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

The EMPRESS collaboration has reported a primordial helium mass fraction $Y_P=0.2402\pm 0.0040$, which lies more than $1\sigma$ below the standard Big Bang Nucleosynthesis prediction. Combined with the deuterium abundance and the Planck baryon density, this corresponds to a mild $\sim 2\sigma$ tension with the standard value of the effective number of relativistic species, or to a $\sim 2\sigma$ 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 ${\alpha }_{22}=0.982$, while the standard values $N_{\mathrm{e}\mathrm{f}\mathrm{f}}=3.044$, ${\xi }_e=0$ remain allowed; the required level of non-unitarity is consistent with current experimental constraints.