{
  "kind": "published-beta-decay-relation",
  "relation": "tau_beta * (1 + 3 * lambda^2) = constant; branch ratio BR = tau_total / tau_beta; exotic branch = 1 - BR",
  "source_version": "1802.01804v3",
  "source_url": "https://arxiv.org/pdf/1802.01804v3",
  "citation": "Andrzej Czarnecki, William J. Marciano and Alberto Sirlin (2018), Neutron Lifetime and Axial Coupling Connection, Physical Review Letters 120, 202002. DOI: 10.1103/PhysRevLett.120.202002. Numerical version: arXiv:1802.01804v3.",
  "constant_s": 5172.0,
  "constant_sigma_s": 1.1,
  "constant_location": "Equation (3); the same constant in Equation (12)",
  "assumption": "V_ud extracted from superallowed beta decays and CKM unitarity agreement are negligibly affected by the exotic new physics (stated before Equation (12)).",
  "authors_inputs": {
    "tau_total_s": 879.4,
    "tau_total_sigma_s": 0.6,
    "tau_beam_s": 888.0,
    "tau_beam_sigma_s": 2.0,
    "gA": 1.2755,
    "gA_sigma": 0.0011,
    "locations": "Abstract; Table I; Equations (8) and (9)"
  },
  "printed": {
    "puzzle_s": 8.6,
    "branch_ratio": 0.9999,
    "branch_ratio_sigma": 0.0007,
    "branch_ratio_slope": 1.3,
    "branch_ratio_location": "Equation (13): BR = 0.9999(7) + 1.30(gA - 1.2755)",
    "exotic_bound": 0.0027,
    "exotic_bound_as_printed": "< 0.27%",
    "bound_confidence": 0.95,
    "bound_convention": "one-sided Gaussian",
    "exotic_bound_location": "Equation (14), phrased as a one sided 95% CL bound",
    "seconds_of_puzzle": 2.4,
    "seconds_location": "Text after Equation (14): more than 2.4 s of the 8.6 s puzzle has less than a 5% chance",
    "gA_example": 1.268,
    "branch_ratio_at_gA_example": 0.99
  },
  "tolerances": {
    "branch_ratio": 5e-05,
    "branch_ratio_sigma": 5e-05,
    "branch_ratio_slope": 0.005,
    "exotic_bound": 5e-05,
    "seconds_of_puzzle": 0.05,
    "puzzle_s": 0.05,
    "branch_ratio_at_gA_example": 0.005,
    "truncated_example_to": 5e-05,
    "reason": "Half a unit in the last printed digit of each printed value."
  },
  "truncation": {
    "citation": "D. Dubbers, H. Saul, B. Märkisch, T. Soldner and H. Abele (2019), Exotic decay channels are not the cause of the neutron lifetime anomaly, Physics Letters B 791, 6-10. DOI: 10.1016/j.physletb.2019.02.013. arXiv:1812.00626v3.",
    "url": "https://arxiv.org/abs/1812.00626v3",
    "convention": "one-sided 95% CL with the distribution truncated at BR <= 100%, that is an exotic branch of at least zero",
    "example_from": 0.0027,
    "example_to": 0.0032,
    "example_location": "Physical PDF pages 8 and 9: truncation raises the bound of Czarnecki et al. from 0.27% to 0.32%",
    "updated_bound": 0.0028,
    "updated_bound_location": "Physical PDF page 9 and Conclusion: BR_X < 0.28%",
    "constant_s": 5172.3,
    "constant_sigma_s": 1.1,
    "constant_location": "Equation (6), from nuclear Ft values, independent of the universal radiative correction",
    "quote": "can be excluded with a high level of confidence",
    "aspect_note": "The then-preliminary aSPECT value was not used; the authors wrote that its inclusion would not significantly change their conclusion (physical PDF page 5)."
  },
  "lambda_sources": {
    "favored2018": {
      "label": "Czarnecki et al. 2018 favoured value",
      "from": "authors_inputs.gA"
    },
    "perkeo3": {
      "label": "PERKEO III, 2019",
      "pdg_document_id": "MAERKISCH 2019",
      "printed": "lambda = -1.27641(45)stat(33)sys",
      "citation": "B. Märkisch, H. Mest, H. Saul, X. Wang, H. Abele, D. Dubbers, M. Klopf, A. Petoukhov, C. Roick, T. Soldner and D. Werder (2019), Measurement of the Weak Axial-Vector Coupling Constant in the Decay of Free Neutrons Using a Pulsed Cold Neutron Beam, Physical Review Letters 122, 242501. DOI: 10.1103/PhysRevLett.122.242501. arXiv:1812.04666v1.",
      "url": "https://arxiv.org/abs/1812.04666v1",
      "measures": "beta asymmetry from electron spectra"
    },
    "aspect": {
      "label": "aSPECT, 2020",
      "pdg_document_id": "BECK 2020",
      "printed": "|lambda| = 1.2677(28)",
      "citation": "M. Beck et al. (aSPECT, 18 authors, 2020), Improved determination of the beta-antineutrino angular correlation coefficient a in free neutron decay with the aSPECT spectrometer, Physical Review C 101, 055506. DOI: 10.1103/PhysRevC.101.055506. arXiv:1908.04785v2.",
      "url": "https://arxiv.org/abs/1908.04785v2",
      "measures": "electron-antineutrino correlation from the proton recoil spectrum"
    },
    "pdg2026": {
      "label": "PDG 2026 average",
      "pdg_summary": "S017AV",
      "url": "https://pdgapi.lbl.gov/summaries/S017AV",
      "listing_url": "https://pdgapi.lbl.gov/listings/S017AV"
    }
  },
  "claimant_reply": {
    "source": "review2023",
    "location": "Fornal 2023, printed pages 15 and 16, Neutron lifetime from beta decay parameters",
    "quote": "The PERKEO III result agrees with the bottle average, but the aSPECT experiment is consistent with the beam average, favoring the neutron dark decay proposal as a solution to the neutron lifetime puzzle."
  },
  "licence": "Factual transcription. Czarnecki et al. (PRL 120, 202002) and Dubbers et al. (PLB 791, 6-10) are published under CC BY 4.0 according to their Crossref records; the other cited works are linked and not redistributed.",
  "licence_url": "https://creativecommons.org/licenses/by/4.0/",
  "retrieved": "2026-09-22"
}