Status of the BeEST Neutrino Experiment Phase-III

The Beryllium Electron-capture in Superconducting Tunnel junctions (BeEST) experiment searches for signatures of heavy neutrino mass eigenstates in the Be-7 electron capture (EC) decay by precisely measuring the nuclear recoil energy using superconducting quantum sensors. In the first release from the Phase-II data in 2021, BeEST has set a world-leading limit on the heavy neutrino mixing in the 100-850 keV mass range using a single detector pixel counting for a month. In 2022, its scaled-up version, the BeEST Phase-III, has initiated with a 36-pixel-array detector and an increased amount of the Be-7 implantation. In this talk, we present the status of the BeEST Phase-III experiment. We will describe the improved Be-7 implantation procedure which reduced the Li-7 contamination and minimized the systematics due to the Auger electron escape. Furthermore, we will describe reduction in systematics associated with the data fitting due to progress on understanding the spectral response. We will also illustrate the improved analysis scheme using the pulse shape discrimination method, enabled by the newly adapted continuous data acquisition system.