The Lattice QCD Equation of State and Implications for Hydrodynamic Modeling of Heavy Ion Collisions

We present results for the QCD Equation of State of at zero baryon density calculated on a lattice of dimensions $32^3 \times 8$ with $m_l = 0.1 m_s$ using two improved staggered fermion actions, p4 and asqtad. Calculations were performed along lines of constant physics with a pion mass of approximately 200~MeV, and were carried out using more than 100M cpu-hours on BG/L supercomputers at LLNL, BNL, and UCSD. Both calculations are consistent with a cross-over transition in the range 185 -- 195~MeV/c. Recent results from the lattice will be compared to those currently used as input to hydrodynamic models. Consequences for calculations of observables such as spectra, flow, and space-time measurements in heavy ion collisions will be discussed.