Evidence of the mixed phase formation in nucleus-nucleus collisions

Abstract

Searchers for various irregularities in the behavior of thermodynamic quantities at chemical freeze-out (CFO) are rather important in a view of experimental studies of quark-gluon plasma (QGP). Using the multicomponent hadron resonance gas model (HRGM), developed in (Sagun, 2014; Bugaev et al., 2015), we performed a high-quality fit of 111 hadronic ratios measured for 14 values of the center of mass collision energies between 2.7 GeV and 200 GeV with the overall fit quality $\chi^2/dof \simeq 0.95$. In addition to previously reported singularities (Bugaev et al., 2015) at CFO we found that the hadron yield ratios $\frac{\Lambda}{p}$, $\frac{K^{+}}{p}$, $\frac{K^{+}}{\Lambda}$, $\frac{\Omega^{-}}{p}$ and $\frac{\Xi^{-}}{p}$ measured in central nuclear collisions demonstrate a significant change of slope in the same range of center of mass collision energy $\sqrt{s_{NN}} = 4.3-4.9$ GeV (Bugaev et al., 2015). This change of slopes is accompanied by a dramatic increase of resonance decays at CFO. Also at CFO the trace anomaly and baryonic density demonstrate the pronounced peaks at the collision energy $\sqrt{s_{NN}} = 4.9 $ GeV. We argue that all these and previously found irregularities provide an evidence for the QGP formation in nuclear collisions at about $\sqrt{s_{NN}} = 4.9 $ GeV.