Longitudinal Momentum Fraction X_L for Two High P_t Protons in pp->ppX Reaction

We present an analysis of new data from Experiment E850 at BNL. We have characterized the inclusive cross section near the endpoint for pp exclusive scattering in Hydrogen and in Carbon with incident beam energy of 6 GeV. We select events with a pair of back-to-back hadrons at large transverse momentum. These cross sections are parameterized with a form $\frac{d \sigma}{d X_{L}}$ $\sim(1-X_{L})^{p}$, where ${X_{L}}$ is the ratio of the longitudinal momentum of the observed pair to the total incident beam momentum. Small value of $p$ may suggest that the number of partons participating in the reaction is large and reaction has a strong dependence on the center-of-mass energy. We also discuss nuclear effects observed in our kinematic region.Comment: 4 pages, 2 figures, to be published in Proceedings of CIPANP2000, Quebec, May 22-28, 2000, requires aipproc.sty(included

Evolution of the Differential Transverse Momentum Correlation Function with Centrality in Au + Au Collisions at √sNN = 200 GeV

We present first measurements of the evolution of the differential transverse momentum correlation function, C, with collision centrality in Au + Au interactions at √sNN = 200 GeV. This observable exhibits a strong dependence on collision centrality that is qualitatively similar to that of number correlations previously reported. We use the observed longitudinal broadening of the near-side peak of C with increasing centrality to estimate the ratio of the shear viscosity to entropy density, η / s , of the matter formed in central Au + Au interactions. We obtain an upper limit estimate of η / s that suggests that the produced medium has a small viscosity per unit entropy

Beam-Energy and System-Size Dependence of Dynamical Net Charge Fluctuations

We present measurements of net charge fluctuations in Au+Au collisions at √sNN=19.6, 62.4, 130, and 200 GeV, Cu+Cu collisions at √sNN=62.4 and 200 GeV, and p+p collisions at √s=200GeV using the dynamical net charge fluctuations measure ν+−,dyn. We observe that the dynamical fluctuations are nonzero at all energies and exhibit a modest dependence on beam energy. A weak system size dependence is also observed. We examine the collision centrality dependence of the net charge fluctuations and find that dynamical net charge fluctuations violate 1/Nch scaling but display approximate 1/Npart scaling. We also study the azimuthal and rapidity dependence of the net charge correlation strength and observe strong dependence on the azimuthal angular range and pseudorapidity widths integrated to measure the correlation

Energy Dependence of \u3cem\u3eKπ\u3c/em\u3e, \u3cem\u3epπ\u3c/em\u3e, and \u3cem\u3eKp\u3c/em\u3e Fluctuations in Au + Au Collisions from √\u3cem\u3e\u3csup\u3es\u3c/sup\u3eNN\u3c/em\u3e=7.7 to 200 GeV

A search for the quantum chromodynamics (QCD) critical point was performed by the STAR experiment at the BNL Relativistic Heavy Ion Collider, using dynamical fluctuations of unlike particle pairs. Heavy ion collisions were studied over a large range of collision energies with homogeneous acceptance and excellent particle identification, covering a significant range in the QCD phase diagram where a critical point may be located. Dynamical Kπ, pπ, and Kp fluctuations as measured by the STAR experiment in central 0–5% Au + Au collisions from center-of-mass collision energies √sNN=7.7 to 200 GeV are presented. The observable νdyn was used to quantify the magnitude of the dynamical fluctuations in event-by-event measurements of the Kπ, pπ, and Kp pairs. The energy dependences of these fluctuations from central 0–5% Au + Au collisions all demonstrate a smooth evolution with collision energy

Parton Energy Loss in Heavy-Ion Collisions Via Direct-Photon and Charged-Particle Azimuthal Correlations

Charged-particle spectra associated with direct photon Ydir) and π0 are measured in p+p and Au+Au collisions at center-of-mass energy √sNN=200 GeV with the STAR detector at the Relativistic Heavy Ion Collider. A shower-shape analysis is used to partially discriminate between Ydirand π0. Assuming no associated charged particles in the γdir direction (near side) and small contribution from fragmentation photons (Yfrag), the associated charged-particle yields opposite to Ydir(away side) are extracted. In central Au+Au collisions, the charged-particle yields at midrapidity (|η|\u3c1) and high transverse momentum (3 \u3c PTassoc \u3c 16 GeV/c) associated with γdir and π0 (|η|\u3c0.9, 8 \u3c PTtrig \u3c16 GeV/c) are suppressed by a factor of 3–5 compared with p+p collisions. The observed suppression of the associated charged particles is similar for Ydir and π0 and independent of the γdirenergy within uncertainties. These measurements indicate that, in the kinematic range covered and within our current experimental uncertainties, the parton energy loss shows no sensitivity to the parton initial energy, path length, or color charge

Measurements of Dielectron Production in Au + Au Collisions at √\u3cem\u3es\u3csub\u3eNN\u3c/sub\u3e\u3c/em\u3e = 200 GeV from the STAR Experiment

We report on measurements of dielectron (e+e−) production in Au + Au collisions at a center-of-mass energy of 200 GeV per nucleon-nucleon pair using the STAR detector at BNL Relativistic Heavy Ion Collider. Systematic measurements of the dielectron yield as a function of transverse momentum (pT) and collision centrality show an enhancement compared to a cocktail simulation of hadronic sources in the low invariant-mass region (Mee \u3c 1 GeV/c2). This enhancement cannot be reproduced by the ρ-meson vacuum spectral function. In minimumbias collisions, in the invariant-mass range of 0.30–0.76 GeV/c2, integrated over the full pT acceptance, the enhancement factor is 1.76 ± 0.06 (stat.) ± 0.26 (sys.) ± 0.29 (cocktail). The enhancement factor exhibits weak centrality and pT dependence in STAR’s accessible kinematic regions,while the excess yield in this invariant-mass region as a function of the number of participating nucleons follows a power-law shape with a power of 1.44 ± 0.10. Models that assume an in-medium broadening of the ρ-meson spectral function consistently describe the observed excess in these measurements. Additionally, we report on measurements of ω- and φ-meson production through their e+e− decay channel. These measurements show good agreement with Tsallis blast-wave model predictions, as well as, in the case of the φ meson, results through its K+K− decay channel. In the intermediate invariant-mass region (1.1\u3cMee \u3c 3 GeV/c2), we investigate the spectral shapes from different collision centralities. Physics implications for possible in-medium modification of charmed hadron production and other physics sources are discussed

Observation of Transverse Spin-Dependent Azimuthal Correlations of Charged Pion Pairs in \u3cem\u3ep\u3c/em\u3e\u3csup\u3e↑\u3c/sup\u3e+\u3cem\u3ep\u3c/em\u3e at √\u3cem\u3es\u3c/em\u3e=200 GeV

We report the observation of transverse polarization-dependent azimuthal correlations in charged pion pair production with the STAR experiment in p↑+p collisions at RHIC. These correlations directly probe quark transversity distributions. We measure signals in excess of 5 standard deviations at high transverse momenta, at high pseudorapidities η\u3e0.5, and for pair masses around the mass of the ρ meson. This is the first direct transversity measurement in p+p collisions

Observation of Charge Asymmetry Dependence of Pion Elliptic Flow and the Possible Chiral Magnetic Wave in Heavy-Ion Collisions

We present measurements of π− and π+ elliptic flow, v2, at midrapidity in Au+Au collisions at √SNN=200, 62.4, 39, 27, 19.6, 11.5, and 7.7 GeV, as a function of event-by-event charge asymmetry, Ach, based on data from the STAR experiment at RHIC. We find that π− (π+) elliptic flow linearly increases (decreases) with charge asymmetry for most centrality bins at √SNN=27  GeV and higher. At √SNN=200  GeV, the slope of the difference of v2 between π− and π+ as a function of Ach exhibits a centrality dependence, which is qualitatively similar to calculations that incorporate a chiral magnetic wave effect. Similar centrality dependence is also observed at lower energies

Azimuthal Anisotropy in U+U and Au+Au Collisions at RHIC

Collisions between prolate uranium nuclei are used to study how particle production and azimuthal anisotropies depend on initial geometry in heavy-ion collisions. We report the two- and four-particle cumulants, v2{2} and v2{4}, for charged hadrons from U+U collisions at √sNN=193  GeV and Au+Au collisions at √sNN=200  GeV. Nearly fully overlapping collisions are selected based on the energy deposited by spectators in zero degree calorimeters (ZDCs). Within this sample, the observed dependence of v2{2} on multiplicity demonstrates that ZDC information combined with multiplicity can preferentially select different overlap configurations in U+U collisions. We also show that v2 vs multiplicity can be better described by models, such as gluon saturation or quark participant models, that eliminate the dependence of the multiplicity on the number of binary nucleon-nucleon collisions