Pseudoscalar and scalar meson masses at finite temperature

The composite operator formalism is applied to QCD at finite temperature to calculate the masses of scalar and pseudoscalar mesons. In particular the ratio of the sigma mass to the pion mass is an interesting measure of the degree of chiral symmetry breaking at different temperatures. We calculate the temperature T* at which M_sigma(T) < 2M_pi(T), above which the sigma partial width into two pions vanishes. We find T*=0.95T_c (where T_c is the critical temperature for the chiral phase transition), within the full effective potential given by the formalism. We find that an expansion a-la Landau of the effective potential around the critical point in the limit of small quark mass provides for a very good determination of T*.