The dual quark condensate in local and nonlocal NJL models: an order parameter for deconfinement?

We study the beahviour of the dual quark condensate $Σ_1$ in the Nambu-Jona-Lasinio (NJL) model and its nonlocal variant. In quantum chromodynamics $Σ_1$ can be realted to the breaking of the center symmetry and is therefore an (approximate) order parameter of confinement. The deconfinement transition is then signaled by a strong rise of $Σ_1$ as a function of temperature. However, a similar behaviour is also seen in the NJL model, which is known to have no confinement. Indeed, it was shown that in this model the rise of $Σ_1$ is triggered by the chiral phase transition. In order to shed more light on this issue, we calculate $Σ_1$ for several variants of the NJL model, some of which have been suggested to be confining. Switching between "confining" and "non-confining" models and parametrizations we find no qualitative difference in the behaviour of $Σ_1$, namely, it always rises in the region of the chiral phase transition. We conclude that without having established a relation to the center symmetry in a given model, $Σ_1$ should not blindly be regarded as an order parameter of confinement.

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