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

Shu-Yuan Guo; Jia-Qi Li

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arXiv · 2610.29069

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

Shu-Yuan Guo·Jia-Qi Li

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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.

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Shu-Yuan Guo, Jia-Qi Li. 2026-10-21. Low Primordial Helium as a Probe of Non-Unitary Neutrino Mixing. https://arxiv.org/abs/2610.29069

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