Existence of a hadron–quark phase transition in neutron star cores

Determine whether dense matter in the cores of neutron stars undergoes a phase transition from hadronic matter to deconfined quark matter.

Background

Understanding the microphysical composition of matter at supranuclear densities is central to neutron star physics. Ab initio theory provides constraints only at relatively low densities via chiral effective field theory and at very high densities via perturbative QCD, leaving the densities realized in neutron star interiors largely unconstrained by first principles. Observational inferences from masses, radii, and tidal deformabilities are thus crucial.

Multiple scenarios are considered plausible for the behavior of matter at these densities, including a smooth crossover to quark matter, a first-order phase transition that may destabilize the star, or more exotic possibilities. This work performs a Bayesian inference over a broad, model-agnostic ensemble of equations of state to compare evidences for these scenarios, but the fundamental question of whether a hadron–quark phase change actually occurs inside neutron stars remains unresolved.

References

It remains an open question of whether the phase transition (PT) from hadronic to quark matter occurs in the cores of NSs - the densest matter found in the Universe.

— First-order phase transitions in the cores of neutron stars  (2404.05637 - Komoltsev, 2024) in Introduction (Section 1)

This overlap leads us to conjecture that it may correspond to a region in which nucleonic and quark degrees of freedom coexist.

— Thermodynamic signatures do not uniquely identify deconfinement in neutron stars  (2610.01701 - Ma et al., 1 Oct 2026) in Main text, paragraph comparing nucleonic, quark-matter, and quarkyonic equations of state

Therefore, the structural characteristics of neutron stars composed of cold and dense matter, as well as whether a hadron-quark phase transition exists in their cores, remain an open question.

— Axion effects on hybrid stars: earlier hadron-quark phase transition and larger quark-matter cores  (2610.09480 - Liu et al., 7 Oct 2026) in Section I, Introduction