The strange border of the QCD phases

We address the flavour composition along the border between the hadronic and the quarkgluon plasma phases of QCD. The ratio of strange to up and down antiquarks (λ^s) produced in particle and nuclear collisions is found to increase in collisions with an initially reached energy density (∈_i) up to ∈_crit ∼ 1 GeV/fm³ . Above this value it decreases approximately linearly and reaches its asymptotic value at zero baryon chemical potential (μ_B). We demonstrate that λ_s in nuclear collisions approaches its asymptotic value at ∈_i ∼ 8-9GeV/fm³, corresponding to s^1/2 ∼ 3-8TeV per nucleon + nucleon pair, which will be reached at the LHC. After correcting for the difference in the chemical potentials of various colliding systems, λ_s universally saturates across the QCD phase boundary, following the temperature. Recent experimental puzzles as the increase in the K⁺/π⁺ ratio in Pb + Pb collisions at 40 GeV per nucleon, its different behaviour at midrapidity, the decrease of the double ratio of K/π(A + A/p + p) in nucleus-nucleus over p+p collisions with increasing s^1/2, and the increase of λ_s in p + A over p+p collisions at the same s_1/2, are naturally explained. We study the approach of thermodynamic observables at μ_B = 0 to the transition point and extract an estimate of the critical temperature.