How much do neutrinos live and weigh?

TL;DR

This paper explores how future large neutrino detectors can set improved bounds on neutrino mass and lifetime by analyzing supernova neutrino signals, including time delays and flux suppression effects.

Contribution

It demonstrates the potential of Hyper-Kamiokande to significantly improve bounds on neutrino properties using supernova neutrino data, especially considering elastic scattering events.

Findings

Lower bound on neutrino lifetime: τ/ m > 4×10^5 s/eV at 95% CL.

Upper bound on neutrino mass: 0.34 eV at 95% CL.

Mass and lifetime bounds are independent of mass ordering.

Abstract

The next-generation water Cherenkov Hyper-Kamiokande detector will be able to detect thousands of neutrino events from a galactic Supernova explosion via Inverse Beta Decay processes followed by neutron capture on Gadolinium. This superb statistics provides a unique window to set bounds on neutrino properties, as its mass and lifetime. We shall explore the capabilities of such a future detector, constraining the former two properties via the time delay and the flux suppression induced in the Supernovae neutrino time and energy spectra. Special attention will be devoted to the statistically sub-dominant elastic scattering induced events, normally neglected, which can substantially improve the neutrino mass bound via time delays. When allowing for a invisible decaying scenario, the $95\%~$C.L. lower bound on $\tau/m$ is almost one order of magnitude better than the one found with SN1987A neutrino events. Simultaneous limits can be set on both $m_\nu$ and $\tau_{\nu}$, combining the neutrino flux suppression with the time-delay signature: the best constrained lifetime is that of $\nu_1$, which has the richest electronic component. We find $\tau_{\nu_1}\gtrsim 4\times 10^5~$s at $95\%~$C.L. The tightest $95\%~$C.L. bound on the neutrino mass we find is $0.34~$eV, which is not only competitive with the tightest neutrino mass limits nowadays, but also comparable to future laboratory direct mass searches. Both mass and lifetime limits are independent on the mass ordering, which makes our results very robust and relevant.