Weyl Fermion Creation by Cosmological Gravitational Wave Background at 1-loop

TL;DR

This paper demonstrates that gravitational waves in the early universe can induce Weyl fermion production at 1-loop, providing a new mechanism that could explain dark matter abundance and other feebly interacting fermions.

Contribution

It introduces a novel 1-loop gravitational mechanism for Weyl fermion production in an expanding universe, breaking conformal invariance via cosmic perturbations.

Findings

Gravitational wave backgrounds induce non-zero fermion self-energy at 1-loop.

This mechanism can dominate over tree-level gravitational production in certain parameter ranges.

Potential implications for dark matter and right-handed neutrino production.

Abstract

Weyl fermions of spin $\frac12$ minimally coupled to Einstein's gravity in 4 dimensions cannot be produced purely gravitationally in an expanding Universe at tree level. Surprisingly, as we showed in a recent letter [1], this changes at gravitational 1-loop when cosmic perturbations, like a gravitational wave background, are present. Such a background introduces a new scale, thereby breaking the fermions' conformal invariance. This leads to a non-vanishing gravitational self-energy for Weyl fermions at 1-loop and induces their production. In this paper, we present an extended study of this new mechanism, explicitly computing this effect using the in-in formalism. We work in an expanding Universe in the radiation-dominated era as a fixed background. Gravitational wave-induced fermion production has rich phenomenological consequences. Notably, if Weyl fermions eventually acquire mass, and assuming realistic - and potentially detectable - gravitational wave backgrounds, the mechanism can explain the abundance of dark matter in the Universe. More generally, gravitational-wave induced freeze-in is a new purely gravitational mechanism for generating other feebly interacting fermions, e.g. right-handed neutrinos. We show that this loop level effect can dominate over the conventional - tree-level - gravitational production of superheavy fermions in a sizable part of the parameter space.