Signatures of nucleon disappearance in large underground detectors

This is the authors' abstract. We don't add key points for this paper.

For neutrons bound inside nuclei, baryon instability can manifest itself as a decay into undetectable particles (e.g., →nννν), i.e., as a disappearance of a neutron from its nuclear state. If electric charge is conserved, a similar disappearance is impossible for a proton. The existing experimental lifetime limit for neutron disappearance is 4--7 orders of magnitude lower than the lifetime limits with detectable nucleon decay products in the final state. In this paper we calculated the spectrum of nuclear de-excitations that would result from the disappearance of a neutron or two neutrons from ¹²C. We found that some de-excitation modes have signatures that are advantageous for detection in the modern high-mass, low-background, and low-threshold underground detectors, where neutron disappearance would result in a characteristic sequence of time- and space-correlated events. Thus, in the KamLAND detector, a time-correlated triple coincidence of a prompt signal, a captured neutron, and a β⁺ decay of the residual nucleus, all originating from the same point in the detector, will be a unique signal of neutron disappearance allowing searches for baryon instability with sensitivity 3--4 orders of magnitude beyond the present experimental limits.