Search for Nucleon Decay vian→ν¯π0andp→ν¯π+in Super-Kamiokande

2Although there is strong theoretical support that na-ture can be described by a grand uni ed theory (GUT) [1,2], there is currently no direct experimental evidence.One of the most powerful ways to test grand uni cationis to look for proton (or bound neutron) decay. MostGUTs have an unstable proton; in the absence of anobservation, setting experimental limits on the protonlifetime can provide useful constraints on the nature ofgrand uni ed theories. Observation, on the other hand,would be tantalizing evidence of new physics beyond theStandard Model.One of the more simple but interesting candidates forgrand uni cation is SO(10), where the Standard Model’sSU(3), SU(2), and U(1) are contained within the largergauge group. The class of models based on SO(10) uni- cation generally make predictions for neutrino massesand mixing that are broadly in accord with all knownneutrino mixing data [3, 4]. The minimal supersym-metric SO(10) model with a 126 Higgs eld describedin Ref. [3] is the particular motivation for the analysispresented here. In addition to predicting neutrino massand mixing in agreement with observations, it leaves R-parity unbroken, which guarantees the existence of stabledark matter. For some region of its allowed parameterspace, this model predicts that the dominant nucleon de-cay modes will be p! ˇ