Gauge Field and Fermion Production during Axion Inflation

TL;DR

This paper investigates how axion inflation can produce helical gauge fields and chiral fermions simultaneously, revealing their mutual influence, resulting bounds, and implications for early universe asymmetries and structure formation.

Contribution

It presents a detailed analysis of gauge and fermion production during axion inflation, highlighting the backreaction effects and constraints on primordial fields and asymmetries.

Findings

Helical gauge fields induce chiral fermion production via Landau levels.

Backreaction imposes upper bounds on gauge field magnitudes.

Primordial chiral asymmetries tend to be erased after inflation.

Abstract

We study the dual production of helical Abelian gauge fields and chiral fermions through the Chern-Simons (CS) coupling with a pseudo-scalar inflaton in the presence of a chiral anomaly. Through the CS term, the motion of the inflaton induces a tachyonic instability for one of the two helicities of the gauge field. We show that the resulting helical gauge field necessarily leads to the production of chiral fermions by deforming their Fermi sphere into discrete Landau levels. The population of the lowest Landau level leads to a chiral asymmetry as inferred from the chiral anomaly, while the higher levels are populated symmetrically through pair production. From the backreaction of the fermions on the gauge field production we derive a conservative but stringent upper bound on the magnitude of the gauge fields. Consequently, we find that the scalar perturbations sourced by these helical gauge fields, responsible for enhanced structure formation on small scales, get reduced significantly. We also discuss the fate of the primordial chiral asymmetry and of the helical gauge fields after inflation, and show that the instability in the chiral plasma tends to erase these primordial asymmetries. This result may impact scenarios where the baryon asymmetry of the Universe is connected to primordial magnetic fields.