Light fermions in quantum gravity

TL;DR

This paper investigates whether quantum gravity can break chiral symmetry in fermions and finds that, within the asymptotic safety framework, chiral symmetry remains intact, supporting the viability of such quantum gravity models.

Contribution

It demonstrates that quantum gravity, specifically asymptotic safety, does not induce chiral symmetry breaking, providing constraints on UV completions of gravity.

Findings

Chiral symmetry remains unbroken at strong gravitational coupling.

Asymptotically safe quantum gravity admits light fermions.

Chiral symmetry breaking is unlikely due to quantum gravity effects.

Abstract

We study the impact of quantum gravity, formulated as a quantum field theory of the metric, on chiral symmetry in a fermionic matter sector. We specifically address the question as to whether metric fluctuations can induce chiral symmetry breaking and bound state formation. Our results based on the functional Renormalization Group indicate that chiral symmetry is left intact even at strong gravitational coupling. In particular, we find that asymptotically safe quantum gravity where the gravitational couplings approach a non-Gaussian fixed point generically admits universes with light fermions. Our results thus further support quantum gravity theories built on fluctuations of the metric field such as the asymptotic-safety scenario. A study of chiral symmetry breaking through gravitational quantum effects may serve as a significant benchmark test also for other quantum gravity scenarios, since a completely broken chiral symmetry at the Planck scale would not be in accordance with the observation of light fermions in our universe. We demonstrate that this elementary observation already imposes constraints on a generic UV completion of gravity.