High sensitivity search for ve's from the Sun and other sources at KamLAND

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