We discuss the role of the U(1) axial symmetry for the phase structure of QCD at finite temperature. We expect that, above a certain critical temperature, also the U(1) axial symmetry will be (effectively) restored. We will try to see if this transition has (or has not) anything to do with the usual chiral transition: various possible scenarios are discussed. In particular, supported by recent lattice results, we analyse a scenario in which a U(1)-breaking condensate survives across the chiral transition. This scenario can be consistently reproduced using an effective Lagrangian model. The effects of the U(1) chiral condensate on the slope of the topological susceptibility in the full theory with quarks are studied: we find that this quantity (in the chiral limit of zero quark masses) acts as an order parameter for the U(1) axial symmetry above the chiral transition. Further information on the new U(1) chiral order parameter is derived from the study (at zero temperature) of the radiative decays of the "light" pseudoscalar mesons in two photons: a comparison of our results with the experimental data is performed. (PACS codes: 12.38.Aw, 12.39.Fe, 11.15.Pg, 11.30.Rd)
IntroductionIt is generally believed that a phase transition which occurs in QCD at a finite temperature is the restoration of the spontaneously broken SU(L) ⊗ SU(L) chiral symmetry in association with L massless quarks. At zero temperature the chiral symmetry is broken spontaneously by the condensation of qq pairs and the L 2 − 1 J P = 0 − mesons are just the Goldstone bosons associated with this breaking [1]. At high temperatures the thermal energy breaks up the qq condensate, leading to the restoration of chiral symmetry. We expect that this property not only holds for massless quarks but also continues for a small mass region. The order parameter for the chiral symmetry breaking is apparently qq ≡ L i=1 q i q i : the chiral symmetry breaking corresponds to the non-vanishing of qq in the chiral limit sup(m i ) → 0. From lattice determinations of the chiral order parameter qq one knows that the SU(L) ⊗ SU(L) chiral phase transition temperature T ch , defined as the temperature at which the chiral condensate qq goes to zero (in the chiral limit sup(m i ) → 0), is nearly equal to the deconfining temperature T c (see, e.g., Ref. [2]). But this is not the whole story: QCD possesses not only an approximate SU(L) ⊗ SU(L) chiral symmetry, for L light quark flavours, but also a U(1) axial symmetry (at least at the classical level) [3,4]. The role of the U(1) symmetry for the finite temperature phase structure has been so far not well studied and it is still an open question of hadronic physics whether the fate of the U(1) chiral symmetry of QCD has or has not something to do with the fate of the SU(L) ⊗ SU(L) chiral symmetry.In the "Witten-Veneziano mechanism" [5,6] for the resolution of the U(1) problem, a fundamental role is played by the so-called "topological susceptibility" in a QCD without quarks, i.e., in a pure Yang-Mills (YM) theory, in...