Oct 2018 - Sep 2019

The sub-MeV dark matter window, and the dark matter-phonon coupling

by Dr Tom Melia (IPMU)

The experimental endeavour to directly detect dark matter in a laboratory on earth must confront a possible dark matter mass range spanning over fifty 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 phonons must precisely vanish in the limit in which the dark matter couples ‘proportional-to-mass’. This is generically the case in dark matter models, and I will discuss how a judicious choice of detector material can avoid this suppression, and thus enhance reach.

Next, I will report on the ongoing detector R&D for recently proposed ‘magnetic bubble chambers’. These detectors utilise newly-discovered magnetic molecular crystals, which are the study of a rapidly progressing field of chemistry. Dark matter induced crystal vibrations initiate a magnetic avalanche that spreads throughout the crystal, providing a mechanism to amplify the initial energy deposit into a detectable signal.