QCD equation of state with 2+1 flavors of improved staggered quarks

We report results for the interaction measure, pressure, and energy density for nonzero-temperature QCD with 2+1 flavors of improved staggered quarks. In our simulations, we use a Symanzik improved gauge action and the Asqtad O(a(2)) improved staggered quark action for lattices with temporal extent N-t=4 and 6. The heavy quark mass m(s) is fixed at approximately the physical strange quark mass and the two degenerate light quarks have masses m(ud)approximate to 0.1m(s) or 0.2m(s). The calculation of the thermodynamic observables employs the integral method where energy density and pressure are obtained by integration over the interaction measure.