Oscillation effects on neutrinos from the early phase of a nearby supernova

Recent observations of atmospheric and solar neutrinos strongly support the phenomenon of neutrino oscillations - a manifestation of a nonzero and non degenerate mass spectrum. Neutrinos emitted during stellar core collapse le ading to a supernova are of the electron neutrino type at source - as for s olar and reactor (anti-)neutrinos and provide another useful tool in the se arch for flavor oscillations. Their propagation to an earth-bound detector involves length scales that can uniquely probe very small neutrino mass dif ferences hitherto unobservable. Although the number of neutrinos emitted du ring the collapse phase is much smaller than that emitted in the post-bounc e epoch tin which all flavors of neutrinos are emitted), a nearby supernova event may nevertheless register a substantial number of detections from th e collapse phase at SuperKamiokande (SK) and the Sudbury Neutrino Observato ry (SNO). The measurement of the fluence of these neutrinos at SNO and the distortion of the spectrum detected at SK can yield valuable information ab out neutrino mass difference and mixing which are illustrated here in terms of two- and three-flavor oscillation models. In particular, we find that R -SNO, the ratio of the calorimetric detection of the neutrino fluence via t he neutral current channel to the total energy integrated fluence observed via the charged current channel at SNO, is a sensitive probe for oscillatio ns. We also find that ai,, the ratio of the nth central moments of the dist ributions seen at SK and SNO (charged current), can be a useful tool (espec ially for n = 3) to look for neutrino oscillations.