Axions in neutron star mergers

Abstract

Supernovae and cooling neutron stars have long been used to constrain the properties of axions, such as their mass and interactions with nucleons and other Standard Model particles. We investigate the prospects of using neutron star mergers as a similar location where axions can be probed in the future. We examine the impact axions would have on mergers, considering both the possibility that they free-stream through the dense nuclear matter and the case where they are trapped. We calculate the mean free path of axions in merger conditions, and find that they would free-stream through the merger in all thermodynamic conditions. In contrast to previous calculations, we integrate over the entire phase space while using a relativistic treatment of the nucleons, assuming the matrix element is momentum-independent. In particular, we use a relativistic mean field theory to describe the nucleons, taking into account the precipitous decrease in the effective mass of the nucleons as density increases above nuclear saturation density. We find that within current constraints on the axion-neutron coupling, axions could cool nuclear matter on timescales relevant to neutron star mergers. Our results may be regarded as first steps aimed at understanding how axions affect merger simulations and potentially interface with observations.

Probing dark gauge boson with observations from neutron stars

“…The dark photon energy emission rate per unit volume for the process N 1 +N 2 → N 3 +N 4 +γ (N = n, p) in a strongly degenerate nuclear matter is given by [30,36]…”

“…Note that the nuclear energies that enter the energy δ-function and the denominator on the right hand side of eq. (3.1) are the effective energies [36]. The fermion phase-space distribution function is given by…”

“…Let us first consider the process n + p → n + p + γ , the dark photon energy emission rate is given by the formula (3.1). Multiply the equation by one in the form [36]…”

“…In this work, we will focus on the production of dark photons in the core of neutron stars (NSs), which are the remnants of the supernova explosion and have long been recognized as excellent laboratories to search for axion-like particles [28][29][30][31][32][33][34][35][36][37][38]. The core of an NS is composed of highly degenerate nuclear matter that has an average density of ∼ 3 × 10 14 g/cm 3 , which corresponds to an average distance of ∼ 1 fm between nucleons.…”