First Limits on Light Dark Matter Interactions in a Low Threshold Two-Channel Athermal Phonon Detector from the TESSERACT Collaboration

We present results of a search for spin-independent dark matter-nucleus interactions in a 1  cm$^2$ by 1 mm thick (0.233 g) high-resolution silicon athermal phonon detector operated above ground. For interactions in the substrate, this detector achieves an rms baseline energy resolution of 361.5⁢(4)  m⁢eV (statistical error), the best for any athermal phonon detector to date. With an exposure of 0.233  g×12 hours, we place the most stringent constraints on dark matter masses between 44 and 87  M⁢eV/c$^2$, with the lowest unexplored cross section of 4⁢×10$^{−32}$  c⁢m$^2$ at 87  M⁢eV/c$^2$. We employ a conservative salting technique to reach the lowest dark matter mass ever probed via direct detection experiment. This constraint is enabled by two-channel rejection of low energy backgrounds that are coupled to individual sensors.