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Why beam and bottle neutron lifetimes disagree

In plain words

Trapping ultracold neutrons in a bottle and counting survivors gives 877.8 seconds; counting decay protons from a neutron beam gives 887.7 seconds. The gap of about 4 standard deviations has no accepted explanation.

Precise statement

Bottle (storage) measurements give $\tau_n = 877.82 \pm 0.22\,(\mathrm{stat}) +0.20/-0.17\,(\mathrm{sys})\,\mathrm{s}$ (UCNtau, arXiv 2409.05560, 2024); proton-counting beam measurements give $887.7 \pm 1.2\,(\mathrm{stat}) \pm 1.9\,(\mathrm{sys})\,\mathrm{s}$ (NIST, 2013). Identify the cause: an unaccounted systematic in either method (proton detection or neutron-flux calibration in beams, unrecognized loss channels in bottles) or a non-standard decay channel. Answer: the identified effect, demonstrated by measurements that bring the methods into agreement or confirm the gap at higher precision.

What would settle it

A next-generation proton-counting beam measurement (BL3) with sub-second uncertainty, plus independent electron-counting and space-based measurements.

Status in the literature

Unverified note

A J-PARC beam measurement counting decay electrons gave 877.2 +/- 1.7 (stat) +4.0/-3.6 (sys) s (arXiv 2412.19519, 2024), consistent with bottles and 2.3 standard deviations from the proton-counting beam average.