Accurate Muonic Interactions in Neutron Star Mergers and Impact on Heavy-element Nucleosynthesis

Abstract The abundances resulting from r -process nucleosynthesis as predicted by simulations of binary neutron star (BNS) mergers remain an open question as the current state of the art is still restricted to three-species neutrino transport. We present the first BNS merger simulations employing a moment-based general-relativistic neutrino transport with five neutrino species, thus including (anti)muons and advanced muonic β -processes, and contrast them with traditional three-neutrino-species simulations. Our results show that a muonic trapped-neutrino equilibrium is established, forming a different trapped-neutrino hierarchy akin to the electronic equilibrium. The formation of (anti)muons and the muonization via muonic β -processes enhance neutrino luminosity, leading to a stronger cooling in the early postmerger phase. Since muonic processes redirect part of the energy otherwise used for protonization by electronic processes, they yield a cooler remnant and disk, together with neutrino-driven winds that are more neutron-rich. Importantly, the unbound ejected mass is smaller than in three-species simulations, and, because of its comparatively smaller temperature and proton fraction, it can enhance lanthanide production and reduce the overproduction of light r -process elements for softer equations of state. This finding underlines the importance of muonic interactions and five neutrino species in long-lived BNS remnants.