Coupling between superfluid neutrons and superfluid protons in the elementary excitations of neutron star matter

Abstract

Several phenomena occurring in neutron stars are affected by the elementary excitations that characterize the stellar matter. In particular, low-energy excitations can play a major role in the emission and propagation of neutrinos, neutron star cooling and transport processes. In this paper, we consider the elementary modes in the star region where both proton and neutron components are superfluid. We study the overall spectral functions of protons, neutrons and electrons on the basis of the Coulomb and nuclear interactions. This study is performed in the framework of the Random Phase Approximation, generalized to superfluid systems. The formalism we use ensures that the Generalized Ward's Identities are satisfied. We focus on the coupling between neutrons and protons. On one hand this coupling results in collective modes that involve simultaneously neutrons and protons, on the other hand it produces a damping of the excitations. Both effects are especially visible in the spectral functions of the different components of the matter. At high density while the neutrons and protons tend to develop independent excitations, as indicated by the spectral functions, the neutron-proton coupling still produces a strong damping of the modes.

Hadron matter in neutron stars in view of gravitational wave observations

“…In the neutron (and proton) superfluid that is often assumed to constitute part of the star, the typical low-energy excitations would be a Goldstone boson near zero excitation energy over the ground state. The fermion excitations corresponding to the breaking of an nn or a pp pair are excitations at twice their respective mass gap [237] ∆, and for temperatures smaller than that gap, their contribution to transport is suppressed by e −∆/T . Likewise, in the asymptotic high-density regime, the CFL phase transport coefficients are dominated by the Goldstone boson of U (1) symmetry breaking [238,239].…”