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

We study the behavior of the dual quark condensate ∑1 in the Nambu–Jona-Lasinio (NJL) model and its nonlocal variant. In quantum chromodynamics ∑1 can be related 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 ∑1 as a function of temperature. However, a similar behavior 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 ∑1 is triggered by the chiral phase transition. In order to shed more light on this issue, we calculate ∑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 behavior of ∑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, ∑1 should not blindly be regarded as an order parameter of confinement.