Search for Extraterrestrial Antineutrino Sources with the KamLAND Detector

We present the results of a search for extraterrestrial electron antineutrinos ((nu) over bar (e)'s) in the energy range 8.3 MeV E-<(nu) over bare < 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 B-8 solar nu(e)'s converting into nu(e)'s at 5.3 x 10(-5) (90% CL), if we assume an undistorted nu(e) shape. This limit corresponds to a solar 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-<(nu)over bar>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.