A search for dinucleon decay into kaons using the Super-Kamiokande water Cherenkov detector

Dinucleon decay is the simultaneous decay of two nucleons bound within the same nucleus. This dissertation describes a search for dinucleon decay into kaons via 16O(pp) → 14C K+ K+ using the first 91.7 kiloton · years of data collected by the Super-Kamiokande water Cherenkov detector. This is an interesting alternative to single proton decay because the process violates only baryon number, not lepton number; single proton decay violates both. In a supersymmetric framework, dinucleon decay into kaons is predicted to be the most sensitive experimental search that can probe the magnitude of the R-parity violating parameter l''uds .;The complex signature of the signal required the creation of several new event reconstruction techniques, including tools for fitting Cherenkov rings produced by charged kaons, a multiple vertex fitter for assigning a unique vertex to each ring, and a ring classification algorithm that incorporated the overall event topology. Another technique new to the Super-Kamiokande collaboration was the use of a boosted decision tree, which was utilized in the final stage of the analysis.;No signal event candidates were found in the data. Monte Carlo studies predicted a background of 0.28 +/- 0.19 events induced by atmospheric neutrino interactions, and a signal detection efficiency of 12.6% +/- 3.2%. Based on these results, the first experimental lower limit on the dinucleon partial lifetime for the decay mode pp → K + K+ has been placed at 1.7 x 1032 years at 90% confidence level. This is about two orders of magnitude better than the only other dinucleon lifetime limits on record, made by the Frejus experiment, which include only decays with final state pions or leptons.;The lifetime limit may be interpreted as an upper limit of 7.8 x 10-9 on l''uds . This new experimental limit is more restrictive than the limit of ∼ 10-7 estimated by J.L. Goity and M. Sher in their paper "Bounds on DeltaB = 1 couplings in the supersymmetric standard model" (Phys. Lett. B 343:1-2, 1995), which was based on typical nucleon and dinucleon lifetime limits of ∼ 1030 years.