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