QCD thermal phase transition in the presence of a small chemical potential

We propose a new method to investigate the thermal properties of QCD with a small quark chemical potential μ. Derivatives of quark and gluonic observables with respect to μ are computed at μ=0 for two flavors of p4 improved staggered fermions with ma=0.1,0.2 on a 16³×4 lattice, and used to calculate the leading order Taylor expansion in μ of the location of the pseudocritical point about μ=0. This expansion should be well behaved for the small values of μ_q/T_c∼0.1 relevant for BNL RHIC phenomenology, and predicts a critical curve T_c(μ) 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 μ≠0 on the equation of state, and comment on the complex phase of the fermion determinant in QCD with μ≠0.