Do hyperons exist in the interior of neutron stars ?

Abstract: In this work we review the role of hyperons on the properties of neutron and proto-neutron stars. In particular, we revise the so-called "hyperon puzzle", go over some of the solutions proposed to tackle it, and discuss the implications that the recent measurements of unusually high neutron star masses have on our present knowledge of hypernuclear physics. We reexamine also the role of hyperons on the cooling properties of newly born neutron stars and on the so-called r-mode instability.

• The paper demonstrates that including hyperons in the equation of state softens it, conflicting with observations of neutron stars over 2 solar masses.
• It applies both experimental hypernuclear studies and theoretical models to investigate repulsive hyperon interactions and three-body forces.
• The study also explores alternative solutions such as phase transitions to quark matter to reconcile dense matter properties with astrophysical data.

Overview of Hyperons in Neutron Stars

The paper entitled "Do hyperons exist in the interior of neutron stars?" by Debarati Chatterjee and Isaac Vidaña presents a thorough discussion on the presence and implications of hyperons within neutron stars. This is intrinsically linked to the long-standing "hyperon puzzle," which emerges from the possibility of hyperons residing in the dense cores of neutron stars and the challenges this poses to theoretical models, particularly in light of observed neutron star masses exceeding 2 solar masses.

Background and Context

Hyperons, baryonic particles containing strange quarks, are known to form stable configurations in hypernuclei and are thought to potentially populate neutron star interiors at high densities, surpassing normal nuclear saturation density. The inclusion of hyperons in the equation of state (EoS) generally leads to a significant softening, reducing the maximum mass of the star. This directly conflicts with the recent precise measurements of neutron star masses, exemplified by PSR J1614-2230 and PSR J0348+0432, which demand a stiffer EoS capable of supporting a mass greater than 2 solar masses. This discrepancy is central to the hyperon puzzle.

Theoretical and Experimental Approaches

The paper delineates both experimental approaches through hypernuclear physics and model-based theoretical studies to address the hypernuclear interactions relevant to the interior of neutron stars. Various experiments, such as hypernuclear production mechanisms using meson exchange, have been undertaken to study the hyperon-nucleon (YN) interaction, albeit limited by sparse scattering data. SU(3) flavor symmetry often guides theoretical models due to these limitations, with refinements available from hypernuclear observations.

Proposed Solutions to the Hyperon Puzzle

1. Repulsive Hyperon-Hyperon Interaction: One approach to address the hyperon puzzle involves introducing enhanced repulsive interactions among hyperons. This is often modeled via the exchange of vector mesons, with studies showing the potential to elevate the maximum mass of neutron stars within observational constraints.
2. Hyperonic Three-Body Forces: The inclusion of repulsive three-body forces involving hyperons (YNN, NNY, YY), an analogous approach to three-nucleon forces in traditional baryonic matter, potentially stiffens the EoS. These have been explored using phenomenological models, yielding promising but varied results regarding their sufficiency in explaining massive neutron stars.
3. Alternate Degrees of Freedom: Considering other baryonic excitations, such as the Δ-isobar and meson condensates, may also shift hyperon onset to higher densities, mitigating their adverse mass-reducing effects.
4. Phase Transition to Quark Matter: A phase transition to deconfined quark matter at densities below hyperon formation thresholds offers another solution pathway. Hybrid stars, possessing both hadronic and quark matter cores, are considered, with the need for strong repulsive interactions among quarks to achieve the 2 solar mass threshold.

Implications and Future Research

The presence of hyperons in neutron stars could significantly impact various stellar properties, including cooling mechanisms via exotic hyperonic processes, thermal evolution, and instabilities such as r-mode oscillations crucial for gravitational wave emission. These insights necessitate refinements in hyperon-nucleon interaction models and heightened observational precision in static and dynamic neutron star properties.

Progressively, future experimental platforms such as FAIR, DANAE, and improved lattice QCD computations are anticipated to provide better insights into hyperon interactions and the dense matter EoS. Observational advancements in neutron star mass and radius measurements and gravitational wave detections are expected to further constrain models and facilitate breakthroughs in solving the hyperon puzzle.

In conclusion, while significant strides have been made, the hyperon puzzle remains an open problem entwined with the depths of hypernuclear and neutron star physics. Researchers continue to strive for a comprehensive model that reconciles the presence of hyperons with the demands of massive neutron star observations.