Search for the proton decay mode p → [bar over v]K[superscript +] with KamLAND

Abstract

We present a search for the proton decay mode p → [bar over ν]K[superscript +] based on an exposure of 8.97 kton-years in the KamLAND experiment. The liquid scintillator detector is sensitive to successive signals from p → [bar over ν]K[superscript +] 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 → [bar over ν]K[superscript +]) > 5.4 × 10[superscript 32] years at 90% C.L.