Measuring Phonon Velocities in CaWO$_{4}$ with Phonon Imaging

A recent search for dark matter by the CRESST experiment seeks to observe dark matter collisions with nuclei of CaWO$_{4}$ using low temperature bolometric detectors which are sensitive to energy deposition on the order of 10 keV with a few percent accuracy. Determination of the amount of energy deposited depends upon details of the non-equilibrium heat flow in the absorber crystals. Our phonon images are sensitive to small angular variations in the phonon flux, and can provide phonon group-velocity data along symmetry and non-symmetry directions.