The Dark Matter - Phonon Coupling

The experimental endeavour to directly detect dark matter in a laboratory on earth must confront a possible dark matter mass range spanning over sixty orders of magnitude. Within the lowest mass regions, the dark matter oscillates as a coherent classical field, which can be leveraged in experiments that search for resonant effects. Conventional WIMP searches, on the other hand, rely on the detection of energy deposited in a scattering event. 

A particularly challenging window exists for dark matter with keV-MeV mass, lying above the coherence regime, but below conventional detector thresholds. For such masses, detectors need to be sensitive to energy deposits between an meV and an eV. One promising avenue is to try and detect dark matter via its scattering off phonon vibrations in crystals, which are natural degrees of freedom to absorb such energies.   

I will present a curious feature of the dark matter-phonon coupling: the leading order scattering off optical crystal phonons must precisely vanish in the limit in which the dark matter couples to atoms proportional to their mass. Such a coupling is in fact generic, and I will discuss how a judicious choice of detector material can avoid this suppression, and thus enhance reach.