Strangeness and pion production as signals of QCD phase transition

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Abstract
A systematic analysis of data on strangeness and pion production in nucleon - nucleon and central nucleus - nucleus collisions is presented. It is shown that at all collision energies the pion/baryon and strangeness/pion ratios indicate saturation with the size of the colliding nuclei. The energy dependence of the saturation level suggests that the transition to the quark - gluon plasma occurs between 15 A GeV c−1 (BNL AGS) and 160 A GeV c−1 (CERN SPS) collision energies. The experimental results interpreted in the framework of a statistical approach show that the effective number of degrees of freedom increases in the course of the phase transition and that the plasma created at CERN SPS energies may have a temperature of about 280 MeV (energy density ≈ 10 GeV fm−3). The presence of the phase transition can lead to the non-monotonic collision energy dependence of the strangeness/pion ratio. After an initial increase the ratio should drop to the characteristic value for the quark - gluon plasma. Above the transition region the ratio is expected to be collision energy independent. Experimental studies of central Pb+Pb collisions in the energy range 20 - 160 A GeV c−1 are urgently needed in order to localize the threshold energy, and study the properties of the QCD phase transition.