Location: TRIUMF
Participating Canadian institutions: TRIUMF
International partners: USA
The search for sterile neutrinos is among the most promising avenues in our quest for understanding the microscopic nature of dark matter in our universe. Sterile neutrinos - unlike the active neutrinos in the Standard Model (SM) - do not interact with normal matter as they move through space, and as such are best observed via their mass-generated effects that result from momentum conservation with SM particles. The BeEST experiment (Beryllium Electron-capture in Superconducting Tunnel junctions) aims to perform the highest-sensitivity search for keV-scale sterile neutrinos to date using the electron capture decay of \(^7\)Be implanted into superconducting quantum sensors. This work has been possible by leveraging existing state-of-the-art superconducting tunnel junction (STJ) detector technology developed at Lawrence Livermore National Laboratory (LLNL), as well as more than a decade of work at TRIUMF-ISAC for the low-energy implantation of pure \(^7\)Be beams into thin films 1. The proposed program employs momentum reconstruction in the electron capture (EC) decay of \(^7\)Be implanted in these STJs to search for missing momentum in the nuclear recoil spectrum that would be generated by heavy neutrinos 2 . Since the neutrino escapes the thin detector without interacting, the spectrum consists of four peaks corresponding to the energy released by the recoiling \(^7\)Li daughter atom for different decay processes. The signature of heavy neutrinos is a small fraction of events whose recoil energy peaks are shifted to lower energies due to the missing momentum. The relative fraction of these events to the total indicates the mixing fraction with the electron neutrino.
Over the coming 5 years, BeEST will scale the experiment using existing 36- and 112-pixel detector arrays, fabricate new 128-pixel Al-based STJ detector arrays deposited on thin membranes, and develop a new target and beam purification techniques at TRIUMF-ISAC and ARIEL to reach \(^7\)Be intensities approaching \(10^{10}\) s\(^{-1}\). The goal is to either find a candidate or provide exclusion limits 10,000 times more stringent than previous tests in the 5-860 keV mass region. The Canadian contribution towards optimal beryllium implantation will make use of the new CFI-funded CANREB facility, and electron-induced photofission at ARIEL.