  We present the results of a search for extraterrestrial electron
antineutrinos ($\bar{\nu}_{e}$'s) in the energy range $8.3 MeV <
E_{\bar{\nu}_{e}} < 31.8 MeV$ using the KamLAND detector. In an exposure of
4.53 kton-year, we identify 25 candidate events. All of the candidate events
can be attributed to background, most importantly neutral current atmospheric
neutrino interactions, setting an upper limit on the probability of $^{8}$B
solar $\nu_{e}$'s converting into $\bar{\nu}_{e}$'s at $5.3 \times 10^{-5}$
(90% C.L.), if we assume an undistorted $\bar{\nu}_{e}$ shape. This limit
corresponds to a solar $\bar{\nu}_{e}$ flux of $93 cm^{-2} s^{-1}$ or an event
rate of $1.6 events (kton-year)^{-1}$ above the energy threshold
$(E_{\bar{\nu}_{e}} > 8.3 MeV)$. The present data also allows us to set more
stringent limits on the diffuse supernova neutrino flux and on the annihilation
rates for light dark matter particles.

High Energy Astrophysical Phenomena;High Energy Physics - Experiment;Nuclear Experiment