Fermion production during and after axion inflation

TL;DR

This paper investigates how derivatively coupled fermions are produced during and after axion-driven inflation, revealing mechanisms for asymmetric helicity production and heavy fermion generation due to the axion's dynamics.

Contribution

It introduces a detailed analysis of fermion production mechanisms during axion inflation, highlighting the role of the axion as an effective chemical potential and the non-adiabatic effects post-inflation.

Findings

Efficient gravitational production of heavy fermions during inflation.

Asymmetric helicity production depending on fermion mass and coupling.

Fermion production during axion oscillations post-inflation.

Abstract

We study derivatively coupled fermions in axion-driven inflation, specifically $m_\phi^2\phi^2$ and monodromy inflation, and calculate particle production during the inflationary epoch and the post-inflationary axion oscillations. During inflation, the rolling axion acts as an effective chemical potential for helicity which biases the gravitational production of one fermion helicity over the other. This mechanism allows for efficient gravitational production of heavy fermion states that would otherwise be highly suppressed. Following inflation, the axion oscillates and fermions with both helicities are produced as the effective frequency of the fermion field changes non-adiabatically. For certain values of the fermion mass and axion-fermion coupling strength, the two helicity states are produced asymmetrically, resulting in unequal number-densities of left- and right-helicity fermions.