A (re)interpretation of the QCD phase transition and of strangeness as QGP signature

Abstract

The temperature at the chemical freeze-out and at zero baryochemical potential has been extracted in a global analysis of $e^+e^-$, $p+p$, $p+ \overline{p}$ and $A+A$ collisions at $\sqrt{s}$=2-1800 GeV per N+N pair. We demonstrate that the temperature at $\mu_B$=0, rises with the initial energy density $\epsilon_i$, and saturates above $\epsilon_i$ $\sim$ 1 GeV/fm$^3$. This behaviour is interpreted as mapping out the QCD phase transition universally in particle and nuclear collisions. The critical energy density is therefore identified to be $\epsilon_{crit}$ $\sim$ 1 $\pm$ 0.3 GeV/fm$^3$. We show that strange particles at $\mu_B$=0, are not significantly enhanced in A+A collisions as compared to $p+ \overline{p}$. The so called 'strangeness suppression factor' ($\lambda_s = \frac{(2 \overline{s})} {(\overline{u} + \overline{d})}$) as a function of $\epsilon_i$ is following the temperature, rising and saturating universally above $\epsilon_{crit}$. This leads to a reinterpretation of strangeness enhancement as QGP signature. Within this interpretation the experimental puzzles with respect to strangeness production can be naturally explained: e.g. the recent measured maximum of $K^+/\pi^+$ in Pb+Pb collisions at 40 A GeV, is explained as due to $\mu_B$. We discuss under which conditions 'strangeness enhancement' and '$J/\Psi$ suppresion' both set in at $\epsilon_{crit}$ $\sim$ 1 GeV/fm$^3$.