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Accepted · first revision · record

Title: Is there a roughly one percent exotic neutron decay?
Plain: 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: 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 $V_{ud}$.
Settled by: Agreement of a sub-second beam lifetime with the bottle value, or direct detection of the invisible branch.
Kind: well-posed
Answer type: yes-no
References: (empty)
Literature status: partially-resolved
Status 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.
Posed since: 2018