Using Neutron Stars to Probe Fundamental Physics

Abstract

Neutron stars present some of the most extreme densities known to be found in the universe. At such densities, astrophysical study of these exotic objects requires high quality nuclear physics input in order to interpret. In this thesis, I will present my recent work addressing a range of questions requiring nuclear calculations that are pertinent to interpretation of astrophysical observations of neutron stars. In particular, I will discuss recent published work using neutron stars to constrain the parameter space of axion-gluon couplings, calculations of rates of neutrino interactions in the presence of strong astrophysical magnetic fields, and the effects that a dense medium of nucleons has on superfluidity at high density. Additionally, I will briefly discuss new progress on addressing whether the QCD axion can be constrained by neutron stars and calculating neutrino opacities in the neutron star merger environment in a computationally efficient formalism. The axion is a well-motivated dark matter candidate and possible solution to the strong CP problem. The formation of an axion condensate can become energetically favorable at high baryon density and neutron star observables may be significantly altered, constraining such a scenario. When the mass of the axion is lighter than the QCD prediction by more than an order of magnitude, this transition occurs at densities accessibly in the crust of neutron stars. We provide a constraint on the parameter space of ``exceptionally light'' QCD axions by tuning a phenomenological model to match the range of predictions of a preliminary calculation in Chiral Effective Field Theory. To access the QCD axion, modifications to nuclear forces in the presence of the axion condensate need to be better understood, in particular in light of large cutoff dependence we find in naively applied Chiral EFT. Isospin breaking effects are also amplified in the axion condensed phase, a component not fully incorporated in our simpler calculation. Presently unconstrained three nucleon forces also contribute to the energy at these densities. Finally, we will also comment on the relevance of chiral symmetry restoration for axion condensation. In the presence of strong magnetic fields, the energy levels of charged particles are quantized. This quantization is expected to be present in neutron stars with strong magnetic field known as magnetars. When the energy levels of charged particles are quantized, the direct Urca process becomes available at lower densities than normally expected. The direct Urca process is responsible for rapidly cooling the heaviest neutron stars, but without strong magnetic fields is strongly suppressed at densities accessible in most neutron stars. The quantization of the energy of charged particles introduces resonances in the density of states that amplify the emissivity in small regions of the neutron star, especially when the temperature is low. At low density and high temperatures, the presence of magnetic fields also has important implications for the opacity of neutrinos. We find greatly amplified opacity for the lowest energy neutrinos in the presence of magnetic field strengths predicted to be found in neutron star merger ejecta. This enhanced opacity results from the large anomalous magnetic moment of the nucleons and modifications to the dispersion relations of electrons and protons. For an unknown range of densities above nuclear density, it is expected that neutrons will become superfluid in the $^3P_2$ channel. At high density it is also expected that short range forces between nucleons are strongly repulsive. When including the one loop correction to the interaction between two nucleons, the Kohn-Luttinger mechanism introduces the possibility that this repulsive force may contribute to attraction in higher partial waves even if it is purely s-wave at tree level. We calculate the size of this effect with a variety of central and non-central potentials to assess the relevance of non-central interactions for induced $^3P_2$ pairing in dense matter.