0 CERN - TH / 2000 - 037 BI - TP 2000 / 01 The Pressure in 2 , 2 + 1 and 3 Flavour QCD

We calculate the pressure in QCD with two and three light quarks on a lattice of size 16 3 × 4 using tree level improved gauge and fermion actions. We argue that for temperatures T > ∼ 2T c systematic effects due to the finite lattice cutoff and non-vanishing quark masses are below 15% in this calculation and give an estimate for the continuum extrapolated pressure in QCD with massless quarks. We find that the flavour dependence of the pressure is dominated by that of the Stefan-Boltzmann constant. Furthermore we perform a calculation of the pressure using 2 light (m u,d /T = 0.4) and one heavier quark (m s /T = 1). In this case the pressure is reduced relative to that of three flavour QCD. This effect is stronger than expected from the mass dependence of an ideal Fermi gas.