Separating double-beta decay events from solar neutrino interactions in a kiloton-scale liquid scintillator detector by fast timing

We present a technique for separating nuclear double beta decay (ββ-decay) events from background neutrino interactions due to 8B decays in the sun. This background becomes dominant in a kiloton-scale liquid-scintillator detector deep underground and is usually considered as irreducible due to an overlap in deposited energy with the signal. However, electrons from 0νββ-decay often exceed the Cherenkov threshold in liquid scintillator, producing photons that are prompt and correlated in direction with the initial electron direction. The use of large-area fast photodetectors allows some separation of these prompt photons from delayed isotropic scintillation light and, thus, the possibility of reconstructing the event topology. Using a simulation of a 6.5 m radius liquid scintillator detector with 100 ps resolution photodetectors, we show that a spherical harmonics analysis of early-arrival light can discriminate between 0νββ-decay signal and 8B solar neutrino background events on a statistical basis. Good separation will require the development of a slow scintillator with a 5 ns risetime.