Probing the Neutrino Mass Hierarchy via Atmospheric ${\nu }_{\mu }+{\bar{\nu }}_{\mu }$ Survival Rates in Megaton Water Cerenkov Detectors

Abstract

The neutrino mass hierarchy, presently unknown, is a powerful discriminator among various classes of unification theories. We show that the ${\nu }_{\mu }+{\bar{\nu }}_{\mu }$ survival rate in atmospheric events can provide a novel method of determining the hierarchy in megaton water Cerenkov detectors. For pathlength and energy ranges relevant to atmospheric neutrinos,this rate obtains significant matter sensitive variations not only from resonant matter effects in $P_{\mu e}$ but also from those in $P_{\mu \tau }$. We calculate the expected muon event rates in the case of matter oscillations with both natural and inverted hierarchy.We identify the energy and pathlength ranges for which resonant matter effects can lead to observable differences between the above two cases. We also estimate the exposure time required to observe this difference and determine the sign of ${\Delta }_{31}$ in a statistically significant manner.

Comments: 18 pages, 4 figures. Sigma sensitivity to the mass hierarchy has been recalculated using the difference in expected events in matter for normal and inverted hierarchies. In addition to absolute event rates, sensitivities for up/down ratios have also been calculated. Figures and tables updated to incorporate the new results. Substantial rewriting of text with clarifying comments. References added