Existing Grand Unified Theories predict a free proton lifetime which would be experimentally accessible. If discovered, a study of the branching modes of the proton could provide indicators to the correct theory. The lifetime of a proton is probably longer in a nucleus, though by no more than an order of magnitude.
A search for nucleon decay was performed for 37 possible branching modes of nucleons in water, and 14 branching modes of free protons from the hydrogen in water. The data was taken from the I.M.B. water Cherenkov detector, which contains 3,300 metric tons in the fiducial volume. The only significant background to proton decay comes from atmospheric neutrino interactions. An initial search used the visible energy, anisotropy, and number of muon decays of events. A more sophisticated search automatically selected events with two clear tracks of opening angle > 115°. The invariant mass and momentum of these events were calculated for 16 nucleon decay hypotheses.
In 417 livedays, 326 events were reconstructed in the fiducial volume at a rate, and with visible characteristics consistent with atmospheric neutrino interactions. No significant excess of events was found for any nucleon decay mode. For the visible energy and anisotropy analysis, partial lifetime limits at the 90% confidence level were set in the range 1031-32 years for nucleons, assuming that the free and bound lifetimes are similar. With the free proton decay analysis, limits were set in the range 1030-32 years. The number of clean two-prong events was found to be 4.0 ± 1.1%. The background estimates gave a mean estimate of 4.1 ± 0.3%. Partial lifetime limits at the 90% confidence level were set for both free protons, and bound plus free nucleons at 1032 years for N → lepton + γ/π, and 1031 years for N → lepton + η/ρ/ω.
A framework for converting the results into model-dependent total lifetime limits is described, and limits for SU(5) are explicitly calculated. Our results imply an SU(5) model-dependent limit of 4 x 1031 years (90% C.L.) on the total proton lifetime, where effects of the nucleus on the decay rate have been accounted for. This is incompatible with theoretical predictions. The derived limits should also provide useful constraints on other Grand Unified Theories.
