First Limits on sub-MeV BSM Physics in the Neutrino Sector from Phase-II of the BeEST Experiment

The lepton sector of the Standard Model (SM) provides perhaps the best window into Beyond Standard Model (BSM) physics through investigations into non-zero neutrino mass states.  Searches for mass states in the keV-MeV range - associated with mostly sterile flavors - are among the most highly motivated since they have the right cosmological properties to explain the observed dark matter in our universe and may offer insight into the baryon asymmetry of the Universe.  The Beryllium Electron-capture with Superconducting Tunnel junctions (BeEST) experiment probes BSM neutrino physics in the sub-MeV mass range using momentum reconstruction in ⁷Be electron capture (EC) decay with quantum sensors.  In this talk, I will present our first limits from Phase-II of the experiment which used a single sensor counting for 28 days at low rate [PRL 126, 021803 (2021)] which are up to an order of magnitude better than all previous decay limits in the 100 - 860 keV region.