Neutron lifetime and low-energy nuclear anomalies

7 problems

• Why beam and bottle neutron lifetimes disagree

  Open

  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.

• Is there a roughly one percent exotic neutron decay?

  Partially resolved

  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.

• Tension between neutron-decay determinations of $g_A/g_V$

  Open

  The ratio $\lambda$ of the two weak couplings in neutron decay can be read from the electron's preferred direction relative to the neutron spin, or from the angle between the electron and the antineutrino. The two most precise such measurements differ by about 3 standard deviations.

• Nuclear-structure corrections in superallowed beta decays

  Open

  Precise decay rates of certain nuclei (superallowed 0+ to 0+ decays) give the strength of the weak coupling between up and down quarks, but only after small nuclear corrections are subtracted. A small deficit that may signal new physics depends on these corrections.

• Origin of the ATOMKI 17 MeV pair-emission anomaly

  Contested

  A group in Hungary (ATOMKI) saw too many electron-positron pairs emitted at large angles from excited beryllium, helium and carbon nuclei, as if a new particle of about 17 MeV were produced. Other experiments have not confirmed it.

• Existence of a 17 MeV boson coupled to electrons

  Contested

  If the X17 particle exists, it should also appear when positrons hit electrons at just the right energy. Dedicated experiments are scanning this energy.

• Can standard nuclear physics produce the ATOMKI angular excess?

  Open

  The extra pairs might come from ordinary nuclear effects, such as interference between different ways an excited nucleus emits a pair, or from the detector response. A complete calculation is needed to decide.