Year: 2026
Type: preprint
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.
Source: arXiv (Cornell University)
Authors: Guo, Shu-Yuan, Li, Jia-Qi
Institution: Yantai University
Language: English
Topic: Neutrino Physics Research
Subfield: Nuclear and High Energy Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Awards: G4073435774, G6869306256
OpenAlex ID: W7223213394