High sensitivity search for (ν)over-bare's from the Sun and other sources at KamLAND -: art. no. 071301

Data corresponding to a KamLAND detector exposure of 0.28 kton yr has been used to search for (v) over bar (e)'s in the energy range 8.3<E-(v) over bare<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 (v) over bar (e) fluxes of any origin. Assuming that all (v) over bar (e) flux has its origin in the Sun and has the characteristic B-8 solar nu(e) energy spectrum, we obtain an upper limit of 3.7x10(2) cm(-2) s(-1) (90% C.L.) on the (v) over bar (e) flux. We interpret this limit, corresponding to 2.8x10(-4) of the standard solar model B-8 nu(e) flux, in the framework of spin-flavor precession and neutrino decay models.