Bag vs. NJL models for color-flavor-locked strange quark matter

TL;DR

This paper compares the predictions of the MIT and NJL models for the mass-radius relationship of strange stars, highlighting differences in how the models handle pairing effects and the implications for the equation of state.

Contribution

It provides a comparative analysis of the MIT and NJL models in describing color-flavor-locked strange quark matter, emphasizing the limitations of the MIT model in capturing BCS-BEC crossover effects.

Findings

MIT model can produce larger star masses by increasing the gap parameter.

NJL model requires a higher diquark-coupling constant, which softens the equation of state.

High diquark coupling in NJL indicates a BCS-BEC crossover not captured by MIT model.

Abstract

We compare the mass-radius relationship of strange stars obtained in two theoretical frameworks describing the color-flavor-locking state of dense quark matter: The semi-empirical MIT model and a self-consistent approach using the Nambu-Jona-Lasinio (NJL) model. In the simplest MIT model extended to include pairing, one can make the equation of state stiffer by increasing the gap parameter so that larger maximum masses for these objects can be reached. In the NJL model, however, such an effect is not possible. To increase the gap parameter within the NJL model to values comparable to those considered in the MIT case, a noticeably increase of the diquark-coupling-constant strength is needed, but this in turn softens the equation of state producing a lower maximum star mass. This behaviour is interpreted as signalling the system crossover at high diquark coupling from a BCS regime to a BEC one, a process that cannot be reproduced within the simple MIT prescription.