Search for dark matter decay of the free neutron from the UCNA experiment: <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>n</mml:mi><mml:mo>→</mml:mo><mml:mi>χ</mml:mi><mml:mo>+</mml:mo><mml:msup><mml:mi>e</mml:mi><mml:mo>+</mml:mo></mml:msup><mml:msup><mml:mi>e</mml:mi><mml:mo>−</mml:mo></mml:msup></mml:mrow></mml:math>
Bibliographic record
Abstract
It has been proposed recently that a previously unobserved neutron decay branch to a dark matter particle ($\ensuremath{\chi}$) could account for the discrepancy in the neutron lifetime observed in experiments that use two different measurement techniques. One of the possible final states discussed includes a single $\ensuremath{\chi}$ along with an ${e}^{+}{e}^{\ensuremath{-}}$ pair. We use data from the UCNA (Ultracold Neutron Asymmetry) experiment to set limits on this decay channel. Coincident electron-like events are detected with $\ensuremath{\sim}4\ensuremath{\pi}$ acceptance using a pair of detectors that observe a volume of stored ultracold neutrons. The summed kinetic energy (${E}_{{e}^{+}{e}^{\ensuremath{-}}}$) from such events is used to set limits, as a function of the $\ensuremath{\chi}$ mass, on the branching fraction for this decay channel. For $\ensuremath{\chi}$ masses consistent with resolving the neutron lifetime discrepancy, we exclude this as the dominant dark matter decay channel at $\ensuremath{\gg}5\ensuremath{\sigma}$ level for $100<{E}_{{e}^{+}{e}^{\ensuremath{-}}}<644\phantom{\rule{4.pt}{0ex}}\text{keV}$. If the $\ensuremath{\chi}+{e}^{+}{e}^{\ensuremath{-}}$ final state is not the only one, we set limits on its branching fraction of $<{10}^{\ensuremath{-}4}$ for the above ${E}_{{e}^{+}{e}^{\ensuremath{-}}}$ range at $>90%$ confidence level.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.002 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.001 |
| Bibliometrics | 0.002 | 0.002 |
| Science and technology studies | 0.002 | 0.000 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.011 | 0.002 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".