Laboratory searches for exotic spin-dependent interactions

The possible existence of exotic spin-dependent interactions with ranges from the subatomic scale to astrophysical scales has been of great theoretical interest for the last few decades. Typically, these exotic interactions are mediated by ultralight bosons with very weak coupling strength. If they indeed exist, such long-range interactions would indicate new physics beyond the Standard Model. A wide variety of experimental tests have been made to search for novel long-range spin-dependent interactions. Most experimental searches have focused on monopole-dipole or dipole-dipole interactions that could be induced by the exchange of ultralight bosons such as axions or axionlike particles. These ultralight bosons could also provide an answer to some of the most challenging problems in modern particle physics and astronomy: for example, the strong-CP problem in quantum chromodynamics (QCD), where C represents the charge conjugate symmetry and P represents the parity symmetry, and the explanation of dark matter and dark energy. In this chapter, we discuss the theoretical motivations as well as experimental searches for exotic spin-dependent interactions mediated by ultralight bosons in recent decades. We also introduce ongoing experimental efforts, such as Axion Resonant InterAction DetectioN Experiment (ARIADNE) and the QUest for AXion (QUAX)-g<Subscript>s</Subscript>g<Subscript>p</Subscript> experiment. The high sensitivities of these tests will allow vast expansion of the discovery potential for exotic spin-dependent interactions.