Search for the Invisible Decay of Neutrons with KamLAND

The Kamioka Liquid scintillator Anti-Neutrino Detector is used in a search for single neutron or two-neutron intranuclear disappearance that would produce holes in the $s$-shell energy level of ${}^{12}\mathrm{C}$ nuclei. Such holes could be created as a result of nucleon decay into invisible modes (inv), e.g., $n\to 3\nu$ or $nn\to 2\nu$. The deexcitation of the corresponding daughter nucleus results in a sequence of space and time-correlated events observable in the liquid scintillator detector. We report on new limits for one- and two-neutron disappearance: $\tau (n\to \mathrm{inv})>5.8\times {10}^{29}$ years and $\tau (nn\to \mathrm{inv})>1.4\times {10}^{30}$ years at 90% C.L. These results represent an improvement of factors of $\sim 3$ and $>{10}^4$ over previous experiments.

Figure 1

$n$ disappearance search: (a) correlated triple background events in the 150–1000 s off-time window for the 3rd hit inside the 5.5 m detector radius; (b)–(d) events inside the 5 m fiducial volume and outside of the 1 m cylinder cut around the central axis are compared to the expected signal for $\tau =4\times {10}^{27}\mathrm{y}$, generated with the Monte Carlo simulation. Dashed lines indicate cuts used in the data analysis.

Figure 2

$nn$ disappearance search: (a) correlated triple background events in the 10–1000 s off-time window for the 3rd hit inside 6 m detector radius; (b)–(d) events inside the 5.5 m fiducial volume and outside of the 1 m cylinder cut around the central axis are compared to the expected signal for $\tau ={10}^{28}\mathrm{y}$, generated with the Monte Carlo simulation. Dashed lines indicate cuts used in the data analysis.

Figure 3

KamLAND Am-Be data compared to the GEANT detector simulation: (a) prompt energy signal; (b) diffusion of the 2.22 MeV delayed $\gamma$ ray from neutron capture on proton.