# (No) neutron star maximum mass constraint from hypernuclei

## TL;DR

This study investigates whether hypernuclei data can constrain neutron star equations of state, finding current data insufficient to limit maximum masses despite models consistent with observed 2 solar mass pulsars.

## Contribution

It demonstrates that existing hypernuclei experimental data and high-density nuclear matter constraints do not sufficiently restrict neutron star maximum mass predictions.

## Key findings

- Hypernuclei data do not constrain neutron star EoS with 2 M_sun observations.
- Lambda potential in symmetric nuclear matter is ~30-32 MeV at saturation.
- Lambda potential in Lambda matter varies between -16 and -8 MeV with SU(6) couplings.

## Abstract

(Abridged) The recent measurement of the mass of two $2\, M_\odot$ pulsars has raised the question whether such large masses allow for the existence of exotic degrees of freedom, such as hyperons, inside neutron stars. In the present work we will investigate how the existing hypernuclei properties may constrain the neutron star equation of state and confront the neutron star maximum masses obtained with equations of state calibrated to hypernuclei properties with the astrophysical $2\,M_\odot$ constraint. The study is performed using a relativistic mean field approach to describe both the hypernuclei and the neutron star equations of state. A set of five models consistent with $2\,M_\odot$ for a purely nucleonic composition are employed. The $\Lambda$-meson couplings are determined for all the models considered. Hyperonic stars with the complete baryonic octet are studied, restricting the coupling of the $\Sigma$ and $\Xi$ hyperons to the $\omega-$, $\rho-$ and $\sigma-$mesons due to the lack of experimental data, and maximum star masses calculated for unified equations of state. We conclude that the currently available hypernuclei experimental data and the lack of constraints on the asymmetric equation of state of nuclear matter at high densities do not allow to further constrain the neutron star matter equation of state using the recent $2\, M_\odot$ observations. It is also shown that the $\Lambda$ potential in symmetric nuclear matter takes a value $\sim 30-32$ MeV at saturation for the $g_{\omega \Lambda}$ coupling given by the SU(6) symmetry, close to values generally used in the literature. However, the $\Lambda$ potential in $\Lambda$ matter varies between -16 and -8 MeV taking for vector mesons couplings the SU(6) values, at variance with generally employed values between $-1$ and $-5$ MeV.