Scale invariant alternatives to general relativity. II. Dilaton properties

TL;DR

This paper explores scale-invariant gravitational theories with a focus on dilaton properties, showing that specific metric dimensions preserve scale invariance and masslessness, and proposing a mechanism for small Higgs and cosmological constant values.

Contribution

It demonstrates that choosing the metric to have mass dimension -2 preserves scale invariance and massless dilaton, and suggests a natural explanation for small Higgs and cosmological constant values.

Findings

Dilaton remains massless and couples only through derivatives.

Scale invariance is preserved with metric dimension -2.

Small values of Higgs mass and cosmological constant may arise nonperturbatively.

Abstract

In the present paper, we revisit gravitational theories which are invariant under TDiffs -- transverse (volume preserving) diffeomorphisms and global scale transformations. It is known that these theories can be rewritten in an equivalent diffeomorphism-invariant form with an action including an integration constant (cosmological constant for the particular case of non-scale-invariant unimodular gravity). The presence of this integration constant, in general, breaks explicitly scale invariance and induces a runaway potential for the (otherwise massless) dilaton, associated with the determinant of the metric tensor. We show, however, that if the metric carries mass dimension $\left[\text{GeV}\right]^{-2}$, the scale invariance of the system is preserved, unlike the situation in theories in which the metric has mass dimension different from $-2$. The dilaton remains massless and couples to other fields only through derivatives, without any conflict with observations. We observe that one can define a specific limit for fields and their derivatives (in particular, the dilaton goes to zero, potentially related to the small distance domain of the theory) in which the only singular terms in the action correspond to the Higgs mass and the cosmological constant. We speculate that the self-consistency of the theory may require the regularity of the action, leading to the absence of the bare Higgs mass and cosmological constant, whereas their small finite values may be generated by nonperturbative effects.