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.

💡 Research Summary

The paper addresses the problem of invisible neutron decay, which in the Standard Model Effective Field Theory (SMEFT) can only be generated by a dimension‑12 operator, making the process extremely suppressed and experimentally inaccessible. Moreover, lower‑dimensional operators that induce neutron decay typically also produce charged‑lepton final states, which are already tightly constrained. To obtain observable invisible neutron decay without accompanying charged‑lepton modes, the authors explore extensions of the SM that introduce light neutral degrees of freedom. Four concrete scenarios are studied: (i) adding at least two right‑handed neutrinos (sterile neutrinos, N_R); (ii) adding a right‑handed neutrino together with an axion‑like particle (ALP); (iii) adding a right‑handed neutrino and a nearly massless singlet scalar φ; and (iv) adding a right‑handed neutrino and a light vector boson Z′.

For each case the authors construct the lowest‑dimensional baryon‑violating operators that mediate invisible neutron decay: dimension‑9 operators for the N_R‑only case, dimension‑8 operators for the N_R+ALP case, and dimension‑7 operators for the N_R+φ and N_R+Z′ cases. They provide explicit forms of these operators, discuss their quantum number assignments, and estimate the neutron‑decay half‑life as a function of the new physics scale Λ. Typical estimates show that Λ of order 10^2–10^3 TeV can yield decay lifetimes near current experimental limits (∼10^29 yr), while the ALP scenario requires a much higher scale (∼10^8 GeV) due to the derivative coupling of the ALP.

The paper then systematically enumerates all possible tree‑level ultraviolet (UV) completions that generate the effective operators. These UV models involve new scalar, fermion, or vector mediators with specific gauge and flavor quantum numbers. The authors present a “black‑box theorem” for ΔL = 3 operators, showing under which symmetry conditions lower‑dimensional operators can be forbidden while allowing the desired higher‑dimensional ones.

Phenomenological implications at the Large Hadron Collider (LHC) are examined. The new mediators can be produced as resonances or in cascade decays, leading to signatures such as multi‑jet plus missing energy, displaced vertices, or dilepton resonances. Existing LHC searches are recast to set bounds on the masses of the mediators, typically in the TeV range for colored scalars or fermions, and in the sub‑TeV range for light Z′ bosons.

Finally, the authors discuss the prospects of upcoming underground detectors (JUNO, THEIA, Hyper‑Kamiokande, DUNE) which aim to improve neutron‑decay lifetime limits by one to two orders of magnitude. Such improvements would probe Λ scales up to several hundred TeV for the dimension‑7 and dimension‑9 scenarios, making invisible neutron decay a powerful indirect probe of light sterile neutrinos, ALPs, ultra‑light scalars, or dark photons.

In summary, the work provides a comprehensive theoretical framework linking invisible neutron decay to a variety of light BSM particles, supplies the effective operators and their UV origins, evaluates current experimental constraints, and outlines the discovery potential of future experiments.