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Particles and Fields

Eur. Phys. J. C 21, 545-555 (2001)
DOI: 10.1007/s100520100728

The strange border of the QCD phases

S. Kabana

Laboratory for High Energy Physics, University of Bern, 3012 Bern, Switzerland

(Received: 17 April 2001 / Published online: 24 August 2001 -© Springer-Verlag / Società Italiana di Fisica 2001 )

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 particle and nuclear collisions is found to increase in collisions with an initially reached energy density ( $\epsilon_i$) up to $\epsilon_{\mathrm {{crit}}} \sim
1$ GeV/fm3. Above this value it decreases approximately linearly and reaches its asymptotic value at zero baryon chemical potential ( $\mu_{\mathrm {B}}$). We demonstrate that $\lambda_s$ in nuclear collisions approaches its asymptotic value at $\epsilon_i \sim 8$-9 GeV/fm3, corresponding to $s^{1/2} \sim 3$-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 s1/2, and the increase of $\lambda_s$ in p+A over p + p collisions at the same s1/2, are naturally explained. We study the approach of thermodynamic observables at $\mu_{\mathrm {B}}=0$ to the transition point and extract an estimate of the critical temperature.

© Società Italiana di Fisica, Springer-Verlag 2001