Model study of the sign problem in a mean-field approximation

We study the sign problem of the fermion determinant at nonzero baryon chemical potential. For this purpose we apply a simple model derived from Quantum Chromodynamics, in the limit of large chemical potential and mass. For SU(2) color, there is no sign problem and the mean-field approximation is similar to data from the lattice. For SU(3) color the sign problem is unavoidable, even in a mean-field approximation. We apply a phase-reweighting method, combined with the mean-field approximation, to estimate thermodynamic quantities. We also investigate the mean-field free energy using a saddle-point approximation.Comment: 7 pages, 2 figures, talk presented at the XXV International Symposium on Lattice Field Theory, July 30 - August 4, 2007, Regensburg, German

Off-diagonal kinetic theory in ultrasoft momentum region at high temperature

We derive a linearized kinetic equation for fermionic excitations with an ultrasoft momentum, g^2T, from the Kadanoff-Baym equation in a Yukawa model and quantum electrodynamics (QED) at extremely high T, where g is the coupling constant. We show that this equation is equivalent to the self-consistent equation in the resummed perturbation theory used in the analysis of the fermion spectrum with the ultrasoft momentum. Furthermore, we derive the equation that determines the n-point function with external lines for a pair of fermions and (n-2) bosons with ultrasoft momenta in QED.Comment: 15 pages, 8 figure

Charged vector mesons in a strong magnetic field

We show that charged vector mesons cannot be condensed by a magnetic field. Although some hadron models predict the charged vector meson condensation in a strong magnetic field, we prove, by means of the Vafa-Witten theorem, that this is not the case in QCD. We also perform the numerical analysis for the meson mass and condensation in lattice QCD. The lattice QCD data confirm no charged vector meson condensation in a magnetic field.Comment: 6 pages, 2 figures; some discussion improved; typos correcte