The strongest bounds on active-sterile neutrino mixing after Planck data

Understand

Light sterile neutrinos can be excited by oscillations with active neutrinos in the early universe.

• Their properties can be constrained by their contribution as extra-radiation, parameterized in terms of the effective number of neutrino species N_ eff, and to the universe energy density today \Omega_\nu h^2.
• Both these parameters have been measured to quite a good precision by the Planck satellite experiment.
• 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.