Remarks on the U(1) axial symmetry and the chiral transition in QCD at finite temperature

Abstract: We discuss the role of the U(1) axial symmetry for the scalar and pseudoscalar meson mass spectrum of QCD at finite temperature, above the chiral transition at T_c, using a chiral effective Lagrangian model, which, in addition to the usual chiral condensate, also includes a (possible) genuine U(1)_A-breaking condensate that (possibly) survives across the chiral transition. The motivations for considering this Lagrangian (and a critical comparison with other effective Lagrangian models existing in the literature) are presented. A detailed comparison between the case N_f=3 (or N_f>3) and the (remarkably different) case N_f=2 is performed. The results obtained in the case N_f=2 are also critically compared with the available lattice results.