QCD equation of state in a virial expansion

We describe recent three-flavor QCD lattice data for the pressure, speed of soun d and interaction measure at nonzero temperature and vanishing chemical potentia l within a virial expansion. For the deconfined phase we use a phenomenological model which includes non-pert urbative effects from dimension two gluon condensates that reproduce the free en ergy of quenched QCD very well. The hadronic phase is parameterized by a generalized resonance-gas model. Furthermore, we extend this approach to finite quark densities introducing an ex plicit $\mu$-dependence of the interaction. We calculate pressure, quark-number density, entropy and energy density and compare to results of lattice calculatio ns. We, additionally, investigate the structure of the phase diagram by calculating the isobaric and isentropic lines as well as the critical endpoint in the ($T, \mu_q$)-plane.Comment: 9 pages, 11 figures. Submitted to Phys. Rev.