A High Sensitivity Search for $\bar{\nu}_{e}$'s from the Sun and Other Sources at KamLAND

Data corresponding to a KamLAND detector exposure of 0.28 kton-year has been used to search for $\bar{\nu}_e$'s in the energy range 8.3 MeV $<$ E$_{\bar{\nu}_e}$ $<$ 14.8 MeV. 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 $\bar{\nu}_{e}$ fluxes of any origin. Assuming that all $\bar{\nu}_e$ flux has its origin in the Sun and has the characteristic $^8$B solar $\nu_e$ energy spectrum, we obtain an upper limit of 3.7 \times 10$^2$ cm$^{-2}$ s$^{-1}$ (90% C.L.) on the $\bar{\nu}_e$ flux. We interpret this limit, corresponding to $2.8{\times}10^{-4}$ of the Standard Solar Model $^8$B $\nu_e$ flux, in the framework of spin--flavor precession and neutrino decay models.