Speaker
Description
We consider fifth forces mediated by an ultralight scalar that arises as a composite pseudo-Nambu-Goldstone boson from a strongly coupled hidden sector. In our framework, the scalar couples to the Standard Model through the hypercharge portal, and its interactions are softened above the compositeness scale by form factors. This can significantly weaken conventional equivalence principle (EP) bounds, especially when the compositeness scale lies between atomic and nuclear scales. We find that clock-based searches can become the leading probe of this scenario, with atomic clocks already more sensitive than traditional EP tests in parts of parameter space — and even outperforming nuclear clocks. These results highlight atomic clock experiments as a powerful, broadly motivated approach to searching for new fifth forces.