Large $N_{c}$, chiral approach to $M_{\eta}'$ at finite temperature

We study the temperature dependence of the eta and eta' meson masses withinthe framework of U(3)_L x U(3)_R chiral perturbation theory, up tonext-to-leading order in a simultaneous expansion in momenta, quark masses andnumber of colours. We find that both masses decrease at low temperatures, butonly very slightly. We analyze higher order corrections and argue that largeN_c suggests a discontinuous drop of M_eta' at the critical temperature ofdeconfinement T_c, consistent with a first order transition to a phase withapproximate U(1)_A symmetry.