ALP-mediated Dark Matter-Nucleon Scattering

We perform a comprehensive analysis of dark matter-nucleon scattering via the exchange of axion-like particles (ALPs). At first sight, this might appear of little practical use, as non-relativistic scattering through pseudo-scalar interactions is momentum-suppressed and spin-dependent, resulting in scattering rates below any experimental sensitivity. We show that the scattering rate can be drastically enhanced in two ways. First, light ALPs with masses below the typical momentum transfer at direct detection experiments lift the momentum suppression by acting as essentially massless mediators. Second, ALP exchange through loops induces coherent spin-independent scattering. If the ALP has flavor-changing couplings to up-type quarks, loop-induced scattering receives an extra strong enhancement by the top-quark mass. We deduce that, contrary to common lore, XENONnT and PandaX-4T are already sensitive to ALP-mediated dark matter-nucleon scattering. The next generation of direct detection experiments will probe far into the parameter space of the ALP effective theory, potentially exceeding the sensitivity of collider searches.