Thermal equilibration and expansion in nucleus-nucleus collisions at the AGS

Abstract

The rather complete data set of hadron yields from central Si + A collisions at the Brookhaven AGS is used to test whether the system at freeze-out is in thermal and hadro-chemical equilibrium. Rapidity and transverse momentum distributions are discussed with regards to the information they provide on hydrodynamic flow.The goal of the ultra-relativistic heavy-ion program at the BNL AGS and CERN SPS is to study highly excited and dense nuclear matter and possibly the transition from hot and dense hadronic matter to deconfined quark-matter with restored chiral symmetry. While future collider experiments will probe a hot quark-gluon plasma with low net baryon density, present fixed target experiments create matter, possibly quark-matter, at very high baryon density and moderate temperature.The present paper is following up on an earlier suggestion by some of us [1], based on the first AGS and SPS data, that a high degree of thermalization is reached and that there is evidence for hydrodynamic expansion of the created fireball. We now use the much larger set of data from central Si + A collisions at the AGS first to discuss quantitatively the 1

Generalized classical and quantum dynamics within a nonextensive approach

“…Such hypothesis is well confirmed by the experimental data and adopted in many theoretical works [13,14,15]. A crucial rôle in the solution of the above NLFPE plays the choice of the diffusion and the drift coefficients.…”

“…Rapidity spectra calculated from (58) will ultimately depend on two parameters: the "interaction" time τ = γt and the non-extensive parameter q. Therefore, no more free parameters are used in this analysis respect to previous theoretical studies [14,15]. The obtained spectra are normalized to 164 protons and the beam rapidity is fixed to y cm = 2.9 (in the c.m.…”

Melting hadrons, boiling quarks

“…The SHARE LHC freeze-out temperature is clearly below the lattice critical temperature range. The results of other groups are obtained with simplified parameter sets: marked GSI [80,81], Florence [82][83][84], THERMUS [85], STAR [86] and AL-ICE [87,88]. These results show the chemical freeze-out temperature T in general well above the lattice T H .…”

“…This complete set is necessary since the observed particle set includes particles arising from [71,72], the SHM-SHARE results (full circles) [7,[73][74][75][76][77][78][79] and results of other groups [80][81][82][83][84][85][86][87][88]. Figure from ref.…”

Relativistic Nuclear Collisions and the QCD Phase Boundary

“…Turning to more detailed information on hadron production we note that hadron yields observed in central nuclear collisions at AGS, SPS, and RHIC energies are found to be described with high precision within a hadro-chemical equilibrium approach [15,16,17,18,19,20,21,22,23], governed by a chemical freeze-out temperature T ch , baryo-chemical potential µ and the fireball volume V ch . A recent review can be found in [24].…”