Axion homeopathy: screening dilaton interactions

Cosmologically active Brans-Dicke (or dilaton) scalar fields are generically ruled out by solar system tests of gravity unless their couplings to ordinary matter much suppressed relative gravitational strength, and this is a major hindrance when building realistic models light dilatons coupled matter. We propose new mechanism for evading such bounds if also couples axion, that exploits nonlinear target-space curvature interactions qualitatively change how the respond gravitating source. find dilaton-matter would be excluded in absence an axion can become acceptable given additional small axion-matter coupling, possible because axion-dilaton end up converting would-be dilaton profile into profile. The trajectories test bodies then controlled weaker easily enough escape detection. call Axion Homeopathy evasion dilaton-coupling persists extremely provided only they nonzero. explore using axio-dilaton equations SL(2,R) invariant (as often appear string compactifications), since these general solutions exterior spherically symmetric source found analytically. use solution compute relevant PPN parameters, $\beta$ $\gamma$, verify difference from unity smaller than have been true so therefore evade experimental bounds.