{
  "abstract": "Weak magnetism, introduced by Gell-Mann as a hallmark consequence of the conserved vector current (CVC) hypothesis, has remained unobserved in the decay of the free neutron, the simplest $β$-decay system. The reason is that the relevant observable, the $β$ asymmetry, cannot separate the weak-magnetism form factor $g_M$ from the axial-to-vector ratio $λ$. We show that this degeneracy is lifted by combining the $β$ asymmetry measurements with the neutron lifetime and the $\\mathcal{F}t$ value of superallowed $0^+\\to0^+$ decays. Current data yield $g_M = 4.47(87)$, in agreement with the CVC prediction of $4.71$ and excluding the absence of weak magnetism at the $5.1σ$ level. Importantly, the significance remains between $5.0σ$ and $6.1σ$ for all considered variations of the neutron dataset, despite its well-known internal tensions. We further show that, under present experimental conditions, a full analysis of the energy dependence of the asymmetry would improve the $g_M$ extraction by only a few percent. These results constitute the first observation of weak magnetism in the decay of the free neutron nearly seven decades after Gell-Mann's prediction.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2610.26792",
  "author_total": 3,
  "authors": [
    {
      "name": "Miriam Camarasa-Domene"
    },
    {
      "name": "Martín González-Alonso"
    },
    {
      "name": "Martín Novoa-Brunet"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/2mtc507d1x63a.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2610.26792",
  "fulltext_url": null,
  "id": "2mtc507d1x63a",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2610.26792",
  "language": null,
  "licence": "CC-BY-4.0",
  "licence_label": "CC BY 4.0",
  "licence_source": "article",
  "licence_url": "http://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "hep-ex"
    ],
    [
      "arxiv",
      "hep-ph"
    ],
    [
      "arxiv",
      "nucl-ex"
    ],
    [
      "arxiv",
      "nucl-th"
    ],
    [
      "openalex",
      "3106|Particle physics theoretical and experimental studies"
    ]
  ],
  "openalex": "W7223009916",
  "pdf_url": "https://arxiv.org/pdf/2610.26792",
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-19",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/2mtc507d1x63a.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:30:52Z",
      "native_id": "2610.26792",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2610.26792"
    }
  ],
  "subjects": [
    {
      "field": "physics",
      "origin": "arxiv:hep-ex",
      "subfield": "high-energy"
    },
    {
      "field": "physics",
      "origin": "arxiv:nucl-ex",
      "subfield": "nuclear"
    }
  ],
  "title": "First observation of weak magnetism in neutron decay",
  "title_text": "First observation of weak magnetism in neutron decay",
  "type": "preprint",
  "updated_at": "2026-10-30T12:33:41Z",
  "url": "https://research.draconitic.com/paper/2mtc507d1x63a",
  "venue": "arXiv",
  "versions": []
}