Balance functions from Au+Au, d+Au, and p+p collisions at root s(NN)=200 GeV

Balance functions have been measured for charged-particle pairs, identified charged-pion pairs, and identified charged-kaon pairs in Au + Au, d + Au, and p + p collisions at root s(NN) = 200 GeV at the Relativistic Heavy Ion Collider using the STAR detector. These balance functions are presented in terms of relative pseudorapidity, Delta eta, relative rapidity, Delta y, relative azimuthal angle, Delta phi, and invariant relative momentum, q(inv). For charged-particle pairs, the width of the balance function in terms of Delta eta scales smoothly with the number of participating nucleons, while HIJING and UrQMD model calculations show no dependence on centrality or system size. For charged-particle and charged-pion pairs, the balance functions widths in terms of Delta eta and Delta y are narrower in central Au + Au collisions than in peripheral collisions. The width for central collisions is consistent with thermal blast-wave models where the balancing charges are highly correlated in coordinate space at breakup. This strong correlation might be explained by either delayed hadronization or limited diffusion during the reaction. Furthermore, the narrowing trend is consistent with the lower kinetic temperatures inherent to more central collisions. In contrast, the width of the balance function for charged-kaon pairs in terms of Delta y shows little centrality dependence, which may signal a different production mechanism for kaons. The widths of the balance functions for charged pions and kaons in terms of q(inv) narrow in central collisions compared to peripheral collisions, which may be driven by the change in the kinetic temperature.

Azimuthal di-hadron correlations in d plus Au and Au plus Au collisions at root s(NN)=200 GeV measured at the STAR detector

Yields, correlation shapes, and mean transverse momenta p(T) of charged particles associated with intermediate-to high-p(T) trigger particles (2.5 < p(T) < 10 GeV/c) in d + Au and Au + Au collisions at root s(NN) = 200 GeV are presented. For associated particles at higher p(T) greater than or similar to 2.5 GeV/c, narrow correlation peaks are seen in d + Au and Au + Au, indicating that the main production mechanism is jet fragmentation. At lower associated particle pT < 2 GeV/c, a large enhancement of the near- (Delta phi similar to 0) and away-side (Delta phi similar to pi) associated yields is found, together with a strong broadening of the away-side azimuthal distributions in Au + Au collisions compared to d + Au measurements, suggesting that other particle production mechanisms play a role. This is further supported by the observed significant softening of the away-side associated particle yield distribution at Delta phi similar to pi in central Au + Au collisions.

Inclusive pi(0), eta, and direct photon production at high transverse momentum in p plus p and d plus Au collisions at root s(NN)=200 GeV

We report a measurement of high-p(T) inclusive pi(0), eta, and direct photon production in p + p and d + Au collisions at root s(NN) = 200 GeV at midrapidity (0 < eta < 1). Photons from the decay pi(0) -> gamma gamma were detected in the barrel electromagnetic calorimeter of the STAR experiment at the Relativistic Heavy Ion Collider. The eta -> gamma gamma decay was also observed and constituted the first eta measurement by STAR. The first direct photon cross-section measurement by STAR is also presented; the signal was extracted statistically by subtracting the pi(0), eta, and omega(782) decay background from the inclusive photon distribution observed in the calorimeter. The analysis is described in detail, and the results are found to be in good agreement with earlier measurements and with next-to-leading-order perturbative QCD calculations.

System size dependence of associated yields in hadron-triggered jets STAR Collaboration

We present results on the system size dependence of high transverse momentum di-hadron correlations at root s(NN) = 200 GeV as measured by STAR at RHIC. Measurements in d + Au, Cu + Cu and Au + Au collisions reveal similar jet-like near-side correlation yields (correlations at small angular separation Delta phi similar to 0, Delta eta similar to 0) for all systems and centralities. Previous measurements have shown Chat the away-side (Delta phi similar to pi) yield is suppressed in heavy-ion collisions. We present measurements of the away-side Suppression as a function of transverse momentum and centrality in Cu + Cu and Au + Au collisions. The suppression is found to be similar in Cu + Cu and An + An collisions at a similar number of participants. The results are compared to theoretical calculations based on the patron quenching model and the modified fragmentation model. The observed differences between data and theory indicate that the correlated yields presented here will further constrain dynamic energy loss models and provide information about the dynamic density profile in heavy-ion collisions. (C) 2009 Elsevier B.V. All rights reserved.

Three-Particle Coincidence of the Long Range Pseudorapidity Correlation in High Energy Nucleus-Nucleus Collisions

We report the first three-particle coincidence measurement in pseudorapidity (Delta eta) between a high transverse momentum (p(perpendicular to)) trigger particle and two lower p(perpendicular to) associated particles within azimuth |Delta phi| < 0.7 in root s(NN) = 200 GeV d + Au and Au + Au collisions. Charge ordering properties are exploited to separate the jetlike component and the ridge (long range Delta eta correlation). The results indicate that the correlation of ridge particles are uniform not only with respect to the trigger particle but also between themselves event by event in our measured Delta eta. In addition, the production of the ridge appears to be uncorrelated to the presence of the narrow jetlike component.

Spectra of identified high-p(T) pi(+/-) and p((p)over-bar ) in Cu + Cu collisions at root s(NN)=200 GeV

We report new results on identified (anti) proton and charged pion spectra at large transverse momenta (3 < p(T) < 10 GeV/c) from Cu + Cu collisions at root s(NN) = 200 GeV using the STAR detector at the Relativistic Heavy Ion Collider (RHIC). This study explores the system size dependence of two novel features observed at RHIC with heavy ions: the hadron suppression at high-p(T) and the anomalous baryon to meson enhancement at intermediate transverse momenta. Both phenomena could be attributed to the creation of a new form of QCD matter. The results presented here bridge the system size gap between the available pp and Au + Au data, and allow for a detailed exploration of the onset of the novel features. Comparative analysis of all available 200 GeV data indicates that the system size is a major factor determining both the magnitude of the hadron spectra suppression at large transverse momenta and the relative baryon to meson enhancement.