Systematic Measurements of Identified Particle Spectra in pp, d+Au and Au+Au Collisions from STAR

Abstract

Identified charged particle spectra of π ± , K ± , p and p at mid-rapidity (|y| < 0.1) measured by the dE/dx method in the STAR-TPC are reported for pp and d+Au collisions at √ s N N = 200 GeV and for Au+Au collisions at 62.4 GeV, 130 GeV, and 200 GeV. Average transverse momenta, total particle production, particle yield ratios, strangeness and baryon production rates are investigated as a function of the collision system and centrality. The transverse momentum spectra are found to be flatter for heavy particles than for light particles in all collision systems; the effect is more prominent for more central collisions. The extracted average transverse momentum of each particle species follows a trend determined by the total charged particle multiplicity density. The Bjorken energy density estimate is at least several GeV/fm 3 for a formation time less than 1 fm/c. A significantly larger net-baryon density and a stronger increase of the net-baryon density with centrality are found in Au+Au collisions at 62.4 GeV than at the two higher energies. Antibaryon production relative to total particle multiplicity is found to be constant over centrality, but increases with the collision energy. Strangeness production relative to total particle multiplicity is similar at the three measured RHIC energies. Relative strangeness production increases quickly with centrality in peripheral Au+Au collisions, to a value about 50% above the pp value, and remains rather constant in more central collisions. Bulk freeze-out properties are extracted from thermal equilibrium model and hydrodynamics-motivated blast-wave model fits to the data. Resonance decays are found to have little effect on the extracted kinetic freeze-out parameters due to the transverse momentum range of our measurements. The extracted chemical freeze-out temperature is constant, independent of collision system or centrality; its value is close to the predicted phase-transition temperature, suggesting that chemical freeze-out happens in the vicinity of hadronization and the chemical freezeout temperature is universal despite the vastly different initial conditions in the collision systems.The extracted kinetic freeze-out temperature, while similar to the chemical freeze-out temperature in pp, d+Au, and peripheral Au+Au collisions, drops significantly with centrality in Au+Au collisions, whereas the extracted transverse radial flow velocity increases rapidly with centrality. There appears to be a prolonged period of particle elastic scatterings from chemical to kinetic freeze-out in central Au+Au collisions. The bulk properties extracted at chemical and kinetic freeze-out are observed to evolve smoothly over the measured energy range, collision systems, and collision centralities.

Hierarchy of Azimuthal Anisotropy Harmonics in Collisions of Small Systems from the Color Glass Condensate

“…Figure 2 shows that the multiplicity distribution as a function of the number of charged hadrons N ch for the rapidity window η < 0.5 in d+Au collisions published by the STAR Collaboration at RHIC [43] is well reproduced in the dilute-dense CGC EFT. The details of the numerical computation on two-dimensional lattices are identical to those articulated previously [17,27,44,45].…”

“…2. The multiplicity distribution of produced particles computed in the dilute-dense CGC framework compared to STAR d+Au data [43].…”

K/pi Fluctuations at Relativistic Energies

“…Nine centrality bins were used corresponding to 0-5% (most central), 5-10%, 10-20%, 20-30%, 30-40%, 40-50%, 50-60%, 60-70%, and 70-80% (most peripheral) of the reaction cross section. To be able to plot our results versus dN/dη, we associate fully corrected values for dN/dη from previously published work with each collision centrality bin [13]. For the 19.6 GeV and 130 GeV data sets, only results from the most central bin are presented.…”

Highlights from STAR: probing the early medium in heavy ion collisions

“…Following Ref. [31], if we consider the RHIC medium to be a classical ultrarelativistic gas of massless particles, then η/s = 0.316 T Rσc s S dN/dy , where the temperature T is obtained by fitting STAR's pion m T slope [32], σ is the isotropic differential cross section of the massless particles, and c s is the speed of sound. The product σc s is determined by fitting v 2 /ǫ with [31].…”