Latest results from the BeEST Phase III

The Beryllium Electron Capture in Superconducting Tunnel Junctions (BeEST) experiment probes the existence of sub-MeV heavy neutrino mass eigenstates through precision measurements of the electron-capture (EC) decay of ⁷Be. This is achieved by directly implanting high doses of ⁷Be into high-resolution superconducting tunnel junction (STJ) cryogenic sensors and precisely measuring the nuclear recoil energy of the ⁷Li daughters. If heavy neutrino states exist, transition to these states would reduce the recoil energy and produce additional low-energy peaks as a distinctive experimental signature.

In Phase III, the BeEST collaboration has collected recoil spectra with ⁷Be activities ranging from 10 to 50 Bq per pixel in a 36-pixel array STJ sensors with unprecedented precision and statistical significance. This presentation will provide an overview of the latest BeEST Phase III updates, including the staged unblinding of the recoil spectrum and improved background modeling. Specifically, improved modeling of Auger electron escape spectrum and electron shake-up/shake-off components will be discussed. The results provide a foundation for setting new constraints on sterile neutrino mixing in the sub-MeV mass range.