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

(Dated: Data corresponding to a KamLAND detector exposure of 0.28 kton-year has been used to search for ¯ ν e ’s in the energy range 8.3 MeV < 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 8 B solar ν e energy spectrum, we obtain an upper limit of 3.7 × 10 2 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 8 B ν e flux, in the framework of spin–flavor precession and neutrino decay models.