Search for sub-GeV dark matter via the Migdal effect with an EDELWEISS germanium detector with NbSi transition-edge sensors

The EDELWEISS collaboration reports on the search for dark matter particle interactions via Migdal effect with masses between 32  MeV⋅c$^{−2}$ to 2  GeV⋅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$_{ee}$ (102.58 eV at 66 V) 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⋅c$^{−2}$. The achieved low threshold with the NbSi sensor shows the relevance of its use for out-of-equilibrium phonon sensitive devices for low-mass dark matter searches.