Light Axiodilatons: Matter Couplings, Weak-Scale Completions and Long-Distance Tests of Gravity

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We consider the physical implications of very light axiodilatons motivated by a novel mechanism to substantially reduce the vacuum energy proposed in arXiv: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 $f_a \sim $ eV (as read from its kinetic term) and that the dilaton is subject to bounds that are relevant to tests of GR once $\rho_{\rm vac} \leq 10^{-80} M_p^4$.
• 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.