High Sensitivity Search forν¯e’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 ${\overline{\nu }}_e$’s in the energy range $8.3<E_{{\overline{\nu }}_e}<14.8\mathrm{M}\mathrm{e}\mathrm{V}$. No candidates were found for an expected background of $1.1\pm 0.4$ events. This result can be used to obtain a limit on ${\overline{\nu }}_e$ fluxes of any origin. Assuming that all ${\overline{\nu }}_e$ flux has its origin in the Sun and has the characteristic ${}^8\mathrm{B}$ solar ${\nu }_e$ energy spectrum, we obtain an upper limit of $3.7\times {10}^2{\mathrm{c}\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$ (90% C.L.) on the ${\overline{\nu }}_e$ flux. We interpret this limit, corresponding to $2.8\times {10}^{-4}$ of the standard solar model ${}^8\mathrm{B}$ ${\nu }_e$ flux, in the framework of spin-flavor precession and neutrino decay models.