BBN with Late Electron-Sterile Neutrino Oscillations - The Finest Leptometer

Understand

A relic lepton asymmetry orders of magnitude bigger than the baryon one may hide in the relic neutrino background.

• No direct theoretical or experimental limitations on its magnitude and sign are known.
• Indirect cosmological constraints exist ranging from $|L|<0.01$ to $L<10$.
• We discuss a BBN model with late electron-sterile neutrino oscillations which is a fine leptometer - it is capable of feeling extremely small relic lepton asymmetry - $|L|>10^{-8}$.