Search for the proton decay mode

We present a search for the proton decay mode p → ¯ ν K þ 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 þ 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 (cid:2) 0 . 2 events. The nonbackground-subtracted limit on the partial proton lifetime is τ =B ð p → ¯ ν K þ Þ > 5 . 4 × 10 32 years at 90% C.L.