The strange border of the QCD phases

Abstract

We address the flavour composition along the border between the hadronic and the quark-gluon plasma phases of QCD. The ratio of strange to up and down antiquarks ($\lambda_s$) produced in partic le and nuclear collisions, is found to increase in collisions with initially reached energy density ($\epsilon_i$) up to $\epsilon_{crit}$ $\sim$ 1 GeV/$fm^3$. Above this value it decreases approximately linearly and reaches its asymptotic value at zero baryon chemical potential ($\mu_B$). We demonstrate that $\lambda_s$ in nuclear collisions is approaching its asymptotic value at $\epsilon_i$ $\sim$ 8-9 GeV/$fm^3$, corresponding to $\sqrt{s}$ $\sim$ 3-8 TeV per nucleon+nucleon pair which will be reached at the LHC. After correcting for the difference in the chemical potentials of various colliding systems, $\lambda_s$ universally saturates across the QCD phase boundary, following the temperature. Recent experimental puzzles as the increase in the $K/\pi$ ratio in Pb+Pb collisions at 40 GeV per nucleon, its different behaviour at midrapidity, the decrease of the double ratio of $K/\pi$(A+A/p+p) in nucleus nucleus over p+p collisions with increasing $\sqrt{s}$, and the increase of $\lambda_s$ in p+A over p+p collisions at the same $\sqrt{s}$, are naturally explained. We study the approach of thermodynamic observables at $\mu_B=0$ to the transition point and extract an estimate of the critical temperature.