Improved constraints on lepton asymmetry from the cosmic microwave background

Understand

A cosmic lepton asymmetry $\eta_{\text{l}}=(n_{\text{l}}-n_{\bar{\text{l}}})/n_{\gamma}$ affects the primordial helium abundance and the expansion rate of the early Universe.

• Both of these effects have an impact on the anisotropies of the cosmic microwave background (CMB).
• We derive constraints on the neutrino chemical potentials from the Planck 2015 data, assuming equal lepton flavour asymmetries and negligible neutrino masses.
• We find $\xi=-0.002 ^{+0.114}_{-0.111}$ (95\% CL) for the chemical potentials, which corresponds to $ -0.085 \leq \eta_{\text{l}} \leq 0.084$.