Weak magnetic field effects on chiral critical temperature in a nonlocal Nambu--Jona-Lasinio model

In this article we study the nonlocal Nambu--Jona-Lasinio model with a Gaussian regulator in the chiral limit. Finite temperature effects and the presence of a homogeneous magnetic field are considered. The magnetic evolution of the critical temperature for chiral symmetry restoration is then obtained. Here we restrict ourselves to the case of low magnetic field values, being this a complementary discussion to the exisiting analysis in nonlocal models in the strong magnetic field regime.

On the magnetic catalysis and inverse catalysis of phase transitions in the linear sigma model

We consider the evolution of critical temperature both for the formation of a pion condensate as well as for the chiral transition, from the perspective of the linear sigma model, in the background of a magnetic field. We developed the discussion for the pion condensate in one loop approximation for the effective potential getting magnetic catalysis for high values of B, i.e. a raising of the critical temperature with the magnetic field. For the analysis of the chiral restoration, we go beyond this approximation, by taking one loop thermo-magnetic corrections to the couplings as well as plasma screening effects for the bosons masses, expressed through the resumation of ring diagrams. Here we found the opposite behavior, i.e. inverse magnetica catalysis, i.e. a decreasing of the chiral critical temperature as function of the intensity of the magnetic field, which seems to be in agreement with recent results form the lattice community.

Thermodynamic instabilities in nonlocal NJL models

It has been recently pointed out, that nonlocal Nambu--Jona-Lasinio models, may present unphysical thermodynamical behavior like negative pressure and oscillating entropy. Here we show how these thermodynamic instabilities can be related to the analytical structure of the poles of the quark propagator in the model. The analysis is carried out for two different regulators and we show, in each case, how the instabilities are related to the pressence of highly unstable poles. We also argue that the softening of these instabilities by the inclusion of the Polyakov loop is related to the effect the latter has on the poles of the propagator.

The Linear Sigma Model and the formation of a charged pion condensate in the presence of an external magnetic field

We discuss the charged pion condensation phenomenon in the linear sigma model, in the presence of an external uniform magnetic field. The critical temperature is obtained as a function of the external magnetic field, assuming the transition is of second order, by considering a dilute gas at low temperature. As a result we found magnetic anti-catalysis in the Bose-Einstein condensation for lower values of the external magnetic field, and catalysis for higher values of the external magnetic field. This behavior confirms previous results with a single charged scalar field.

The nNJL model with a fractional Lorentzian regulator in the real time formalism

In this article we study the finite temperature and chemical potential effects in a nonlocal Nambu-Jona-Lasinio (nNJL) model in the real time formalism. We make the usual Wick rotation to get from imaginary to real time formalism. In doing so, we need to define our regulator in the complex plane q^2. This deffinition will be crucial in our later analysis. We study the poles in the propagator of this model and conclude that only some of them are of interst to us. Once we have a well defined model in real time formalism, we look at the chiral condensate to find the temperature at which chiral symmetry restoration will occur. We find a second order phase transition that turns to a first order one for high enough values of the chemical potential.