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 ${ϵ}_i$, and saturates above ${ϵ}_i$ $\sim$ 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 ${ϵ}_{crit}$ $\sim$ 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 ${ϵ}_i$ is following the temperature, rising and saturating universally above ${ϵ}_{crit}$. 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 ${ϵ}_{crit}$ $\sim$ 1 GeV/fm${}^3$.

Comments: 8 pages, 5 figures, XXXVIth Rencontres de Moriond on 'QCD and high energy hadronic interactions', 17-24 March 2001, Les Arcs 1800, France