Alessandro Mirizzi, Gianpiero Mangano, Ninetta Saviano, Enrico Borriello, Carlo Giunti, Gennaro Miele, Ofelia Pisanti
Light sterile neutrinos can be excited by oscillations with active neutrinos in the early universe and contribute as extra-radiation, parameterized in terms of the effective number of neutrino species N_eff. This parameter has been measured to quite a good precision by the Planck satellite experiment, yielding N_eff=3.30 \pm 0.27 at 68 % C.L. We use this result to update the bounds on the parameter space of (3+1) sterile neutrino scenarios, with an active-sterile neutrino mass squared splitting in the range (10^{-5} - 10^2) eV^2, in both normal and inverted mass hierarchies for the active and sterile states. For the first time we take into account the possibility of two non-vanishing active-sterile mixing angles. We find that the bounds are stronger than those obtained in laboratory experiments. In fact, we get active-sterile mixing angles sin^2 \theta_{i4} < 10^{-2.5} for mass splittings \Delta m^2_{41} > 10^{-1} eV^2. This result leads to a strong tension with the short-baseline hints of light sterile neutrinos. In order to relieve this disagreement, modifications of the standard cosmological scenario, e.g. large primordial neutrino asymmetries, are required.
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http://arxiv.org/abs/1303.5368
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