Conformal Metric-Affine Gravities

TL;DR

This paper explores how conformal (scaling) symmetry can be incorporated as a gauge symmetry in metric-affine gravity theories, introducing a dynamical gauge field and a dilaton to generate mass scales through spontaneous symmetry breaking.

Contribution

It demonstrates the inclusion of local conformal symmetry as a gauge symmetry in a broad class of metric-affine gravity theories, linking the gauge field to torsion and extending conformal invariance in Riemann-Cartan frameworks.

Findings

The gauge field of conformal symmetry can be identified with the torsion vector.

A dilaton field induces spontaneous symmetry breaking, generating a mass scale.

The approach generalizes conformal invariance to Ricci-based and other gravity theories.

Abstract

We revisit the gauge symmetry related to integrable projective transformations in metric-affine formalism, identifying the gauge field of the Weyl (conformal) symmetry as a dynamical component of the affine connection. In particular, we show how to include the local scaling symmetry as a gauge symmetry of a large class of geometric gravity theories, introducing a compensator dilaton field that naturally gives rise to a St\"uckelberg sector where a spontaneous breaking mechanism of the conformal symmetry is at work to generate a mass scale for the gauge field. For Ricci-based gravities that include, among others, General Relativity, $f(R)$ and $f(R,R_{\mu\nu}R^{\mu\nu})$ theories and the EiBI model, we prove that the on-shell gauge vector associated to the scaling symmetry can be identified with the torsion vector, thus recovering and generalizing conformal invariant theories in the Riemann-Cartan formalism, already present in the literature.