Neutron Stars May Avoid Exotic Matter Thanks to Delayed Hyperon Appearance

Scientists are continuing to probe the enduring ‘hyperon puzzle’ in neutron star physics, seeking to understand the observed absence of certain particles despite theoretical predictions. Bikai Gao from the Research Center for Nuclear Physics (RCNP), Osaka University, and colleagues, in collaboration with researchers across multiple institutions, present new findings utilising a parity doublet model with chiral representation to investigate hyperon emergence within these extreme stellar environments. Their work demonstrates a strong correlation between the parameter and the density at which hyperons appear, revealing that a larger value can significantly delay hyperon onset, potentially even beyond the density range where quark deconfinement occurs. This mechanism offers a natural solution to the hyperon puzzle by preventing excessive equation of state softening and aligning with observational data from massive neutron stars, highlighting the crucial role of chiral dynamics in determining the composition of these dense objects.

Scientists are tackling a long-standing puzzle in nuclear astrophysics concerning the behaviour of hyperons, baryons containing strange quarks, within the incredibly dense environment of neutron stars. This work investigates how and when these hyperons emerge inside neutron stars, utilising a sophisticated model known as the parity doublet model with chiral representation.

This framework inherently accounts for the restoration of chiral symmetry, a fundamental principle in particle physics, and systematically describes the varying masses of baryons at extreme densities through the interplay of chiral and chiral-invariant mass components. The research reveals a striking sensitivity of hyperon emergence to a key parameter, the chiral invariant mass, with significant implications for understanding the internal structure of these stellar objects.

Specifically, the study demonstrates that for a chiral invariant mass of 500 MeV, hyperons begin to appear at a density of 1.9times the nuclear saturation density (n0), a standard measure of density in nuclear physics. However, when the chiral invariant mass is increased to 750 MeV or higher, hyperon emergence is delayed, occurring only at densities exceeding 5n0.

This postponement arises from a weakened dependence of baryon masses on density at larger values of the chiral invariant mass. Crucially, when the hyperon onset density surpasses the range where a transition to deconfined quark matter is anticipated, between 2 and 5n0, the material undergoes this phase change before hyperons can significantly populate the star.

This avoids the softening of the equation of state (EoS) that typically accompanies hyperon appearance, and aligns with observations of massive neutron stars. The research team constructed a model based on the SU parity doublet framework, employing a linear realisation of chiral symmetry to describe both nucleons and hyperons (Lambda, Sigma, and Xi) alongside their negative-parity counterparts.

By incorporating scalar and pseudoscalar mesons through a 3×3 matrix field, and vector mesons via hidden local symmetry, the model accurately reproduces observed baryon masses and nuclear matter saturation properties. Numerical analysis revealed the critical role of the chiral invariant mass in regulating hyperon onset, offering a potential resolution to the hyperon puzzle without resorting to artificial repulsive interactions.

The findings suggest that chiral dynamics provides a natural explanation for the absence of significant EoS softening due to hyperons, thereby resolving a key discrepancy between theoretical predictions and observational constraints on neutron star masses. This work not only advances our understanding of matter under extreme conditions but also opens avenues for further exploration of the interplay between chiral symmetry restoration and the emergence of exotic phases in neutron stars.

A parity doublet model, incorporating chiral representation, underpins this work investigating hyperon emergence within neutron stars. To establish the model’s parameters, experimentally measured baryon masses, specifically the nucleon (939 MeV and 1535 MeV), lambda (1116 MeV), and sigma (1193 MeV), were used as input values, sourced from the Particle Data Group.

The model treats the ground state nucleon and its excited state as a chiral partner pair, while predicting the masses of chiral partners for lambda, sigma, and xi baryons. Baryon masses within the model are calculated using equations defining relationships between nucleon, lambda, sigma, and xi species, incorporating four free parameters: g1, g2, m1, and m2.

These parameters were determined through a fitting procedure across a range of chiral invariant masses, from 500 to 900 MeV, with the resulting values detailed in Table Further refinement involved examining the scalar meson sector, beginning with a Lagrangian describing kinetic energy, potential interactions, and explicit chiral symmetry breaking. A mean-field approximation was then applied, simplifying the Lagrangian and allowing for the calculation of meson masses and couplings.

The coefficients for explicit chiral symmetry breaking were fixed using the pion and kaon decay constants, ensuring consistency with established experimental values. Hyperon onset densities exhibit a pronounced sensitivity to the chiral invariant mass, m0. Specifically, for m0 equalling 500 MeV, hyperons first appear at a density of 1.9n0. However, when m0 is greater than or equal to 750 MeV, hyperon emergence is delayed, occurring only above 5n0.

This delayed onset is directly attributable to a weakened density dependence of baryon masses at larger m0 values. Crucially, when the hyperon onset density exceeds the expected range for quark-hadron transitions (2, 5n0), the model predicts a transition to deconfined quark matter before hyperons significantly populate the neutron star. This preemptive deconfinement effectively avoids the equation of state softening traditionally associated with hyperon appearance, thereby maintaining consistency with observations of massive neutron stars.

Parameter values, determined by fitting to vacuum baryon masses and nuclear matter saturation properties, reveal specific coupling constants. For m0 of 500 MeV, g1 is 15.47 and g2 is 9.02, while for m0 of 900 MeV, these values decrease to 12.41 and 5.96 respectively. Scientists have long grappled with the “hyperon puzzle” in neutron star physics, and recent work offers a compelling resolution.

The core of the problem lies in predicting the behaviour of hyperons, exotic particles containing strange quarks, at the immense densities found within these stellar remnants. Conventional models suggested their early appearance would drastically soften the neutron star’s equation of state, contradicting observations of remarkably massive neutron stars.

This discrepancy demanded either a revision of our understanding of hyperon interactions, or a mechanism to delay their onset. This latest research, employing a sophisticated parity doublet model, demonstrates a pathway to that delay. By carefully accounting for chiral dynamics, the subtle interplay of quantum effects governing particle masses, the team shows that hyperons may not appear until densities high enough to preclude this problematic softening.

The strength of this approach isn’t simply in matching observations, but in providing a theoretically grounded explanation rooted in fundamental symmetries. It sidesteps the need for artificial “repulsive” forces previously invoked to keep hyperons at bay. However, this isn’t a complete closure of the debate. The model relies on specific parameters, and uncertainties in those values inevitably translate into uncertainties in the predicted hyperon onset density. Furthermore, the precise point of transition between neutron star matter and a quark-gluon plasma remains poorly understood, and its interplay with hyperon emergence requires further investigation.