Using Taylor expansions of the pressure obtained previously in studies of 2-flavor QCD at nonzero chemical potential we calculate expansion coefficients for the energy and entropy densities up to $\mathcal{O}({\mu }_q^6)$ in the quark chemical potential. We use these series in ${\mu }_q/T$ to determine lines of constant entropy per baryon number ($S/N_B$) that characterize the expansion of dense matter created in heavy ion collisions. In the high temperature regime these lines are found to be well approximated by lines of constant ${\mu }_q/T$. In the low temperature phase, however, the quark chemical potential is found to increase with decreasing temperature. This is in accordance with resonance gas model calculations. Along the lines of constant $S/N_B$ we calculate the energy density and pressure. Within the accuracy of our present analysis we find that the ratio $p/ϵ$ for $T>T_0$ as well as the softest point of the equation of state, $(p/ϵ{)}_{\min }\simeq 0.075$, show no significant dependence on $S/N_B$.

• Received 27 December 2005

DOI:https://doi.org/10.1103/PhysRevD.73.054506