Search for Daily Modulation of MeV Dark Matter Signals with DAMIC-M

Dark matter (DM) particles with sufficiently large cross sections may scatter as they travel through Earth's bulk. The corresponding changes in the DM flux give rise to a characteristic daily modulation signal in detectors sensitive to DM -electron interactions. Here, we report results obtained from the first underground operation of the DAMIC-M prototype detector searching for such a signal from DM with MeV-scale mass. A model -independent analysis finds no modulation in the rate of 1 e- events with sidereal period, where a DM signal would appear. We then use these data to place exclusion limits on DM in the mass range [0.53; 2.7] MeV/c2 interacting with electrons via a dark photon mediator. Taking advantage of the time -dependent signal we improve by similar to 2 orders of magnitude on our previous limit obtained from the total rate of 1 e- events, using the same dataset. This daily modulation search represents the current strongest limit on DM -electron scattering via ultralight mediators for DM masses around 1 MeV/c2.