We study the thermodynamics of massless phi-fourth theory using screened perturbation theory. In this method, the perturbative expansion is reorganized by adding and subtracting a thermal mass term in the Lagrangian. We calculate the free energy through four loops expanding in a double power expansion in m/T and g^2, where m is the thermal mass and g is the coupling constant. The expansion is truncated at order g^7 and the loop expansion is shown to have better convergence properties than the weak-coupling expansion. The free energy at order g^6 involves the four-loop triangle sum-integral evaluated by Gynther, Laine, Schroeder, Torrero, and Vuorinen using methods developed by Arnold and Zhai. The evaluation of the free energy at order g^7 requires the evaluation of a nontrivial three-loop sum-integral, which we calculate by the same methods.Comment: 34 pages, 6 figures, RevTe

We study pion condensation and the phase structure in a twoflavour Nambu-Jona-Lasinio model in the presence of baryon chemical potential µ and isospin chemical potential µ I at zero and finite temperature. There is a competition between the chiral condensate and a Bose-Einstein condensate of charged pions. In the chiral limit, the chiral condensate vanishes for any finite value of the isospin chemical potential, while there is a charged pion condensate that depends on the chemical potentials and the temperature. At the physical point, the chiral condensate is always nonzero, while the charged pion condensate depends on µ I and T . For T = µ = 0, the critical isospin chemical potential µ c I for the onset of Bose-Einstein condensation is always equal to the pion mass. For µ = 0, we compare our results with chiral perturbation theory, sigma-model calculations, and lattice simulations.Finally, we examine the effects of imposing electric charge neutrality and weak equilibrium on the phase structure of the model. In the chiral limit, there is a window of baryon chemical potential and temperature where the charged pions condense. At the physical point, the charged pions do not condense.PACS numbers: 14.40. Aq, 11.30.Qc,21.65.-f ‡ The diquark condensate in two-colour QCD does not break any local symmetries, only global ones. The system is therefore a superfluid but not a colour superconductor.

Here, we study the flow of energy between coupled simulators in a co-simulation environment using the concept of power bonds. We introduce energy residuals which are a direct expression of the coupling errors and hence the accuracy of co-simulation results. We propose a novel EnergyConservation-based Co-Simulation method (ECCO) for adaptive macro step size control to improve accuracy and efficiency. In contrast to most other co-simulation algorithms, this method is noniterative and only requires knowledge of the current coupling data. Consequently, it allows for significant speed ups and the protection of sensitive information contained within simulator models. A quarter car model with linear and nonlinear damping serves as a co-simulation benchmark and verifies the capabilities of the energy residual concept: Reductions in the errors of up to 93 % are achieved at no additional computational cost.

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The average phase factor of the QCD fermion determinant signals the strength of the QCD sign problem. We compute the average phase factor as a function of temperature and baryon chemical potential using a two-flavor NJL model. This allows us to study the strength of the sign problem at and above the chiral transition. It is discussed how the $U_A(1)$ anomaly affects the sign problem. Finally, we study the interplay between the sign problem and the endpoint of the chiral transition.Comment: 9 pages and 9 fig