Spin-dependent exotic interactions

Novel interactions beyond the four known fundamental forces in nature (electromagnetic, gravitational, strong, and weak interactions) may arise due to "new physics" beyond the standard model, manifesting as a "fifth force." This review focuses on spin-dependent fifth forces mediated by exotic bosons such as spin-0 axions and axionlike particles and spin-1 𝑍′ bosons, dark photons, and paraphotons. Many of these exotic bosons are candidates to explain the nature of dark matter and dark energy, and their interactions may violate fundamental symmetries. Spin-dependent interactions between fermions mediated by the exchange of exotic bosons have been investigated in a variety of experiments, particularly at the low-energy frontier. Experimental methods and tools used to search for exotic spin-dependent interactions, such as atomic comagnetometers, torsion balances, nitrogen-vacancy spin sensors, and precision atomic and molecular spectroscopy, are described. A complete set of interaction potentials, derived based on quantum field theory with minimal assumptions and characterized in terms of reduced coupling constants, are presented. A summary of existing experimental and observational constraints on exotic spin-dependent interactions is given, in the process illustrating the current research landscape and promising directions of further research.