Search for the proton decay mode p -> ¯νK+ with KamLAND

We present a search for the proton decay mode p→ν¯K+ based on an exposure of 8.97 kton-years in the KamLAND experiment. The liquid scintillator detector is sensitive to successive signals from p→ν¯K+ with unique kinematics, which allow us to achieve a detection efficiency of 44%, higher than previous searches in water Cherenkov detectors. We find no evidence of proton decays for this mode. The expected background, which is dominated by atmospheric neutrinos, is 0.9±0.2 events. The nonbackground-subtracted limit on the partial proton lifetime is τ/B(p→ν¯K+)>5.4×1032 years at 90% C.L.