The hyperon puzzle in neutron stars

Resolve the hyperon puzzle by establishing whether equations of state for dense baryonic matter that include Λ hyperons at supranuclear densities can remain consistent with the existence of approximately two-solar-mass neutron stars, thereby reconciling the theoretically expected softening from hyperon onset with astrophysical mass observations.

Background

At densities above roughly twice nuclear saturation density, baryonic matter in neutron stars is expected to include hyperons produced via weak processes. Their appearance generally softens the equation of state, which in many models reduces the maximum neutron star mass below the ≈2 M⊙ objects observed astronomically.

This tension between theoretical expectations for hyperon-rich matter and astrophysical mass measurements is commonly termed the hyperon puzzle. The paper highlights this as a central challenge at the interface of nuclear physics and astrophysics, motivating the need for an ab initio framework that can simultaneously describe hypernuclei and dense hypernuclear matter consistent with observations.

References

This inconsistency, known as the hyperon puzzle, has become one of the central open problems at the interface of nuclear physics and astrophysics.

— Multi-strangeness matter from ab initio calculations  (2509.26148 - Tong et al., 30 Sep 2025) in Introduction

Determining whether additional repulsion from genuine hyperonic three-body interactions is required in stellar matter will require their consistent implementation in $\beta$-equilibrated matter and subsequent NS structure calculations.

— Hyperonic Softening versus Nucleonic Three-Body Repulsion in Hypernuclear Matter within a microscopic approach  (2609.00851 - Shahrbaf, 1 Sep 2026) in Section 3.1, “Energy per baryon: Direct comparison with the two-body baseline”