Mapping the boundary of the first order finite temperature restoration   of chiral symmetry in the m_pi-m_K--plane with a linear sigma model
  The phase diagram of the three-flavor QCD is mapped out in the low mass corner of the m_pi-m_K--plane with help of the SU_L(3) x SU_R(3) linear sigma model (LSM). A novel zero temperature parametrization is proposed for the mass dependence of the couplings away from the physical point based on the the three-flavor chiral perturbation theory (U(3) ChPT). One-loop thermodynamics is constructed by applying optimized perturbation theory. The unknown dependence of the scalar spectra on the pseudoscalar masses limitates the accuracy of the predictions. Results are compared to lattice data and similar investigations with other variants of effective chiral models. The critical value of the pion mass is below 65 MeV for all m_K values <= 800 MeV. Along the diagonal m_pi=m_K, we estimate m_crit(diag)=40+-20 MeV. 
T. Herpay, A. Patkos, Zs. Szep, P. Szepfalusy