Search for Multitrack Nucleon Decay

Abstract

We report new limits on the nucleon lifetime obtained with the Irvine-Michigan-Brookhaven water Cherenkov detector. Techniques in track fitting, particle identification, and invariant-mass reconstruction provide information about multitrack events which leads to a significant reduction in background for most decay modes. Data obtained during the initial 417-day run and during the subsequent 69-day run with enhanced light collection are discussed. A background subtraction is made, yielding 90%-confidence-level lower bounds on the nucleon partial lifetime per branching fraction in the range (0.6-46.0)×${10}^{31}$ yr for 23 decay channels.