Directional Detectability of Dark Matter With Single Phonon Excitations:
  Target Comparison
  Single phonon excitations are sensitive probes of light dark matter in the
keV-GeV mass window. For anisotropic target materials, the signal depends on
the direction of the incoming dark matter wind and exhibits a daily modulation.
We discuss in detail the various sources of anisotropy, and carry out a
comparative study of 26 crystal targets, focused on sub-MeV dark matter
benchmarks. We compute the modulation reach for the most promising targets,
corresponding to the cross section where the daily modulation can be observed
for a given exposure, which allows us to combine the strength of DM-phonon
couplings and the amplitude of daily modulation. We highlight Al$_2$O$_3$
(sapphire), CaWO$_4$ and h-BN (hexagonal boron nitride) as the best polar
materials for recovering a daily modulation signal, which feature
$\mathcal{O}(1 - 100)\%$ variations of detection rates throughout the day,
depending on the dark matter mass and interaction. The directional nature of
single phonon excitations offers a useful handle to mitigate backgrounds, which
is crucial for fully realizing the discovery potential of near future
experiments.
