Physics
Scientific paper
Jan 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993phdt.........6q&link_type=abstract
PhD Dissertation, California Univ. San Diego, CA United States
Physics
7
Flavor (Particle Physics), Cosmology, Neutrinos, Supernovae, Antineutrinos, Neutron Stars, Nuclear Fusion, Stellar Winds, Baryons
Scientific paper
Massive neutrinos are interesting in cosmology, astrophysics, and particle physics. The current experimental upper limits on neutrino masses are very loose (m(nue) is less than 7.2 eV, m(numu) is less than 250 keV, and m(nutau) is less than 31 MeV). The existence of cosmologically significant neutrinos (mnu = 1-100 eV) has to be tested by other means. Once neutrinos have mass, mixing between different neutrino flavors occurs. In particular, this mixing can be greatly enhanced by the Mikheyev-Smirnov-Wolfenstein (MSW) mechanism when neutrinos are propagating through matter with appropriate densities. The MSW mechanism can be operating for cosmologically significant neutrinos in core-collapse driven supernovae, where the central part of a massive star evolves into a proto-neutron star, releasing about 1053 erg in all three flavors of neutrinos and anti-neutrinos. Because of their different interactions with matter, numu, nutau, and their anti-neutrinos decouple deeper in the core and therefore have higher average energies than nue and -nue. For the late time supernova mechanism, which depends on the energy deposit by nue and -nue absorption on free nucleons above the proto-neutron star to revive the stalled shock, mixing between a cosmologically significant nu(su mu) or nutau and nue will increase the energy deposit rate, and may help bridge the gap between the simulated and the observed explosion energy. After the stalled shock gets outgoing again, a high-entropy, evacuated region above the proto-neutron star is created by the previous and still on-going neutrino heating. Later, the expansion of this region develops into a neutrino-driven wind. And this wind is a promising site for the r-process nucleosynthesis of heavy elements. The outcome of the r-process nucleosynthesis depends on three physical quantities baryon, the neutron-to-proton ratio, and the dynamical timescale. All three quantities are closely related to the neutrino processes involved in the formation of the wind. The most important neutrino processes in this regard are nue and -nue absorption on free nucleons. The physics of the neutrino-driven wind is studied, with all the important neutrino processes and the neutrino flavor-mixing effects taken into account. The evolution of the electron fraction Ye (the net number of electrons per baryon, an alternate way to indicate the neutron-richness other than the neutron-to-proton ratio) is discussed in detail, leading to a probe of mixing properties of cosmologically significant neutrinos. Neutrino flavor-mixing effects on detection of a galactic supernova neutrino burst are also discussed, and a possible signature of supernova neutrino flavor-mixing in water Cerenkov detectors is suggested. One of the appendices deals with Fermi-Dirac integrals encountered in evaluating certain neutrino processes and the equations of state for a general electron-positron gas.
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