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Scalar-tensor theories with the shift symmetries required by light scalarsare well-explored modifications to GR. For these, two-derivative scalarself-interactions usually dominate at low energies and interestingly competewith the two-derivative metric interactions of GR itself. Although much efforthas been invested in single scalars (on grounds of simplicity) these happen tohave no two-derivative interactions, requiring such models to explorehigher-derivative interactions (that usually would be less important atlow-energies). This suggests multiple-scalar sigma models as well-motivatedcandidates for finding new phenomena in tests of gravity. We identify a newmulti-field screening mechanism appropriate for two light scalar fields (anaxion and a Brans-Dicke style dilaton) that relies on their mutualtwo-derivative interactions. We show how very weak axion-matter couplings canintroduce axion gradients that can reduce the apparent coupling of theBrans-Dicke scalar to macroscopic matter sources. We further identify arelaxation mechanism that allows this reduction to be amplified to asuppression by the ratio of the axion gradient's length scale to the source'sradius (similar in size to the suppression found in Chameleon models). Unlikesome screening mechanisms our proposal is technically natural and works deepwithin the regime of control of the low-energy EFT. It uses only ingredientsthat commonly appear in the low-energy limit of string vacua and so is likelyto have wider applications to models that admit UV completions. We brieflydiscuss phenomenological implications and challenges for this scenario, whichsuggests re-examination of decay loss bounds and the value ofequivalence-principle tests for different-sized objects.

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