Study of invisible nucleon decay, n→νν, and a forbidden nuclear transition in the Kamiokande detector

A search was made for anomalous γ-rays in the energy range 19 and 50 MeV in the large underground water Cherenkov detector Kamiokande. We found no significant excess above the known backgrounds. The result was used to set a limit on invisible nucleon decay through n→ν μ ν μ ν μ , n→ν e ν e ν e and n→ν τ ν τ ν τ . A lower limit on the partial life time τB is given by τB/Br(γ)=1.8×1031 yr(90% CL), where Br(γ)=Γγ/Γtot is the branching fraction of the γ-decay to the particle emission of the deexcitation of 15O∗. The life time limit becomes τB=4.9×1026 yr(90% CL) for a conservative estimate of Br(γ)=2.7×10−5 assuming a nuclear single particle model. The result was also used for studying forbidden transitions in the 16O nucleus. The limit for the relative strength of the forbidden transition to the normal transition is