility of dangerous strangelet production.

We present the first measurement of the left-right cross section asymmetry (${\mathit{A}}_{\mathit{L}\mathit{R}}$) for Z boson production by ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ collisions. The measurement was performed at a center-of-mass energy of 91.55 GeV with the SLD detector at the SLAC Linear Collider which utilized a longitudinally polarized electron beam. The average beam polarization was (22.4\ifmmode\pm\else\textpm\fi{}0.6)%. Using a sample of 10 224 Z decays, we measure ${\mathit{A}}_{\mathit{L}\mathit{R}}$ to be 0.100\ifmmode\pm\else\textpm\fi{}0.044(stat)\ifmmode\pm\else\textpm\fi{}0.004(syst), which determines the effective weak mixing angle to be ${\mathrm{sin}}^{2}$${\mathrm{\ensuremath{\theta}}}_{\mathit{W}}^{\mathit{e}\mathit{f}\mathit{f}}$=0.2378 \ifmmode\pm\else\textpm\fi{}0.0056(stat)\ifmmode\pm\else\textpm\fi{}0.0005(syst).

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

Average multiplicities and pseudorapidity distributions for 100-GeV/c $p$-, ${K}^{+}$-, and ${\ensuremath{\pi}}^{+}$-nucleus collisions are presented. The average multiplicities increase with nuclear thickness. The fractional increase is independent of incident particle species, provided that nuclear thicknesses are calculated in units of mean free path of the incident hadron. This scaling behavior suggests that the immediate product of a hadron-nucleon collision is a state similar to the incident hadron.

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.

Results on forward charged hadrons in 490 GeV deep-inelastic muon scattering are presented. The transverse momenta, azimuthal asymmetry, and energy flow of events with four or more forward charged hadrons are studied. The range of the invariant hadronic mass squared 300<W2<900 GeV2/c4 extends higher than previous deep-inelastic muon scattering experiments. Data are compared to the predictions of the Lund Monte Carlo model with perturbative QCD simulated by matrix elements, parton showers, and color dipole radiation. All of the QCD-based models are consistent with the data while a model without QCD processes is not. Correlations with the multiplicity-independent event variable Π≃Σ|pT| are studied. The relationship between the azimuthal asymmetry and transverse momentum of forward hadrons is also presented. The data are most consistent with intrinsic parton transverse momentum squared k2T of 0.25 GeV2/c2.Received 14 June 1993DOI:https://doi.org/10.1103/PhysRevD.48.5057©1993 American Physical Society

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

${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}p$ elastic differential cross sections in the momentum range 1.72-2.80 GeV/c have been measured at the proton synchrotron "NIMROD" of the Rutherford High Energy Laboratory. The results are tabulated, and analyses of the differential cross sections employing optical models and Legendre polynomial expansions are advanced. A critical discussion of a recent interpretation of differential-cross-section structure in terms of interference between resonant and background amplitudes is presented.

We have measured large-angle electron-positron pairs from the reaction $\ensuremath{\gamma}+\mathrm{Be}\ensuremath{\rightarrow}\mathrm{Be}+{e}^{+}+{e}^{\ensuremath{-}}$ in the ${e}^{+}{e}^{\ensuremath{-}}$ invariant-mass region $610&lt;m&lt;850 \frac{\mathrm{MeV}}{{c}^{2}}$. The phase of the photoproduction amplitude of the $\ensuremath{\rho}$ meson at 4.1-6.1 GeV was found to deviate from pure imaginary by 11.8\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}4.4\ifmmode^\circ\else\textdegree\fi{} which corresponds to a ratio of the real to imaginary $\ensuremath{\rho}$-nucleon amplitude of $\ensuremath{\beta}=0.2\ifmmode\pm\else\textpm\fi{}0.1$.

We report a new measurement of Rb = Gamma(Z0->b-bbar) / Gamma(Z0->hadrons) using a double tag technique, where the b hemisphere selection is based on the reconstructed mass of the B hadron decay vertex. The measurement was performed using a sample of 130k hadronic Z0 events, collected with the SLD at the SLC. The method utilizes the 3-D vertexing abilities of the CCD pixel vertex detector and the small stable SLC beams to obtain a high b-tagging efficiency and purity. We obtain Rb=0.2142+/-0.0034(stat.)+/-0.0015(syst.)+/-0.0002(R_c).

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.

We have determined the electroweak parameters ${{sin}^{2}\ensuremath{\theta}}_{\mathrm{W}}$ and $\ensuremath{\rho}$ by a measurement of deep-inelastic neutrino-nucleon scattering used a fine-grained neutrino detector exposed to a narrowband neutrino beam at Fermilab. The sampling properties of our detector have permitted neutralcurrent and charged-current events to be well separated over a wide kinematic range, thereby allowing a determination of ${{sin}^{2}\ensuremath{\theta}}_{\mathrm{W}}$ and $\ensuremath{\rho}$ to be made with good statistics and small systematic errors. We have found ${{sin}^{2}\ensuremath{\theta}}_{\mathrm{W}}=0.246\ifmmode\pm\else\textpm\fi{}0.012\ifmmode\pm\else\textpm\fi{}0.013$ in a single-parameter fit.

Based on trends seen at lower energies we 'predict' the multiplicities and pseudorapidity distributions of particle density and elliptic flow that will be seen in PbPb and pp collisions at the LHC. We argue that, if these predictions turn out to be correct, either these quantities are insensitive to the state of matter created in high-energy heavy-ion collisions or the observed simplicity and universality of the data must be telling us something profound about the mechanism of particle production, which to this date is not well understood.

We present high-statistics results on the reactions $a+p\ensuremath{\rightarrow}c+X$ where $a$ and $c$ can be any of ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$, ${K}^{\ifmmode\pm\else\textpm\fi{}}$, $p$, or $\overline{p}$. The data were taken at 100 and 175 GeV/c incident momenta using the Fermilab Single-Arm Spectrometer operated over the kinematic range $0.2&lt;x&lt;1.0$ and ${p}_{t}&lt;~1.0$ GeV/c. Investigating the $x$ dependence of the data, we find agreement with a quark-parton picture, namely the cross sections have a power-law behavior in $1\ensuremath{-}x$ independent of ${p}_{\mathrm{beam}}$ and ${p}_{t}$.

We have studied p-cu interference in di-pion photoproduction in the energy range 5-7

We present a new measurement of the left-right cross section asymmetry $({A}_{\mathrm{LR}})$ for $Z$ boson production by ${e}^{+}{e}^{\ensuremath{-}}$ collisions. The measurement was performed at a center-of-mass energy of 91.28 GeV with the SLD detector at the SLAC Linear Collider (SLC). The luminosity-weighted average polarization of the SLC electron beam was $(77.23\ifmmode\pm\else\textpm\fi{}0.52)%$. Using a sample of 93 644 $Z$ decays, we measure the pole value of the asymmetry, ${A}_{\mathrm{LR}}^{0}$, to be $0.1512\ifmmode\pm\else\textpm\fi{}0.0042(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.0011(\mathrm{syst})$, which is equivalent to an effective weak mixing angle of $\mathrm{sin}{}^{2}{\ensuremath{\theta}}_{W}^{\mathrm{eff}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.23100\ifmmode\pm\else\textpm\fi{}0.00054(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.00014\left(\mathrm{syst}\right)$.

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

3 JzNuARv 1972 in which all low-lying hadron states have zero isospin.Below the threshold for production of states with nonze- ro isospin, only the isoscalar component of the electromagnetic current would be observed.This isoscalar probe would not distinguish between.proton and neutron.Low-energy experiments would indicate a deuteron made of two identical spin-2 nucleons, each with charge 2, and peculiar statistics allowing two identical parti- cles in a symmetric state.Only after the excitation of the isovector states of the two-nucleon system would it be clear that there was a hidden degree of freedom and two kinds of nucleons with integral charges.This result is expected, since the three-triplet mod- el was constructed to give all the Gell-Mann-Zweig properties of observed particles.Given a set of hadron states in the conventional quark model, corresponding hadron states in the three-triplet model can be con- structed for which the matrix elements of the electromagnetic current operator are identical to those of the quark model.

We have measured the cross section for the photoproduction of di-pions in the mass range 0.9-2.2 $\frac{\mathrm{BeV}}{{c}^{2}}$. The experiment was performed with a 16-BeV bremsstrahlung beam on a Be target. Upper limits on the cross section for photoproduction of high-mass vector mesons are determined.

In this paper the spectrometer constructed by the E665 Collaboration is described. The spectrometer was built during the period 1982–87 and the first data were taken during the 1987–88 Fixed Target run of the Fermi National Accelerator Laboratory (FNAL) Tevatron. This is the first of a series of runs in which a comprehensive program of high energy muon scattering experiments will be performed.

We have used the Fermilab 30-in. bubble-chamber hybrid spectrometer to study multiparticle production in the interactions of 200-GeV/c protons and π+ and K+ mesons with nuclei of gold, silver, and magnesium. We find that the multiplicities of produced particles and negative particles increase linearly with the number of projectile collisions, with no beam or target dependence. The number of secondary collisions in the nucleus increases significantly less rapidly with the number of projectile collisions than has been reported by a streamer chamber experiment. The properties of secondary collisions suggest that they arise from rescattering of recoil nucleons rather than intranuclear cascade of produced particles. Dispersions of multiplicity distributions at fixed impact parameter are in better agreement with a model of independent sources than with Koba-Nielsen-Olesen scaling.Received 14 November 1988DOI:https://doi.org/10.1103/PhysRevD.39.2484©1989 American Physical Society

We present the first experimental study of the ratio of cumulant to factorial moments of the charged-particle multiplicity distribution in high-energy particle interactions, using hadronic Z$^0$ decays collected by the SLD experiment at SLAC. We find that this ratio, as a function of the moment-rank $q$, decreases sharply to a negative minimum at $q=5$, which is followed by quasi-oscillations. These features are insensitive to experimental systematic effects and are in qualitative agreement with expectations from next-to-next-to-leading-order perturbative QCD.

Inelastic scattering of 490 GeV μ+ from deuterium and xenon nuclei has been studied for xBj>s.001. The ratio of the xenon/deuterium cross section per nucleon is observed to vary with xBj, with a depletion in the kinematic range 0.001 < xBj < 0.025 which exhibits no significant Q2 dependence. An electromagnetic calorimeter was used to verify the radiative corrections.

Fermilab Experiment-665 measured deep-inelastic scattering of 490 GeV muons off deuterium and xenon targets. Events were selected with a range of energy exchange ν from 100 GeV to 500 GeV and with large ranges of Q2 and xBj: 0.1 GeV2/c2<Q2<150 GeV2/c2 and 0.001<xBj<0.5. The fractional energy (z) distributions of forward-produced hadrons from the two targets have been compared as a function of the kinematics of the scattering; specifically, the kinematic region of ‘‘shadowing’’ has been compared to that of nonshadowing. The dependence of the distributions upon the order of the hadrons, determined by the fractional energies, has been examined as well; a strong degree of similarity has been observed in the shapes of the distributions of the different order hadrons. These z distributions, however, show no nuclear dependence, even in the kinematic region of shadowing.Received 4 August 1993DOI:https://doi.org/10.1103/PhysRevD.50.1836©1994 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.

The real part of the forward amplitude for Compton scattering on protons was measured through the interference between the Compton and Bethe-Heithler amplitudes by detecting the zero-degree electron pairs asymmetrically. The measurement was made at an average photon energy of $〈k〉=2.2$ GeV, and an average momentum transfer to the recoil proton $〈t〉=\ensuremath{-}0.027$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$. The result confirms the prediction of the Kramers-Kronig relation.

tion of the mass and width was that of the 1.51-GeV isobar since it lies on a relatively slowly varying background.The 1.23-GeV isobar lies on top of the rapidly rising 1m and 2~nonresonant background and the 1.67-GeV isobar lies near the limit of the instrumental acceptance.These factors introduce uncertainties in fitting the back- ground under these peaks and hence also in their masses and widths.In Table I the results of the present experiment are compared with the most recent preceding PP missing-mass experiment, ' a recent rp missing- mass experiment, ' and the results of mP phase- shift analyses.' The three missing-mass experi- ments are in good agreement, demonstrating that widths obtained in missing-mass experiments are narrower than widths obtained from ~P phaseshift analyses.

We have measured the distributions of the jet energies in ${e}^{+}$${e}^{\ensuremath{-}}$\ensuremath{\rightarrow}qq\ifmmode\bar\else\textasciimacron\fi{}g events, and of the three orientation angles of the event plane, using hadronic ${Z}^{0}$ decays collected in the SLD experiment at SLAC. We find that the data are well described by perturbative QCD incorporating vector gluons. We have also studied models of scalar and tensor gluon production and find them to be incompatible with our data.

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.

We present a design for a tagged photon-electron facility for the National Accelerator Laboratory. This facility is designed to provide tagged photons with energy from 15–300 GeV and electrons up to 300 GeV with energy resolution of ± ≈ 2.0%. For 1013 incident 500 GeV protons we expect typically 1.5 × 108 electrons at 150 GeV and 5 × 105 photons with 100 GeV < E < 150 GeV. Photons from π°'s produced in a primary target by proton interactions are separated from charged secondaries and then allowed to convert to e+e- in a Pb radiator. The electrons are transported either to an experimental target or to a second radiator where they may produce photons with tagged energy. Hadronic contamination is kept below a few tenths of a percent.

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.

Using an impact parameter tag to select an enriched sample of Z0→bb¯ events, and the net momentum-weighted track charge to identify the sign of the charge of the underlying b quark, we have measured the left-right forward-backward asymmetry for b quark production as a function of polar angle. Based on 1.8pb−1 of Z0 decay data produced with a mean electron beam polarization of Pe=63%, this yields a direct measurement of the extent of parity violation in the Zbb coupling of Ab=0.87±0.11(stat)±0.09(syst).Received 3 October 1994DOI:https://doi.org/10.1103/PhysRevLett.74.2890©1995 American Physical Society

Measurements of inclusive cross sections at 100 GeV/c are presented for the double-charge-exchange reactions a+p→π−X with a=π, K, or p. The measurements covered a kinematic range in the Feynman x variable of 0.3<~x<~0.9 at transverse momenta of 0.3 and 0.5 GeV/c. A model summing the contributions from resonance production and from inclusive central-region π− production is used to fit the data and demonstrates the importance of resonance production via one-pion exchange for large values of the Feynman x.Received 4 November 1977DOI:https://doi.org/10.1103/PhysRevLett.40.141©1978 American Physical Society

The pion yield produced by 16-GeV electrons in a 0.6-radiation-length beryllium target was studied at laboratory angles of 0.2\ifmmode^\circ\else\textdegree\fi{} to 1.1\ifmmode^\circ\else\textdegree\fi{}. The experimental yields are somewhat larger than those estimated using the Drell and $\ensuremath{\rho}$-diffraction models for pion photoproduction, and they do not appear to decrease at small angles as expected from the theoretical models.

The parity-violating parameters Ab and Ac are directly measured by the SLD experiment at the SLAC Linear Collider in e+e− collisions with polarized electrons at the Z0 resonance. Leptons with distinctive total and transverse momenta are used to select and analyze Z0→bb¯ and Z0→cc¯ events. Ab and Ac are extracted by forming the left-right forward-backward asymmetry in electron beam polarization and quark polar angle. From our 1993 sample of 1.8 pb–1 of Z0 decay data with an average electron beam polarization of 63% we find Ab=0.91±0.14 (stat) ±0.07 (syst) and Ac=0.37±0.23 (stat) ±0.21 (syst).Received 4 October 1994DOI:https://doi.org/10.1103/PhysRevLett.74.2895©1995 American Physical Society

The lifetimes of B 1 and B 0 mesons are measured using a sample of 150 000 hadronic Z 0 decays collected by the SLD experiment at the SLAC Linear Collider between 1993 and 1995.Two analyses are presented in which the decay length and charge of the B meson are reconstructed.The first method uses a novel topological vertexing technique while the second uses semi-inclusively reconstructed semileptonic decays.The topological analysis yields a sample of 6033 ( 3665) charged (neutral) vertices with good charge purity, whereas the semileptonic analysis yields a smaller sample of 634 ( 584) charged (neutral) decays with excellent charge purity.Combining the results from both analyses, we find t B 1 1.66 6 0.06͑stat͒ 6 0.05͑syst͒ ps, t B 0 1.64 6 0.08͑stat͒ 6 0.08͑syst͒ ps, and t B 1 ͞t B 0 1.01 6 0.07͑stat͒ 6 0.06͑syst͒.[S0031-9007(97)

Bhabha scattering with polarized electrons at the ${Z}^{0}$ resonance has been measured with the SLD experiment at the SLAC Linear Collider. The first measurement of the left-right asymmetry in Bhabha scattering is presented, yielding the effective weak mixing angle of ${sin}^{2}{\ensuremath{\theta}}_{W}^{\mathrm{eff}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.2245\ifmmode\pm\else\textpm\fi{}0.0049\ifmmode\pm\else\textpm\fi{}0.0010$. The effective electron couplings to the ${Z}^{0}$ are extracted from a combined analysis of polarized Bhabha scattering and the left-right asymmetry previously published: ${\ensuremath{\upsilon}}_{e}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{-}0.0414\ifmmode\pm\else\textpm\fi{}0.0020$ and ${a}_{e}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{-}0.4977\ifmmode\pm\else\textpm\fi{}0.0045$.

Abstract A large hadron calorimeter and muon tracking device using plastic streamer tubes has been constructed in the iron flux-return structure for the SLD detector at SLAC. Various studies of the operating characteristics of the streamer tubes of this system are presented. Emphasis is placed on the tracking capabilities of the device and on the optimization of the high voltage and readout electronics.

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.

Measurements of complete angular distributions of elastic K+ p scattering at closely spaced incident momenta from 1368 to 2259 MeV/c are presented and discussed. A PDP-8 computer controlled system of scintillation counters and core-readout wire spark chambers was used for the detection of elastic events. Diffractive behaviour is already present at the lowest measured momentum and becomes more prominent as the incident momentum increases. An expansion of the angular distributions in terms of Legendre polynomials shows no marked structure of the expansion coefficients as functions of the incident momentum. Our measurements can be adequately described by a number of existing phase shift solutions within 5% of their published values. Also Regge pole extrapolations represent our data satisfactorily.

In June 2005 the PHOBOS Collaboration completed data taking at RHIC. In five years of operation PHOBOS recorded information for Au+Au at sNN=19.6, 62.4, 130, and 200GeV, Cu+Cu at 22.4, 62.4 and 200GeV, d+Au at 201GeV, and p+p at 200 and 410GeV, altogether more than one billion collisions. Using these data we have studied the energy and centrality dependence of the global properties of charged particle production over essentially the full 4π solid angle and (for pions near mid rapidity) charged particle spectra down to transverse momenta below 30MeV/c. We have also studied correlations of particles separated in pseudorapidity by up to 6 units. We find that the global properties of heavy ion collisions can be described in terms of a small number of simple dependencies on energy and centrality, and that there are strong correlations between the produced particles. To date no single model has been proposed which describes this rich phenomenology. In this talk I summarize what the data is explicitly telling us.

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.

Multiparticle production of charged particles at high energies exhibit the phenomenon of Limiting Fragmentation. Furthermore, the region in rapidity over which the production of particles appears to be independent of energy, increases with energy. It is argued that this phenomenon, known as Extended Longitudinal Scaling, is a direct manifestation of some kind of saturation, akin to that in the Color Glass Condensate picture of particle production.

Using an impact parameter tag to select an enriched sample of ${\mathit{Z}}^{0}$\ensuremath{\rightarrow}bb\ifmmode\bar\else\textasciimacron\fi{} events, we have measured the difference between the average charged multiplicity of ${\mathit{Z}}^{0}$\ensuremath{\rightarrow}bb\ifmmode\bar\else\textasciimacron\fi{} and ${\mathit{Z}}^{0}$\ensuremath{\rightarrow} hadrons to be n${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathit{b}}$-n${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathrm{had}}$=2.24\ifmmode\pm\else\textpm\fi{}0.30 ( stat )\ifmmode\pm\else\textpm\fi{}0.33 ( syst ) tracks per event. From this, we have derived n${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathit{b}}$-n${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathit{u}\mathit{d}\mathit{s}}$=3.31\ifmmode\pm\else\textpm\fi{}0.41\ifmmode\pm\else\textpm\fi{}0.79. Comparing this measurement with those at lower center-of-mass energies, we find no evidence that n${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathit{b}}$-n${\mathrm{\ifmmode\bar\else\textasciimacron\fi{}}}_{\mathit{u}\mathit{d}\mathit{s}}$ depends on energy. This result is in agreement with a precise prediction of perturbative QCD, and supports the notion that QCD remains asymptotically free down to the scale ${\mathit{M}}_{\mathit{b}}^{2}$.

Average charged multiplicities have been measured separately in b, c and light quark (u, d, s) events from Z0 decays measured in the SLD experiment. Impact parameters of charged tracks were used to select enriched samples of b and light quark events, and reconstructed charmed mesons were used to select c quark events. We measured the charged multiplicities: nuds= 20.21 ± 0.10(stat.) ± 0.22(syst.), nc= 21.28 ± 0.46(stat.)−0.36+0.41syst.) and nb= 23.14 ± 0.10(stat.)−0.37+0.38(syst.), from which we derived the differences between the total average charged multiplicities of c or b quark events and light quark events: Δnc= 1.07 ± 0.47(stat.)−0.30+0.36(syst.) and Δnb= 2.93 ± 0.14(stat.)−0.29+0.30(syst.). We compared these measurements with those at lower center-of-mass energies and with perturbative QCD predictions. These combined results are in agreement with the QCD expectations and disfavor the hypothesis of flavor-independent fragmentation.