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 163×4 using tree level improved gauge and fermion actions. We argue that, for temperatures T≳2Tc, systematic 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-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 two light (mu,d/T=0.4) and one heavier quark (ms/T=1). In this case the pressure is reduced with respect to that of 3-flavour QCD. This effect is stronger than expected from the mass dependence of an ideal Fermi gas.