We present results of a simulation of two flavor QCD on a 16 3 ×4 lattice using p4-improved staggered fermions with bare quark mass m/T = 0.4. Derivatives of the thermodynamic grand canonical partition function Z(V, T, µ u , µ d ) with respect to chemical potentials µ u,d for different quark flavors are calculated up to sixth order, enabling estimates of the pressure and the quark number density as well as the chiral condensate and various susceptibilities as functions of µ q = (µ u + µ d )/2 via Taylor series expansion. Furthermore, we analyze baryon as well as isospin fluctuations and discuss the relation between the radius of convergence of the Taylor series and the chiral critical point in the QCD phase diagram. We argue that bulk thermodynamic observables do not, at present, provide direct evidence for the existence of a chiral critical point in the QCD phase diagram. Results are compared to high temperature perturbation theory as well as a hadron resonance gas model.

We propose a new method to investigate the thermal properties of QCD with a small quark chemical potential . Derivatives of quark and gluonic observables with respect to are computed at ϭ0 for two flavors of p4 improved staggered fermions with maϭ0.1,0.2 on a 16 3 ϫ4 lattice, and used to calculate the leading order Taylor expansion in of the location of the pseudocritical point about ϭ0. This expansion should be well behaved for the small values of q /T c ϳ0.1 relevant for BNL RHIC phenomenology, and predicts a critical curve T c () in reasonable agreement with estimates obtained using exact reweighting. In addition, we contrast the case of isoscalar and isovector chemical potentials, quantify the effect of 0 on the equation of state, and comment on the complex phase of the fermion determinant in QCD with 0.