Bulk viscosity of nuclear matter with pions in the neutrino-trapped regime

TL;DR

This paper investigates how thermal pions influence the bulk viscosity of dense, neutron-rich matter in neutrino-trapped environments like neutron star mergers, revealing that pions significantly enhance bulk viscosity at various temperatures.

Contribution

It provides the first detailed analysis of pion effects on bulk viscosity in neutrino-trapped nuclear matter, highlighting their impact on reaction pathways and the equation of state.

Findings

Pions increase bulk viscosity at low temperatures by a factor of a few.

At higher temperatures, pions can boost bulk viscosity by an order of magnitude.

Bulk viscosity remains much smaller than its peak value despite pion enhancement.

Abstract

Recent work [B. Fore and S. Reddy, Phys. Rev. C 101 035809 (2020)] has shown that the population of thermal pions could modify the equation of state and transport properties of hot and dense neutron-rich matter and introduce new reaction pathways to change the proton fraction. In this article we study their impact on the bulk viscosity of dense matter, focusing on the neutrino-trapped regime that would be realized in neutron star mergers and supernovae. We find that the presence of a thermal population of pions alters the bulk viscosity by modifying the EoS (via the susceptibilities) and by providing new reaction pathways to achieve beta-equilibrium. In neutron star merger conditions, the bulk viscosity in neutrino-trapped $npe\mu$ matter (without pions) has its peak at temperatures of at most a couple MeV and is quite small at temperatures of tens of MeV. We find that thermal pions enhance the low-temperature peak of the bulk viscosity by a factor of a few and shift it to slightly lower temperatures. At higher temperatures, where the pion abundance is large but the bulk viscosity is traditionally small, pions can increase the bulk viscosity by an order of magnitude or more, although it is still orders of magnitude smaller than its peak value.