Search for sub-GeV Dark Matter via Migdal effect with an EDELWEISS
  germanium detector with NbSi TES sensors
  The EDELWEISS collaboration reports on the search for Dark Matter (DM)
particle interactions via Migdal effect with masses between $32$
MeV$\cdot$c$^{-2}$ to $2$ GeV$\cdot$c$^{-2}$ using a $200$ g cryogenic Ge
detector sensitive to simultaneously heat and ionization signals and operated
underground at the Laboratoire Souterrain de Modane in France. The phonon
signal was read out using a Transition Edge Sensor made of a NbSi thin film.
The detector was biased at $66$ V in order to benefit from the
Neganov-Trofimov-Luke amplification and resulting in a resolution on the energy
of electron recoils of $4.46$ eV$_{ee}$ (RMS) and an analysis threshold of $30$
eV$_{ee}$. The sensitivity is limited by a dominant background not associated
to charge creation in the detector. The search constrains a new region of
parameter space for cross-sections down to $10^{-29}$ cm$^2$ and masses between
$32$ and $100$ MeV$\cdot$c$^{-2}$. The achieved low threshold with the NbSi
sensor shows the relevance of its use for athermal-phonon sensitive devices for
low-mass DM searches.
