Invisible neutron decay and light BSM particles

In Standard Model Effective Field Theory (SMEFT), invisible neutron decay arises from d = 12 operators. Adding new light particles to the field content of the SM, such as right-handed neutrinos, allows one to construct operators for invisible neutron decay at much lower dimensions. Observing invisible neutron decay, if nucleon decays with charged leptons remain absent, would therefore point towards the existence of new neutral degrees of freedom. Here we discuss four cases: (i) adding right-handed neutrinos to the SM; (ii) a right-handed neutrino and an axion-like particle; (iii) a right-handed neutrino and a nearly massless singlet scalar; and (iv) a right-handed neutrino and a light Z'. We give the general tree-level decomposition for the resulting d = 7-9 operators for invisible neutron decay and briefly discuss LHC searches related to the exotic states found in these UV completions.