Large lepton asymmetry from axion inflation and helium abundance hinted by ACT

TL;DR

This paper explores how axion inflation can generate lepton asymmetry and influence helium abundance, linking early universe magnetic fields, lepton asymmetry, and recent CMB observations.

Contribution

It introduces a mechanism for generating large lepton asymmetry via axion inflation with gauged lepton flavor symmetry, overcoming previous limitations.

Findings

Lepton asymmetry can be significantly enhanced by suppressing fermion production during inflation.

Generated lepton asymmetry can be large enough to affect primordial helium abundance.

The model aligns with recent ACT CMB observations suggesting helium abundance hints.

Abstract

The generation of helical magnetic fields and the associated chiral asymmetry via the chiral anomaly is a generic feature in pseudoscalar inflation. In the presence of a Chern--Simons coupling between the inflaton and a U(1) gauge field, the homogeneous evolution of the inflaton induces a tachyonic instability in one circular polarization of the gauge field, resulting in the production of helical magnetic fields. In this work, we show that, in the case of a gauged lepton flavor symmetry, U(1)$_{L_i-L_j}$, this mechanism can lead to the generation of a sizable lepton asymmetry. In a simple setup, however, the resulting lepton asymmetry is typically too small to have an observational consequences, even setting aside constraints from baryon overproduction via sphaleron processes, due to the backreaction of the produced gauge fields and fermions on the inflationary dynamics. We demonstrate that this limitation can be overcome by implementing a mechanism to suppress fermion production during inflation. As a result, a much larger lepton asymmetry can be generated from the subsequent decay of magnetic helicity. Remarkably, for the gauged U(1)$_{L_\mu-L_\tau}$ symmetry, the generated asymmetry can be sufficiently large to suppress the primordial helium abundance, as may be inferred from recent cosmic microwave background observations by ACT.