Composite Ultralight Scalars, Fifth Forces, and Atomic Clocks

We study fifth forces from an ultralight scalar that arises as 
a pseudo-Nambu-Goldstone-boson of a strongly coupled hidden sector. A 
low compositeness scale protects its mass from radiative corrections 
induced by its non-derivative couplings to the Standard Model (SM). We 
present simple realizations in which an approximate conformal hidden 
sector couples to the SM through the hypercharge portal, leading to a 
form-factor suppression of couplings of the scalar to the SM fields at 
momenta above the compositeness scale. This suppression can weaken 
conventional equivalence principle (EP) bounds relative to atomic clock 
searches. We identify regions of parameter space where existing clock 
tests are already more sensitive than standard EP probes, and where 
atomic clocks can outperform nuclear clocks. These results motivate 
broad fifth force searches with atomic clocks over a wide range of 
distance scales.