A new interpretation of the QCD phase transition and of strangeness as QGP signature

Abstract

We address the question of how to identify the QCD phase transition using measured light (u,d,s-structured) hadrons, without invoking comparison to the QCD $ε_c$ predictions, and extract $ε_c$ from the data. We analyse several particle and nuclear collisions and extract their chemical freeze-out temperature $T$ at zero baryochemical potential ($μ_B$). We find at $μ_B=0$ a universal rise and saturation of both the $T$ and of the strangeness suppression factor $λ_s$ (=$\frac {2\bar{s}} {\bar{u} + \bar{d}} $) with increasing initial energy density ($ε_i$). The onset of saturation of both $T$ and $λ_s$, is interpreted as due to the event of the QCD phase transition. The critical energy density is estimated to be $ε_c$ $\sim$ 1 +0.3 -0.5 GeV/fm$^3$, corresponding approximately to a $\sqrt{s}$ of $\sim$ 8.8 GeV for central Pb+Pb collisions. Concerning the role of strangeness, we identify trivial and non-trivial sources of strangeness enhancement: The peak of $λ_s$ in Pb+Pb collisions at $\sqrt{s}$=8.8 GeV and other phenomena of 'strangeness enhancement' defined with respect to p+p data, are trivially traced back to the different baryochemical potentials and $ε_i$ of the compared systems. A non trivial redefined '$λ_s$ enhancement' is however also present. The netbaryonfree $λ_s$ limit is estimated to be approximately reached in Au+Au collisions at the LHC.