Monte Carlo Study of Supernova Neutrino Spectra Formation

Abstract

The neutrino flux and spectra formation in a supernova core is studied by using a Monte Carlo code. The dominant opacity contribution forl is elastic scattering on nucleonslN ! Nl, wherel always stands for eitherl or � � . In addition, we switch on or off a variety of processes that allow for the exchange of energy or the creation and destruction of neutrino pairs, notably nucleon bremsstrahlung NN ! NNl � � l, the pair annihilation processes e þ e � ! � l � � l ande � � e ! � ll, recoil and weak magnetism in elastic nucleon scatter- ing, elastic scattering on electronsle � ! e � � l, and elastic scattering on electron neutrinos and antineutri- nosle ! � el andl � � e ! � � el. The least important processes are neutrino-neutrino scattering and e + e � annihilation. The formation of the spectra and fluxes ofl is dominated by the nucleonic processes, i.e., bremsstrahlung and elastic scattering with recoil, but alsoee annihilation andlescattering contribute significantly. When all processes are included, the spectral shape of the emitted neutrino flux is always '' pinched,'' i.e., the width of the spectrum is smaller than that of a thermal spectrum with the same average energy. In all of our cases we find that the averagel energy exceeds the average � � e energy by only a small amount, 10% being a typical number. Weak-magnetism effects cause the opacity ofl to differ slightly from that of � � l, translating into differences of the luminosities and average energies of a few percent. Depending on the density, temperature, and composition profile, the flavor-dependent luminosities Le , Le , and Ll can mutually differ from each other by up to a factor of 2 in either direction. Subject headings: diffusion — neutrinos — scattering — supernovae: general