Search for the proton decay mode $p \rightarrow \overline{\nu} K^{+}$ with KamLAND

TL;DR

This study searches for proton decay into a neutrino and a kaon using KamLAND's liquid scintillator, achieving higher detection efficiency but finding no evidence, thus setting a new lower limit on proton lifetime.

Contribution

First to utilize KamLAND's liquid scintillator for this decay mode, improving detection efficiency over water Cherenkov detectors and establishing a new proton lifetime limit.

Findings

No proton decay observed in the data

Detection efficiency of 44% for the decay mode

Lower limit on proton lifetime set at 5.4 x 10^{32} years

Abstract

We present a search for the proton decay mode $p \rightarrow \overline{\nu} K^{+}$ based on an exposure of 8.97 kton-years in the KamLAND experiment. The liquid scintillator detector is sensitive to successive signals from $p \rightarrow \overline{\nu} K^{+}$ 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 $0.9 \pm 0.2$ events. The nonbackground-subtracted limit on the partial proton lifetime is $\tau / B(p \rightarrow \overline{\nu} K^{+}) > 5.4 \times 10^{32}$ years at 90% C.L.