Axio-Chameleons: A Novel String-Friendly Multi-field Screening Mechanism

TL;DR

This paper introduces a new multi-field screening mechanism involving axions and dilatons that reduces scalar-matter couplings, leveraging mutual two-derivative interactions, and is natural within low-energy effective field theories.

Contribution

It proposes a novel, technically natural screening mechanism for two light scalar fields that operates deep within the low-energy regime and is compatible with string theory-inspired models.

Findings

Weak axion-matter couplings can significantly reduce scalar interactions.

The mechanism achieves suppression proportional to the ratio of axion gradient length scale to source radius.

The approach is natural and applicable within the low-energy effective field theory regime.

Abstract

Scalar-tensor theories with the shift symmetries required by light scalars are well-explored modifications to GR. For these, two-derivative scalar self-interactions usually dominate at low energies and interestingly compete with the two-derivative metric interactions of GR itself. Although much effort has been invested in single scalars (on grounds of simplicity) these happen to have no two-derivative interactions, requiring such models to explore higher-derivative interactions (that usually would be less important at low-energies). This suggests multiple-scalar sigma models as well-motivated candidates for finding new phenomena in tests of gravity. We identify a new multi-field screening mechanism appropriate for two light scalar fields (an axion and a Brans-Dicke style dilaton) that relies on their mutual two-derivative interactions. We show how very weak axion-matter couplings can introduce axion gradients that can reduce the apparent coupling of the Brans-Dicke scalar to macroscopic matter sources. We further identify a relaxation mechanism that allows this reduction to be amplified to a suppression by the ratio of the axion gradient's length scale to the source's radius (similar in size to the suppression found in Chameleon models). Unlike some screening mechanisms our proposal is technically natural and works deep within the regime of control of the low-energy EFT. It uses only ingredients that commonly appear in the low-energy limit of string vacua and so is likely to have wider applications to models that admit UV completions. We briefly discuss phenomenological implications and challenges for this scenario, which suggests re-examination of decay loss bounds and the value of equivalence-principle tests for different-sized objects.