Pions in Neutron-rich Matter: Implications for Neutron Stars and Supernovae

Abstract

The effects of pions on astrophysical phenomena have been studied for decades. In the 1970s, Migdal and Sawyer independently proposed that pions could condense in dense matter due to attractive p-wave interactions with nucleons. Many other studies have investigated the possibility of pion condensation in dense neutron-rich matter. However, due to the strong interactions between nucleons and pions, there is a large degree of uncertainty in condensate formation. We have considered the effects of a thermal population of negatively charged pions, where the enhancement from strong interactions with nucleons is taken into account with a virial approximation and find that even without the formation of a condensate, pions are likely to have a large effect on neutron-rich matter at high densities and temperatures, such as the conditions in the proto-neutron star of a core-collapse supernova or inside neutron star mergers. This model is also used for calculations of neutrino mean free path in the processes $\nu_\mu+\pi^-\rightarrow\mu^-$ and $\bar{\nu}_\mu+\mu^-\rightarrow\pi^-$ as well as calculations of axion emissivity through a pion induced reaction $\pi^-p\rightarrow na$. Due to the pions repulsive s-wave interaction with nucleons and strongly attractive p-wave interactions, the majority of studies have focused on investigations of pion condensation at finite momentum. Early studies concluded that a condensed state at finite momentum was favored, but later more sophisticated analysis found that these results may not be robust at the densities encountered in neutron stars. In this thesis we reevaluate the possibility of s-wave pion condensation through improved calculations of the pion self-energy using Chiral Perturbation Theory.