  Tree-level and complete one-loop parametrisation of the linear sigma model
(LSM) is performed and the phase boundary between first order and crossover
transition regions of the m_pi-m_K-plane is determined using the optimised
perturbation theory (OPT) as a resummation tool of perturbative series. Away
from the physical point the parameters of the model were determined by making
use of chiral perturbation theory (ChPT). The location of the phase boundary
for m_pi=m_K and of the tricritical point (TCP) on the m_pi=0 were estimated.
