Chameleons with Field Dependent Couplings

TL;DR

This paper demonstrates that scalar-tensor theories with field-dependent couplings and power-law potentials can exhibit the chameleon mechanism, affecting experimental searches for fifth forces and Casimir effects.

Contribution

It extends the chameleon mechanism to theories with field-dependent couplings, analyzing their implications for experimental tests of gravity.

Findings

The theory admits thin-shell solutions for spherical bodies.

Scalar field coupling is sensitive to a mass scale near the current Hubble scale.

Constraints from gravitational tests limit the coupling strength.

Abstract

Certain scalar-tensor theories exhibit the so-called chameleon mechanism, whereby observational signatures of scalar fields are hidden by a combination of self-interactions and interactions with ambient matter. Not all scalar-tensor theories exhibit such a chameleon mechanism, which has been originally found in models with inverse power run-away potentials and field independent couplings to matter. In this paper we investigate field-theories with field-dependent couplings and a power-law potential for the scalar field. We show that the theory indeed is a chameleon field theory. We find the thin-shell solution for a spherical body and investigate the consequences for E\"ot-Wash experiments, fifth-force searches and Casimir force experiments. Requiring that the scalar-field evades gravitational tests, we find that the coupling is sensitive to a mass-scale which is of order of the Hubble scale today.