Multi-Field Dilaton Screening Beyond the Thin-Shell Mechanism

TL;DR

This paper explores multi-field scalar-tensor theories with dilaton and axion fields, revealing new screening mechanisms that allow strong matter couplings without conflicting with solar system gravity tests.

Contribution

It introduces a novel multi-field screening regime where axion backreaction suppresses scalar charges, bypassing thin-shell limitations and expanding viable parameter space.

Findings

Full screening can occur without fine-tuning.

Axion backreaction can suppress or enhance scalar charges.

Multi-field dynamics decouple screening from thin-shell localization.

Abstract

We analyse screening in multi-field scalar-tensor theories, focusing on systems with a dilaton coupled to matter and an axion with a dilaton-dependent kinetic term, in the presence of both planetary and stellar density profiles. Using analytic arguments and fully coupled numerical solutions, we identify a regime in which full screening for a dark-energy-light, effectively unpinned string-dilaton, can occur without fine-tuning. The backreaction of the dilaton's partnered axion field can suppress the exterior scalar charge by selecting a minimum-energy configuration (the BBQ mechanism), yielding robust screening for generic axion gradients. In this regime screening is achieved by cancelling the dilaton's gradient rather than localising it. This reduces the exterior scalar charge and allows for gravity tests in the solar system to be passed. We then show that the more familiar thin-shell intuition need not apply in the multi-field setting. Axion surface gradients can drastically reshape the dilaton profile and drive a more localised transition without generically suppressing the fifth force. The exterior charge can remain essentially unchanged or even be enhanced as the shell is made thinner by a kinetically coupled field. Multi-field two-derivative dynamics therefore decouple localisation in thin shells from screening, evade single-field no-go arguments, and reopen viable parameter space for cosmologically light dilaton-like scalars with strong couplings to matter.