Electron Neutrino Pair Annihilation: A New Source for Muon and Tau Neutrinos in Supernovae

We show that in a supernova core the annihilation process nu_e nu_e-bar -> numu,tau numu,tau-bar is always more important than the traditional reaction e^+ e^- -> numu,tau numu,tau-bar as a source for muon and tau neutrino pairs. We study the impact of the new process by means of a Monte Carlo transport code with a static stellar background model and by means of a self-consistent hydrodynamical simulation with Boltzmann neutrino transport. Nucleon bremsstrahlung NN -> NN numu,tau numu,tau-bar is also included as another important source term. Taking into account nu_e nu_e-bar -> numu,tau numu,tau-bar increases the nu_mu and nu_tau luminosities by as much as 20% while the spectra remain almost unaffected. In our hydrodynamical simulation the shock was somewhat weakened. Elastic numu,tau nu_e and numu,tau nu_e scattering is not negligible but less important than numu,tau e^+ or e^- scattering. Its influence on the numu,tau fluxes and spectra is small after all other processes have been included.