# Improved constraints on lepton asymmetry from the cosmic microwave background

## TL;DR

This paper uses Planck 2015 data to place tight, robust constraints on the cosmic lepton asymmetry, improving upon previous limits and aligning with primordial element measurements.

## Contribution

It provides the first strong, robust CMB-based constraints on lepton asymmetry assuming equal flavor asymmetries and negligible neutrino masses.

## Key findings

- Lepton asymmetry constrained to -0.085 to 0.084 at 95% CL.
- Constraints are more robust than those from primordial light element abundances.
- Results are consistent with direct measurements of primordial helium and deuterium.

## Abstract

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$. Our constraints on the lepton asymmetry are significantly stronger than previous constraints from CMB data analysis and we argue that they are more robust than those from primordial light element abundances. The resulting constraints on the primordial helium and deuterium abundances are consistent with those from direct measurements.