Search for multitrack nucleon decay

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) x 10/sup 31/ yr for 23 decay channels.