ΔB = 2 neutron decay into antiproton mode n→p¯e+ν(ν¯)

We discuss the unique baryon number violation by two units neutron decay mode n→p¯e+χ, with χ being the standard model (SM) neutrino ν or antineutrino ν¯ or any beyond the SM light fermion, in the framework of effective field theory. This mode is kinematically allowed but rarely discussed theoretically or searched for experimentally. We estimate the lower bound on its partial lifetime from that of the dinucleon decay np→e+χ per oxygen nucleus 16O set by the Super-Kamiokande experiment, with a conservative bound Γn→p¯e+χ−1>5.7×1039yrs. We also discuss its characteristic signature for the future experimental search and astrophysical implications.