Abstract
Modifications of general relativity often involve coupling additional scalar fields to the Ricci scalar, leading to scalar-tensor theories of Brans-Dicke type. If the additional scalar fields are light, they can give rise to long-range fifth forces, which are subject to stringent constraints from local tests of gravity. In this talk, we show that Yukawa-like fifth forces only arise for the Standard Model (SM) due to a mass mixing of the additional scalar with the Higgs field, and we emphasise the pivotal role played by discrete and continuous symmetry breaking. Quite remarkably, if one assumes that sufficiently light, non-minimally coupled scalar fields exist in nature, the non-observation of fifth forces has the potential to tell us about the structure of the SM Higgs sector and the origin of its symmetry breaking. Moreover, with these observations, we argue that certain classes of scalar-tensor theories are, up to and including their dimension-four operators, equivalent to Higgs-portal theories. In this way, ultra-light dark matter models may also exhibit fifth-force phenomenology, and we consider the impact on the dynamics of disk galaxies as an example.
Highlights
Many extensions of general relativity (GR) involve additional scalar fields that are coupled non-minimally to the gravitational sector, falling into the class of scalar-tensor theories
We show that Yukawa-like fifth forces only arise for the Standard Model (SM) due to a mass mixing of the additional scalar with the Higgs field, and we emphasise the pivotal role played by discrete and continuous symmetry breaking
If one assumes that sufficiently light, non-minimally coupled scalar fields exist in nature, the non-observation of fifth forces has the potential to tell us about the structure of the SM Higgs sector and the origin of its symmetry breaking
Summary
Many extensions of general relativity (GR) involve additional scalar fields that are coupled non-minimally to the gravitational sector, falling into the class of scalar-tensor theories. The resulting modifications of gravity are heavily constrained by local tests of gravity These constraints can be avoided by appealing to screening mechanisms, wherein the fifth force is suppressed dynamically near to dense sources of matter due to the interplay between the environmental dependence and non-linearities of the governing field equations [1, 2]. In this talk, we describe how a certain class of scalar-tensor theories, involving a coupling of the additional scalar to the Ricci scalar of GR, are equivalent to Higgs-portal theories [3, 4, 5, 6, 7].
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