S parameter at nonzero temperature and chemical potential

We compute the finite-temperature and matter density corrections to the S-parameter at the one-loop level. At nonzero temperature T and matter density μ Lorentz symmetry breaks and therefore we suggest a suitable generalization of the S-parameter. By computing the plasma correction, we discover a reduction of the S-parameter in the physically relevant region of small external momenta for any nonzero μ and T. In particular, the S-parameter vanishes at small m/T, where m is the mass of the fermions, due to the finite extent of the temporal direction. Our results are directly applicable to the determination of the S-parameter via first-principle lattice simulations performed with antiperiodic boundary conditions in the temporal direction. © 2011 American Physical Society.