The Axion-Electron Coupling: Prospects and Pitfalls - Kevin Zhou (SLAC)

Abstract: Most axion dark matter experiments search for the axion's coupling to photons, but the axion also acts directly on electrons. Understanding the effect of this coupling is surprisingly subtle: many recent papers have overestimated sensitivity to it by up to fifteen orders of magnitude. Such issues occur because the axion-electron coupling can be written in two equivalent forms, analogous to the pseudoscalar and pseudovector forms of the pion-nucleon coupling, which in turn implies an ambiguity in the definition of nonrelativistic physical observables. I will explain this point and show several new ways to probe the true effects of the axion-electron coupling, which include spin torques and spin-dependent forces. In particular, by considering how the axion excites currents in magnetized media, one can probe far into unexplored parameter space at micro-eV masses