How to use plasmas to discover new particles

Many theories of physics beyond the standard model predict the existence of new bosons, such as axions and dark photons, which mix with ordinary photons. In the presence of a dilute plasma, the probability of conversion between the photons and these new states is greatly enhanced by the presence of level crossings. In this Colloquium I will give a few examples of astrophysical systems, such as neutron stars and the ionosphere, which can be used to discover new physics in the near future.