Two astroparticle physics problems: Solar neutrinos and primordial 4He

In this dissertation two astrophysical environments are used to constrain possible extensions of the standard model of particle physics. The first environment considered is the sun, the second is the big bang. With the sun as a neutrino source, neutrino masses as small as 10-6 eV, and mixing angles as small as θu ≅ 10-4, can be probed. The measured solar neutrino fluxes of the Davis experiment, Kamiokande, Gallex and Sage and the predictions of standard solar models is used to investigate neutrino oscillation solutions to the solar neutrino problem, and thus constrain neutrino mass matrix elements. The author considers flavor and sterile open-quotes just-soclose quotes and MSW neutrino oscillation solutions to the solar neutrino problem. The author shows which commonly used approximations are valid in the context of calculating numerically the neutrino oscillation solutions. This thesis demonstrates that the Kamiokande experiment was not sensitive to the semi-annual variations predicted by just-so oscillation theory, and that the proposed Borexino experiment has a promising chance to see the semi-annual variations. The solutions for the just-so and MSW scenarios are shown to be largely constrained by the Davis experiment. For sterile MSW the author predicts the event rate in the planned Sudbury Neutrino Observatory neutral current experiment. The big bang nucleosynthesis prediction for 4He is sensitive to additional contributions to the energy density of the universe when the temperature is order 1 MeV. Using observational upper bounds on D + 3He and 4He the number of light degrees of freedom can be constrained. This thesis considers the following in determining the 3rd significant figure for the predicted 4He abundance: improved numerical integration of the nuclear abundances, order α (coulomb, radiative and finite temperature) and M-1N corrections to the weak rates