Search for extraterrestrial antineutrino sources with the KamLAND detector

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 $93c{m}^{-2}{s}^{-1}$ or an event rate of $1.6events(kton-year{)}^{-1}$ above the energy threshold $(E_{{\bar{\nu }}_e}>8.3MeV)$. 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.