High Sensitivity Search for ν¯e's from the Sun and Other Sources at KamLAND

Data corresponding to a KamLAND detector exposure of 0.28kton yr has been used to search for ν¯_e’s in the energy range 8.3<E_ν¯e<14.8 MeV. No candidates were found for an expected background of 1.1±0.4 events. This result can be used to obtain a limit on ν¯_e fluxes of any origin. Assuming that all ν¯_e flux has its origin in the Sun and has the characteristic ⁸B solar ν_e energy spectrum, we obtain an upper limit of 3.7×10² cm^-2 s^-1 (90% C.L.) on the ν¯_e flux. We interpret this limit, corresponding to 2.8×10^-4 of the standard solar model ⁸B ν_e flux, in the framework of spin-flavor precession and neutrino decay models.