Search for invisible modes of nucleon decay in water with the SNO+ detector

This paper reports results from a search for nucleon decay through ‘invisible’ modes, where no visible energy is directly deposited during the decay itself, during the initial water phase of SNO+. However, such decays within the oxygen nucleus would produce an excited daughter that would subsequently de-excite, often emitting detectable gamma rays. A search for such gamma rays yields limits of $2.5\times {10}^{29}$ y at 90% Bayesian credibility level (with a prior uniform in rate) for the partial lifetime of the neutron, and $3.6\times {10}^{29}$ y for the partial lifetime of the proton, the latter a 70% improvement on the previous limit from SNO. We also present partial lifetime limits for invisible dinucleon modes of $1.3\times {10}^{28}$ y for $nn$, $2.6\times {10}^{28}$ y for $pn$ and $4.7\times {10}^{28}$ y for $pp$, an improvement over existing limits by close to three orders of magnitude for the latter two.