Experimental Limits on the Free-Proton Lifetime for Two- and Three-Body Decay Modes

Lower partial-lifetime limits for the decay of the free proton are obtained in the range 10³⁰ to 10³² yr, from a 417-d exposure of a water Cherenkov detector with a fiducial mass of 370 metric tons of hydrogen. In this time, 401 contained events were observed, at a rate consistent with atmospheric neutrino interactions. Eleven of these events are compatible with both neutrino interactions and the neutrinoless decay of the free proton. These results are of the same order of magnitude as previously published limits for bound plus free protons, but are independent of models for proton decay in oxygen.