Early-Universe constraints on the electron mass

We investigate the impact of a nonstandard electron mass m e on early-Universe thermal history, focusing on neutrino decoupling and big bang nucleosynthesis (BBN). In the standard cosmology, neutrino-electron interactions keep neutrinos in thermal contact with the electromagnetic plasma until shortly before e ± annihilation. Varying m e shifts the decoupling epoch and the entropy transfer from e ± annihilation, thereby modifying the neutrino energy density and the inferred effective number of relativistic species, N eff . Independently, during BBN, the rates of charged-current weak processes, and hence the neutron-to-proton ratio, depend on m e . By confronting BBN predictions for the primordial light-element abundances with observations and imposing cosmological constraints on N eff , we obtain the following 1 σ bounds on m e in the early Universe: m e = 0.50 5 − 0.007 + 0.006 MeV (for the NACRE II nuclear reaction network) or m e = 0.50 9 − 0.004 + 0.005 MeV (for the PRIMAT nuclear reaction network). These bounds have been derived by adopting the recent determination of the primordial helium-4 abundance by the Large Binocular Telescope observations of 54 metal-poor H regions. If instead we adopt the Particle Data Book helium-4 abundance, the bounds are m e = 0.50 3 − 0.015 + 0.011 MeV (NACRE II) or m e = 0.52 1 − 0.007 + 0.009 MeV (PRIMAT) The obtained allowed ranges are close to the present laboratory value at the level of ∼ 0.4 % – 2 % , depending on the dataset and nuclear network, thus supporting the constancy of the electron mass over cosmological timescales.