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Abstract |
We investigate the impact of a nonstandard electron mass me 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 et annihilation. Varying me shifts the decoupling epoch and the entropy transfer from et annihilation, thereby modifying the neutrino energy density and the inferred effective number of relativistic species, Neff. Independently, during BBN, the rates of charged-current weak processes, and hence the neutron-to-proton ratio, depend on me. By confronting BBN predictions for the primordial light-element abundances with observations and imposing cosmological constraints on Neff, we obtain the following 1 sigma bounds on me in the early Universe: me = 0.505+0.006 -0.007 MeV (for the NACRE II nuclear reaction network) or -0.004 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 II regions. If instead we adopt the Particle Data Book helium-4 abundance, the bounds are me = 0.503 & thorn;0.011 -0.015 MeV (NACRE II) or me = 0.521 & thorn;0.009 obtained allowed ranges are close to the present laboratory value at the level of similar to 0.4%-2%, depending on the dataset and nuclear network, thus supporting the constancy of the electron mass over cosmological timescales. |
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Address |
[Garramone, Michela; Fornengo, Nicolao; Gariazzo, Stefano] Univ Turin, Phys Dept, Via Pietro Giuria 1, I-10125 Turin, Italy, Email: michela.garramone@unito.it; |
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