A Chameleon Primer

TL;DR

This paper reviews chameleon theories, focusing on their density-dependent properties that allow them to evade gravitational tests and potentially address experimental discrepancies.

Contribution

It provides a comprehensive overview of chameleon properties, including mass variation and screening effects, and discusses their implications for experimental physics.

Findings

Chameleon fields have density-dependent masses that suppress fifth forces in dense environments.

Thin shell effects shield compact bodies from fifth forces, explaining their weak experimental signatures.

Extensions of chameleon theories may resolve PVLAS versus CAST experimental discrepancies.

Abstract

We review some of the properties of chameleon theories. Chameleon fields are gravitationally coupled to matter and evade gravitational tests thanks to two fundamental properties. The first one is the density dependence of the chameleon mass. In most cases, in a dense environment, chameleons are massive enough to induce a short ranged fifth force. In other cases, non-linear effects imply the existence of a thin shell effect shielding compact bodies from each other and leading to an irrelevant fifth force. We also mention how a natural extension of chameleon theories can play a role to solve the PVLAS versus CAST discrepancy.