Skip to content
DraconiticResearch
Search Sources API
203,234 studies31 sourcesUpdated 49 min ago
PreprintCC BY 4.0arXiv

Low Primordial Helium as a Probe of Non-Unitary Neutrino Mixing

  • Shu-Yuan Guo
  • Jia-Qi Li
Open at sourcePDFDOICite

Abstract

The EMPRESS collaboration has reported a primordial helium mass fraction YP = 0.2402 ± 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 ∼ 2σ tension with the standard value of the effective number of relativistic species, or to a ∼ 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 YP. A full numerical BBN calculation, combined with the EMPRESS YP determination and the deuterium abundance gives the best fit point at α22=0.982, while the standard values Neff=3.044, ξe=0 remain allowed; the required level of non-unitarity is consistent with current experimental constraints.

Related

  1. Towards a Measurement of the Primordial Helium Isotope Ratio
    Ryan J Cooke, James W Johnson, Pasquier Noterdaeme +4
    The Astrophysical JournalPeer-reviewedCC BY 4.0Full text1 citedCosmologyGeneral physicsHAL
  2. Supernova microlensing as a probe of ultracompact minihalos and primordial cosmology
    Djuna Croon, Sergio Sevillano Muñoz, Miguel Zumalacarregui
    arXivPreprintarXiv licenceFull textCosmologyHigh-energy astrophysicsarXiv
  3. A Unitary Fixed Point Away from Threshold: Momentum-Surface EFT for Strong Mixing
    Lorenzo De Ros, Javier Reig Navarro
    arXivPreprintarXiv licenceFull textCondensed matterHigh-energy & particle physicsarXiv
  4. Cosmological Evolution of Primordial Black Holes: UV/IR Decoupling and the KM3NeT 220 PeV Neutrino Anomaly
    Debdatta Dey, Tausif Parvez, S. Shankaranarayanan
    arXivPreprintCC BY 4.0Full textHigh-energy astrophysicsGravitation & general relativityarXiv
  5. A New Probe of Strongly-Interacting Dark Sector using Neutrino Telescopes
    Jose A. Macias Cruz, Christopher V. Cappiello, P. S. Bhupal Dev +1
    arXivPreprintCC BY 4.0Full textHigh-energy astrophysicsHigh-energy & particle physicsarXiv

Cite

BibTeXRIS
BibTeX
@misc{guo2026primordial,
  title = {{Low Primordial Helium as a Probe of Non-Unitary Neutrino Mixing}},
  author = {Shu-Yuan Guo and Jia-Qi Li},
  howpublished = {arXiv},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2610.29069},
  eprint = {2610.29069},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2610.29069},
  note = {CC BY 4.0},
}
RIS
TY  - UNPB
TI  - Low Primordial Helium as a Probe of Non-Unitary Neutrino Mixing
AU  - Shu-Yuan Guo
AU  - Jia-Qi Li
PY  - 2026
DA  - 2026/10/21
PB  - arXiv
DO  - 10.48550/arxiv.2610.29069
UR  - https://arxiv.org/abs/2610.29069
AB  - The EMPRESS collaboration has reported a primordial helium mass fraction Y_P = 0.2402 ± 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 ∼ 2σ tension with the standard value of the effective number of relativistic species, or to a ∼ 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_eff=3.044, ξ_e=0 remain allowed; the required level of non-unitarity is consistent with current experimental constraints.
ER  -