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 analyze the screening of static diquark sources in 2-flavor QCD and compare results with the screening of static quark-antiquark pairs. We show that a two quark system in a fixed color representation is screened at short distances like a single quark source in the same color representation whereas at large distances the two quarks are screened independently. At high temperatures we observe that the relative strength of the interaction in diquark and quark-antiquark systems, respectively, obeys Casimir scaling. We use this result to examine the possible existence of heavy quark-quark bound states in the high temperature phase of QCD. We find support for the existence of bb states up to about 2Tc while cc states are unlikely to be formed above Tc.

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We analyze the dependence of heavy quark free energies on the baryon chemical potential µ b in 2-flavour QCD using improved (p4) staggered fermions with a bare quark mass ofm/T = 0.4. By performing a 6 th order Taylor expansion in the chemical potential which circumvents the sign problem. The Taylor expansion coefficients of colour singlet and colour averaged free energies are calculated and from this the expansion coefficients for the corresponding screening masses are determined. We find that for small µ b the free energies of a static quark anti-quark pair decrease in a medium with a net excess of quarks and that screening is well described by a screening mass which increases with increasing µ b . The µ b -dependent corrections to the screening masses are well described by perturbation theory for T > ∼ 2Tc. In particular, we find for all temperatures above Tc that the expansion coefficients for singlet and colour averaged screening masses differ by a factor 2.