Technically Natural Suppression of Fifth Force

Abstract

Light scalars generically mediate a fifth force incompatible with local tests of gravity unless their couplings are parametrically suppressed or screening mechanisms are introduced. We demonstrate that such suppression can arise from symmetry. We propose a $Z_2$-symmetric mirror extension of the Standard Model within a bi-conformal gravity construction, where spontaneous breaking of scale invariance produces a light scalaron as a pseudo-Nambu-Goldstone boson. This scalaron couples to the difference of trace anomalies between the Standard Model and mirror sectors. We find a parameter-independent correlation between the fifth-force strength $\alpha$ and the scalaron mass $m_\sigma$, with the proportionality set by QCD observables and the electroweak scale. The Standard Model predicts $\alpha \lesssim 10^{-3}$ above 10~cm scales for $m_\sigma \lesssim 10^{-6}$ eV, which is directly in the target window of next-generation experiments. In contrast to environmental screening mechanisms, this suppression mechanism follows directly from symmetry rather than nonlinear scalar dynamics.