How much strangeness production is there in ultrarelativistic nucleus-nucleus collisions?

Abstract

Strangeness production has been proposed as a signal for the formation of a quark-gluon plasma in ultrarelativistic nucleus-nucleus collisions. We address this problem in the framework of Bjorken’s model for the longitudinal hydrodynamic expansion of baryon-free matter. Different scenarios are considered for the assumed first-order phase transition from QCD plasma into hadrons, including a slow transition via the Maxwell construction of coexisting phases, and a rapid transition from supercooling followed by reheating. The rate equations for strange-quark and kaon abundances are solved numerically. With the most likely set of parameters, the $K^{\mathrm{-}}$/${\pi }^{\mathrm{-}}$ ratio is about three times larger than in p¯p collisions at the same energy. This results primarily from the large rate constant in the hadron phase and from the long time spent in this phase. Thus, the $K^{\mathrm{-}}$/${\pi }^{\mathrm{-}}$ ratio may not be a direct signal for the formation of quark-gluon plasma; however, it may be an indirect signal, since the system would not live nearly as long if the quark-gluon plasma had not been formed initially.