Abstract

Quark matter with only $u$ and $d$ quarks ($ud$QM) might be the ground state of baryonic matter at large baryon number $A>A_{\rm min}$. With $A_{\rm min}\gtrsim 300$, this has no direct conflict with the stability of ordinary nuclei. An intriguing test of this scenario is to look for quantum nucleation of $ud$QM inside neutron stars due to their large baryon densities. In this paper, we study the transition rate of cold neutron stars to $ud$ quark stars ($ud$QSs) and the astrophysical implications, considering the relevant theoretical uncertainties and observational constraints. It turns out that a large portion of parameter space predicts an instantaneous transition, and so the observed neutron stars are mostly $ud$QSs. We find this possibility still viable under the recent gravitational wave and pulsar observations, although there are debates on its compatibility with some observations that involve complicated structure of quark matter. The tension could be partially relieved in the two-families scenario, where the high-mass stars ($M\gtrsim2 M_{\odot}$) are all $ud$QSs and the low-mass ones ($M\sim1.4\, M_{\odot}$) are mostly hadronic stars. In this case, the slow transition of the low-mass hadronic stars points to a very specific class of hadronic models with moderately stiff EOSs, and $ud$QM properties are also strongly constrained.

Highlights

  • Quark matter, a state consisting purely of quark and gluon degrees of freedom without confining into individual nucleons, is expected to form at high density or high temperature

  • In our recent study [4], with this being adequately included in a phenomenological quark-meson model, we demonstrated that u, d quark mater is in general more stable than strange quark matter (SQM), and it can be more stable than the ordinary nuclear matter when the baryon number A is sufficiently large above Amin ≳ 300

  • III, by adopting the standard calculation formalism for quantum nucleation, we identify the hadronic matter and udQM features essential for the determination of the transition rate

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Summary

INTRODUCTION

A state consisting purely of quark and gluon degrees of freedom without confining into individual nucleons, is expected to form at high density or high temperature. “the hyperon puzzle”, motivates an alternative explanation of these heavy pulsars as being pure quark stars This possibility has been extensively studied in the context of the SQM hypothesis [13,14,15,16,17,18], while it is a more natural option for the stable udQM scenario. We conduct a comprehensive and systematic study for the above two possibilities in the context of the stable udQM scenario, taking into account various uncertainties on the hadronic matter and quark matter properties, and the most recent observational constraints. III, by adopting the standard calculation formalism for quantum nucleation, we identify the hadronic matter and udQM features essential for the determination of the transition rate The relevance of these two possibilities becomes clear.

PROPERTIES OF udQM AND udQS
QUANTUM NUCLEATION OF udQM IN COLD NEUTRON STAR MATTER
CONVERSION OF NEUTRON STARS AND ASTROPHYSICAL OBSERVATIONS
All compact stars being udQSs
Coexistence of hadronic stars and udQSs
Findings
SUMMARY

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