We consider vector and axial-vector mesons in the framework of a gauged linear sigma model with chiral UN f R UN f L symmetry. For N f 2, we investigate the behavior of the chiral condensate and the meson masses as a function of temperature by solving a system of coupled Dyson-Schwinger equations derived via the 2PI formalism in double-bubble approximation. We find that the inclusion of vector and axial-vector mesons tends to sharpen the chiral transition. Within our approximation scheme, the mass of the meson increases by about 100 MeV towards the chiral transition.

We investigate the implications of a tetraquark field on chiral symmetry restoration at nonzero temperature. In order for the chiral phase transition to be cross-over, as shown by lattice QCD studies, a strong mixing between scalar quarkonium and tetraquark fields is required. This leads to a light ($\sim0.4$ GeV), predominantly tetraquark state, and a heavy ($\sim1.2$ GeV), predominantly quarkonium state in the vacuum, in accordance with recently advocated interpretations of spectroscopy data. The mixing even increases with temperature and leads to an interchange of the roles of the originally heavy, predominantly quarkonium state and the originally light, predominantly tetraquark state. Then, as expected, the scalar quarkonium is a light state when becoming degenerate in mass with the pion as chiral symmetry is restored at nonzero temperature.Comment: 4 pages, 2 figure

We consider the O(N ) linear σ model and introduce an auxiliary field to eliminate the scalar self-interaction. Using a suitable limiting process this model can be continuously transformed into the nonlinear version of the O(N ) model. We demonstrate that, up to two-loop order in the CJT formalism, the effective potential of the model with auxiliary field is identical to the one of the standard O(N ) linear σ model, if the auxiliary field is eliminated using the stationary values for the corresponding one-and two-point functions. We numerically compute the chiral condensate and the σ− and π−meson masses at nonzero temperature in the one-loop approximation of the CJT formalism. The order of the chiral phase transition depends sensitively on the choice of the renormalization scheme. In the linear version of the model and for explicitly broken chiral symmetry, it turns from crossover to first order as the mass of the σ particle increases. In the nonlinear case, the order of the phase transition turns out to be of first order. In the region where the parameter space of the model allows for physical solutions, Goldstone's theorem is always fulfilled.

We study the implications of a light tetraquark on the chiral phase transition at nonzero temperature T : The behavior of the chiral and fourquark condensates and the meson masses are studied in the scenario in which the resonance f 0 (600) is described as a predominantly tetraquark state. It is shown that the critical temperature is lowered and the transition softened. Interesting mixing effects between tetraquark, and quarkonium configurations take place.