Delta baryons and diquark formation in the cores of neutron stars

TL;DR

This paper explores how Delta baryons and quark matter influence neutron star properties, showing that Delta baryons can significantly affect radii and that phase transitions to quark matter can produce stable, smaller-radius neutron stars consistent with recent observational constraints.

Contribution

It demonstrates the potential prominence of Delta baryons in neutron star cores and analyzes the impact of hadron-quark phase transitions on star stability and observable properties.

Findings

Delta baryons may constitute a large fraction of neutron star baryons.

Hadron-quark phase transition occurs near the maximum mass in neutron stars.

Stable compact objects with smaller radii can exist beyond the traditional maximum mass.

Abstract

We investigate the hadron-quark phase transition in cold neutron stars in light of (i) the observed limits on the maximum-mass of heavy pulsars, (ii) constraints on the tidal properties inferred from the gravitational waves emitted in binary neutron-star mergers, and (iii) mass and radius constraints derived from the observation of hot spots on neutron star observed with NICER. Special attention is directed to the possible presence of $\Delta(1232)$ baryons in neutron star matter. Our results indicate that this particle could make up a large fraction of the baryons in neutron stars and thus have a significant effect on the properties of such objects, particularly on their radii. This is partially caused by the low density appearance of $\Delta$s for a wide range of theoretically defensible sets of meson-hyperon, SU(3) ESC08 model, and meson-$\Delta$ coupling constants. The transition of hadronic matter to quark matter, treated in the 2SC+s condensation phase, is found to occur only in neutron stars very close to the mass peak. Nevertheless, quark matter may still constitute an appreciable fraction of the stars' total matter if the phase transition is treated as Maxwell-like (sharp), in which case the neutron stars located beyond the gravitational mass peak would remain stable against gravitational collapse. In this case, the instability against gravitational collapse is shifted to a new (terminal) mass different from the maximum-mass of the stellar sequence, giving rise to stable compact objects with the same gravitational masses as those of the neutron stars on the traditional branch, but whose radii are smaller by up to 1 km. All models for the equation of state of our study fall comfortably within the bound established very recently by Annala {\it et al.} (Nature Physics, 2020)