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 cm2 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) meV (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 MeV/c2, with the lowest unexplored cross section of 4×10-32 cm2 at 87 MeV/c2. 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.