Date of Award

Winter 1995

Project Type

Dissertation

Program or Major

Physics

Degree Name

Doctor of Philosophy

First Advisor

J M Ryan

Abstract

We have performed a sensitive search for a massive neutrino species. The premise of the search is the existence of an unstable massive neutrino that decays radiatively. Because of its electromagnetic daughter product, a radiative decay may be the only mode that can be observed directly. Using core collapse supernovae--known to be copious producers of neutrinos--as a cosmic neutrino laboratory, a detailed model has been developed to predict the decay-produced photon energy spectrum as a function of neutrino mass $m\sb{\nu}$ and lifetime $\tau\sb{\nu}$. Using the COMPTEL instrument aboard the Compton Gamma-Ray Observatory, observations of two recent nearby type-II supernovae (SN1987A, SN1993J) were made to search for the characteristic photon emission in the MeV energy range.

This search was sensitive to neutrino masses $m\sb{\nu}\ >$ 100 eV, lifetimes $\sim$10$\sp5\ \le\ \tau\sb{\nu} \le$ 10$\sp{15}$ seconds, and radiative branching ratios $B\sb{\gamma} \ge$ 10$\sp{-5}.$ We have found no evidence for a radiative decay mode and thus exclude a new region of $m\sb{\nu}$/$\tau\sb{\nu}$/$B\sb{\gamma}$-parameter space. In the context of the neutrino decay hypothesis, the $\gamma$-ray observation of SN1987A made with the Gamma-Ray Spectrometer aboard the Solar Maximum Mission satellite has been re-analyzed.

A complimentary analysis was made to determine the potential contribution an isotropic sea of radiatively decaying neutrinos might make to the cosmic diffuse $\gamma$-ray flux, specifically to the so-called MeV-bump. Two possible sources were considered here: the sea of neutrinos produced by the continuous generation of supernovae in the Universe, and the flux of relic neutrinos created in the Big Bang. A predictive model was developed for both cases. In the supernova scenario, the neutrino decay emission was parameterized as a function of supernova rate and the epoch of galaxy formation; while in the case of relic neutrinos the photon flux was parameterized as a function of the relic neutrino abundance. Cosmological factors were included in both models. The predicted emission (spectral shape and flux level) from neutrino decay was compared to the measurements of the cosmic flux of $\gamma$-rays, for a wide range of neutrino mass and lifetime. The spectra from both the supernova and relic scenarios were inconsistent with the cosmic diffuse measurements in the range 0.1 to 30 MeV, leading to a conservative limit $B\sb{\gamma}\ \le$ 10$\sp{-1}.$.

The predicted emission from relic neutrino decay was found to be consistent with the feature known as the MeV-bump--for a narrow range of $m\sb{\nu}$ and $\tau\sb{\nu}$. This range of parameter values, however, is excluded by mass density arguments and the assumption of an $\Omega$ = 1 Universe.

This work represents a systematic search for massive neutrinos using an astrophysical source other than our Sun. Although negative results are obtained from this search, the observational results are used to constrain fundamental particle, astrophysical, and cosmological phenomena.

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