{
  "abstract": "The EMPRESS collaboration has reported a primordial helium mass fraction \\(Y_P = 0.2402 \\pm 0.0040\\), which lies more than \\(1σ\\) below the standard Big Bang Nucleosynthesis prediction. Combined with the deuterium abundance and the Planck baryon density, this corresponds to a mild \\(\\sim 2σ\\) tension with the standard value of the effective number of relativistic species, or to a \\(\\sim 2σ\\) indication of a non-zero electron-neutrino degeneracy. We show that it can be explained by non-unitary three-neutrino mixing, a generic feature of seesaw extensions of the Standard Model. Non-unitarity makes the Fermi constant in beta decay exceed that in muon decay, enhancing the neutron-to-proton conversion rate and lowering \\(Y_P\\). A full numerical BBN calculation, combined with the EMPRESS \\(Y_P\\) determination and the deuterium abundance gives the best fit point at \\(α_{22}=0.982\\), while the standard values \\(N_{\\mathrm{eff}}=3.044\\), \\(ξ_e=0\\) remain allowed; the required level of non-unitarity is consistent with current experimental constraints.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2610.29069",
  "author_total": 2,
  "authors": [
    {
      "name": "Shu-Yuan Guo"
    },
    {
      "name": "Jia-Qi Li"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/0wypn7nd0hgw8.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2610.29069",
  "fulltext_url": null,
  "id": "0wypn7nd0hgw8",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2610.29069",
  "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-ph"
    ],
    [
      "openalex",
      "3106|Neutrino Physics Research"
    ]
  ],
  "openalex": "W7223213394",
  "pdf_url": "https://arxiv.org/pdf/2610.29069",
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-21",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/0wypn7nd0hgw8.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:34:19Z",
      "native_id": "2610.29069",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2610.29069"
    }
  ],
  "subjects": [
    {
      "field": "physics",
      "origin": "arxiv:hep-ph",
      "subfield": "high-energy"
    }
  ],
  "title": "Low Primordial Helium as a Probe of Non-Unitary Neutrino Mixing",
  "title_text": "Low Primordial Helium as a Probe of Non-Unitary Neutrino Mixing",
  "type": "preprint",
  "updated_at": "2026-10-30T12:34:19Z",
  "url": "https://research.draconitic.com/paper/0wypn7nd0hgw8",
  "venue": "arXiv",
  "versions": []
}