Search for Proton Decay via p -> e+ pi0 in a Large Water Cherenkov Detector

Abstract

We have searched for proton decay via p -> e+ pi0 using data from a 25.5 kton-year exposure of the Super-Kamiokande detector. We find no candidate events with an expected background induced by atmospheric neutrinos of 0.1 events. From these data, we set a lower limit on the partial lifetime of the proton to be 1.6E33 years at a 90% confidence level.

Figures (3)

• Figure 1: The total invariant mass and total momentum distributions after criteria (A)--(E) (see text) for 3 samples: (a) $p \rightarrow e^+ \pi^0$ Monte Carlo, (b) atmospheric neutrino Monte Carlo corresponding to 900 kton$\cdot$year, (c) data corresponding to 25.5 kton$\cdot$year. The boxed region in each figure shows the criterion (F) for the $p \rightarrow e^+ \pi^0$ signal.
• Figure 2: The total invariant mass distributions of data (circles), normalized atmospheric neutrino Monte Carlo corresponding to 10 years (unshaded histogram), and $p \rightarrow e^+ \pi^0$ Monte Carlo normalized to one event (shaded histogram) which satisfy the criteria (B)--(E) (see text) and have a total reconstructed momentum $<$ 250 MeV/$c$.
• Figure 3: The event rate after each proton decay selection criterion (see text) for data (filled circles) and atmospheric neutrino Monte Carlo (empty circles). There is no event in the data after criterion F and only the 90% C.L. upper limit is shown in the last bin.