Thermodynamics of Four-Flavour QCD with Improved Staggered Fermions

We have calculated the pressure and energy density in four-flavour QCD using improved fermion and gauge actions. We observe a strong reduction of finite cut-off effects in the high temperature regime, similar to what has been noted before for the SU(3) gauge theory. Calculations have been performed on ${16}^3\times 4$ and 16^4 lattices for two values of the quark mass, $ma=0.05$ and 0.1. A calculation of the string tension at zero temperature yields a critical temperature $T_c\mathrm{/}\sqrt{\sigma }=0.407\pm 0.010$ for the smaller quark mass value.