Fifth forces, Higgs portals and broken scale invariance

TL;DR

This paper explores how fifth forces mediated by light scalars relate to the origin of scale breaking in the Standard Model, linking scalar-tensor gravity modifications with Higgs-portal theories and their experimental implications.

Contribution

It establishes the equivalence between conformally coupled scalar-tensor gravity and Higgs-portal models, highlighting how fifth forces depend on the Higgs mass generation mechanism.

Findings

Fifth forces only appear with mass mixing between scalar and Higgs.

Solar System tests can constrain the Higgs sector's scale-breaking nature.

The strength of fifth forces depends on explicit versus spontaneous symmetry breaking.

Abstract

We study the relationship between the strength of fifth forces and the origin of scale breaking in the Standard Model (SM) of particle physics. We start with a light scalar field that is conformally coupled to a toy SM matter sector through a Weyl rescaling of the metric. After appropriately normalizing the fields, the conformally coupled scalar only interacts directly with the would-be Higgs field through kinetic-mixing and Higgs-portal terms. Thus, for the first time, we describe the equivalence of conformally coupled scalar-tensor modifications of gravity and Higgs-portal theories, and we find that the usual tree-level fifth forces only emerge if there is mass mixing between the conformally coupled scalar and the Higgs field. The strength of the fifth force, mediated by the light scalar, then depends on whether the mass of the Higgs arises from an explicit symmetry-breaking term or a spontaneous mechanism of scale breaking. Solar System tests of gravity and the non-observation of fifth forces therefore have the potential to provide information about the structure of the Higgs sector and the origin of its symmetry breaking, setting an upper bound on the magnitude of any explicit scale-breaking terms. These results demonstrate the phenomenological importance (both for cosmology and high-energy physics) of considering how scalar-tensor modifications of gravity are embedded within extensions of the SM.