Chemical equilibration in relativistic heavy ion collisions

Abstract

In the hadronic sector of relativistic heavy ion physics, the ρ ⇆ 2 π reaction is the strongest one, strong enough to equilibrate the ρ with the pions throughout the region from chemical freezeout to thermal freezeout when free-particle interactions (with no medium-dependent effects) are employed. Above the chiral restoration temperature, only ρ's and π's are present, in that the chirally restored A 1 is equivalent to the ρ and the mesons have an SU(4) symmetry, with no dependence on isospin and negligible dependence on spin. In the same sense the σ and π are “equivalent” scalars. Thus the chirally restored ρ ⇆ 2 π exhaust the interspecies transitions. We evaluate this reaction at T c and find it to be much larger than below T c , certainly strong enough to equilibrate the chirally restored mesons just above T c . When emitted just below T c the mesons remain in the T c + ε freezeout distribution, at least in the chiral limit because of the Harada–Yamawaki “vector manifestation” that requires that mesonic coupling constants go to zero (in the chiral limit) as T goes to T c from below. Our estimates in the chiral limit give deviations in some particle ratios from the standard scenario (of equilibrium in the hadronic sector just below T c ) of about double those indicated experimentally. This may be due to the neglect of explicit chiral symmetry breaking in our estimates. We also show that the instanton molecules present above T c are the giant multipole vibrations found by Asakawa, Hatsuda and Nakahara and of Wetzorke et al. in lattice gauge calculations. Thus, the matter formed by RHIC can equivalently be called: chirally restored mesons, instanton molecules, or giant collective vibrations. It is a strongly interacting liquid.