Axions: Theory and experiment

Axions (and axion-like particles) are a hypothetical class of particle arising as predictions of theories ranging from the nuclear force to dark matter. They are pseudo-Goldstone bosons, and classically possess only derivative interactions. Quantum e ects, including the axial current anomaly and instantons, break this symmetry and cause axions to couple to topological operators and gain a mass. These oparators suggest speci c experimental means to search for axions, for ex- ample by vacuum birefringence, “shining light through a wall”, and time variation fo the neutron electric dipole moment. I will describe a small selection of labo- ratory, astrophysical, and cosmological constraints on axions. I will also describe the exciting future for axion experiments caused by renewed global interest and investment, driven by a wide range of new theoretical and experimental ideas.