NUC In the literature: partially resolved

Is there a roughly one percent exotic neutron decay?

In plain words

If about 1 percent of neutrons decayed into invisible particles, bottles would see neutrons vanish faster than beams see protons appear, explaining the gap. Searches have found no such decay so far.

Precise statement

Test whether the neutron has an exotic branch $n \to \chi + \text{anything}$ (Fornal and Grinstein, PRL 120, 191801, 2018) with branching ratio about 1 percent and $937.9\,\mathrm{MeV} < m_{\chi} < 938.8\,\mathrm{MeV}$, as needed to explain the beam-bottle gap. Answer: yes or no; if no, a limit on any invisible branch below 0.1 percent from direct searches and from consistency of $\tau_{n}$ with $\lambda = g_{A}/g_{V}$ and $V_{ud}$.

What would settle it

Agreement of a sub-second beam lifetime with the bottle value, or direct detection of the invisible branch.

Status in the literature

Unverified note

Direct searches (2018 onward) and neutron-star mass limits exclude the simplest versions, and the Standard Model relation with $\lambda$ from PERKEO III (2019) favors the bottle value, as of 2026.