Systematics of kinetic freeze-out properties in high energy collisions from STAR

Abstract

The main aim of the RHIC Beam Energy Scan (BES) program is to explore the QCD phase diagram which includes search for a possible QCD critical point and the phase boundary between QGP and hadronic phase. We report the collision energy and centrality dependence of kinetic freeze-out properties from the measured mid-rapidity (|y| < 0.1) light hadrons (pions, kaons, protons and their anti-particles) for Au+Au collisions at the center-of-mass energy √ s NN = 7.7, 11.5, 19.6, 27, and 39 GeV. The STAR detector, with a large uniform acceptance and excellent particle identification is used in the data collection and analysis. The kinetic freeze-out temperature T kin and average collective velocity β parameters are extracted from blast-wave fits to the identified hadron spectra and systematically compared with the results from other collision energies including those at AGS, SPS and LHC. It is found that all results fall into an anti-correlation band in the 2-dimension (T kin , β ) distribution: the largest value of collective velocity and lowest temperature is reached in the most central collisions at the highest collision energy. The energy dependence of these freeze-out parameters are discussed.

Recent STAR heavy-ion results

“…The spectra are measured using TPC+TOF up to 4 GeV/c for strange baryons and less than 2 GeV/c for π, K, p,p. The similar spectra for identified particles at other BES energies were presented in [25][26][27]. Multi-strange hadrons are important probes for the search of the phase boundaries of QGP (see [28] and references therein).…”

“…The kinetic freeze-out temperature and average collective velocity parameters are extracted from blast-wave fits to the identified hadron spectra. STAR spectra of particles π, K, p, Λ, Ξ and their anti-particles were used to determine the different particle ratios and to obtain the phase diagram in {T ch , μ B } and {T kin , < β >} planes [24][25][26][27]. Figure 6(a) demonstrates the dependence of T ch and μ B on centrality (number of participants N part ) at different energies √ s NN = 7.7, 11.5, 14.5, 19.6, 27, and 39 GeV.…”

Exploring the QCD Phase Structure with Beam Energy Scan in Heavy-ion Collisions

“…The chemical freeze-out temperature (T ch ) is obtained by fitting the particle yield or particle ratios with thermal model [11], whereas the kinetic freeze-out temperature (T kin ) is obtained from the blast-wave model fitting of the particle p T spectra [12]. Figure 2 left shows the energy dependence of the chemical and kinetic freeze-out temperature T ch and T kin for central heavy-ion collisions from different experiments [13]. The chemical freeze-out temperature monotonically increases with √ s NN and then saturates at about √ s NN =10 GeV, with a limiting temperature T limt ∼160 MeV.…”

Dynamical evolution of critical fluctuations and its observation in heavy ion collisions

“…In our calculation we set the initial temperature T 0 = 220 MeV at proper time τ 0 = 1.0 fm, the critical temperature of the QCD critical point T c = 160 MeV, and the kinetic freeze-out temperature T f = 100 MeV [46], where we stop the evolution. The parameters for the critical region in Eq.…”