The pressure in 2, 2+1 and 3 flavour QCD

We calculate the pressure in QCD with two and three light quarks on a latti ce of size 16(3) X 4 using tree level improved gauge and fermion actions. W e argue that, for temperatures T greater than or similar to 2T(c), systemat ic effects due to the finite lattice cut-off and non-vanishing quark masses are below 15% in this calculation and give an estimate for the continuum-e xtrapolated pressure in QCD with massless quarks. We find that the flavour dependence of the pressure is dominated by that of the Stefan-Boltzmann con stant. Furthermore we perform a calculation of the pressure using two light (m(u,d)/T = 0.4) and one heavier quark (m(s)/T = 1). In this case the pres sure is reduced with respect to that of 3-flavour QCD. This effect is stron ger than expected from the mass dependence of an ideal Fermi gas. (C) 2000 Elsevier Science B.V. All rights reserved.