Bulk viscosity from Urca processes: $npe\mu$ matter in the neutrino-trapped regime

Understand

In this work, we extend our previous study of the bulk viscosity of hot and dense $npe$ matter induced by the Urca processes in the neutrino trapped regime to $npe\mu$ matter by adding the muonic Urca processes as well as the purely leptonic electroweak processes involving electron-muon transition.

• The nuclear matter is modeled in a relativistic density functional approach with two different parametrizations which predict neutrino dominated matter (DDME2 model) and antineutrino dominated matter (NL3 model) at temperatures for which neutrinos/antineutrinos are trapped.
• In the case of neutrino-dominated matter, the main equilibration mechanism is lepton capture, whereas in the case of antineutrino-dominated matter this is due to neutron decay.
• We find that the equilibration rates of Urca processes are higher than that of the pure leptonic processes, which implies that the Urca-process-driven bulk viscosity can be computed with the leptonic reactions assumed to be frozen.