Hyperons: the strange ingredients of the nuclear equation of state

TL;DR

This review explores hyperons' roles in nuclear systems, their interactions, and their impact on neutron star properties, addressing the hyperon puzzle and recent advances in theoretical and experimental studies.

Contribution

It provides a comprehensive overview of hyperon properties, interactions, and their effects on neutron stars, including recent theoretical developments and solutions to the hyperon puzzle.

Findings

Hyperons significantly soften the nuclear equation of state.

Recent measurements of massive pulsars challenge existing hyperon models.

Proposed solutions aim to reconcile hyperon effects with observed neutron star masses.

Abstract

In this article we will review the role and properties of hyperons in finite and infinite nuclear systems. In particular, we will revise different production mechanisms of hypernuclei, as well as several aspects of hypernuclear $\gamma$-ray spectroscopy, and the weak decay modes of hypernuclei. Then we will discuss the construction of hyperon-nucleon and hyperon-hyperon interactions on the basis of the meson-exchange and chiral effective field theories. Recent developments based on the so-called V$_{low\,\, k}$ approach and lattice QCD will also be adressed. Finally, we will go over some of the effects of hyperons on the properties of neutron and proto-neutron stars with an emphasis on the so-called "hyperon puzzle", {\it i.e.,} the problem of the strong softening of the equation of state, and the consequent reduction of the maximum mass, induced by the presence of hyperons, a problem which has become more intringuing and difficult to solve due the recent measurements of $\sim 2M_\odot$ millisecond pulsars. We will discuss some of the solutions proposed to tackle this problem. We will also re-examine the role of hyperons on the cooling properties of newly born neutron stars and on the development of the so-called r-mode instability.