Light axiodilatons: matter couplings, weak-scale completions and long-distance tests of gravity

We consider the physical implications of very light axiodilatons motivated by a novel mechanism to substantially reduce the vacuum energy proposed in 2110.10352. We address the two main problems concerning the light axiodilaton that appears in the low-energy limit, namely that the axion has a very low decay constant fa ∼ eV (as read from its kinetic term) and that the dilaton is subject to bounds that are relevant to tests of GR once ρ vac ≲ 10-80 Mp 4. We show that eV scale axion decay constants need not be a problem by showing how supersymmetric extra dimensions provide a sample unitarization for axion physics above eV scales for which non-anomalous matter/axiodilaton couplings can really have gravitational strength, showing how naive EFT reasoning can mistakenly overestimates axion interactions at eV. When axions really do couple strongly at eV scales we identify the dimensionless interaction in the UV completion that is also O(1), and how axion energy-loss bounds map onto known extra-dimensional constraints. We find a broad new class of exact exterior solutions to the vacuum axiodilaton equations and knowledge of axiodilaton-matter couplings also allows us to numerically search for interior solutions that match to known exterior solutions that can evade solar-system tests. We find no examples that do so, but also identify potential new candidate mechanisms for reducing the effective dilaton-matter coupling to gravitating objects without also undermining the underlying suppression of ρ vac.