Recent results from high temperature lattice QCD
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ecent Results from High Temperature Lattice QCD1 E. Laermann Fakultät für Physik, Universität Bielefeld, D33615 Bielefeld, Germany email: [email protected]bielefeld.de Abstract—Recent results obtained from numerical computations in lattice regularized QCD are summa rized. The writeup of the talk concentrates on the liberation of strange quarks in the vicinity of the chiral QCD transition and on certain ratios of cumulants of net electric charge fluctuations which can be used to determine freezeout parameters by a comparison of experimental data from heavy ion collisions with lattice QCD results. DOI: 10.1134/S1063779615050147 1
1. INTRODUCTION Fluctuations and correlations of conserved charges like baryon number B, electric charge Q or strangeness S are sensitive to the composition of hot and dense QCD matter [1, 2]. Measures of them in general take quite different values in a phase where the carriers of the quantum numbers are hadrons opposed to a phase where those are quarks. It is therefore of interest to compute fluctuation and correlation quantities from QCD to study the properties of stronginteraction matter at high temperature and density. Moreover, fluctuations measured in a heavy ion collision experiment may reflect thermal conditions at the time where the generated medium has expanded, cooled down and diluted sufficiently so that hadrons form again. Although it may be questioned whether the thermal medium at this time is in equilibrium and how well hadronization is localized in time, the suc cess of hadron resonance gas (HRG) model calcula tions, performed to describe properties of the medium at the time of freezeout [3], seems to suggest that the thermal conditions are well characterized by a freeze out temperature Tf and a baryon chemical potential f
f
μ B . The values Tf and μ B at freezeout are usually determined by comparing experimental data on parti cle yields with a HRG model calculation [3, 4]. How ever, it clearly is desirable to extract the freezeout parameters also by comparing experimental data directly with a QCD calculation. In the following writeup of the talk I will concen trate on strangeness liberation and on determining freezeout parameters with the help of lattice QCD. Both topics rely on our measurements of fluctuation and correlation variables performed for two degener ate light quarks and a strange quark. The quark mass 1 The article is published in the original.
values have been tuned such that the Kaon mass acquires its physical value and that the (Goldstone) pion is of mass 160 MeV. The simulations are based on the socalled hisq action [5] for the quarks, a highly improved discretization with small discretization errors (taste violations). At aspect ratios Nσ/Nτ ≥ 4, the 3
temporal extents of the N σ × Nτ lattices have been chosen as Nτ = 6, 8 and 12 to address finite lattice spacing effects ~(NτT)–1 with T being the temperature. 2. STRANGENESS LIBERATION The basic quantities calculated on the lattice arise from a Taylor expansion of the pressure, or equiva
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