Grant: Subatomic Physics Envelope - Project

Sterile neutrinos (SN) are mysterious new particles predicted by theory that offer an intriguing possibility in the quest for understanding the nature of dark matter (DM), which contributes ~85% of the total matter in our universe. SNs could have far-reaching cosmological implications as their properties might be linked to the matter-antimatter asymmetry, which is an outstanding puzzle in cosmology. Unlocking these connections could provide insights into the universe's origins and evolution including galaxy formation. Unlike the known "active" neutrinos in the Standard Model of particle physics, SN do not interact with normal matter, making them very difficult to detect. However, their presence can be indirectly inferred through their mass-generated effects from momentum conservation with SM particles. The BeEST (Beryllium Electron-capture with Superconducting Tunnel junctions (STJs)) experiment employs advanced STJ quantum sensors implanted with 7Be to study its nuclear electron capture (EC) decay, a scenario where the final state contains only the neutrino and a recoiling atom. This experiment aims to search for SN in the keV-mass range, which are of particular interest due to the potential implications for "warm" dark matter and the matter-antimatter asymmetry problem. BeEST precisely measures the 7Li recoil energy spectrum in the quantum sensors. Any signal from SN would manifest as a subtle shift in this spectrum, resulting from the conservation of momentum following neutrino emission. This approach is a powerful, model-independent method in the search for beyond SM (BSM) physics and is the most sensitive of its kind to date. The detection and characterization of eV-scale energies at the precision required to provide competitive limits on BSM physics pose an extreme technical challenge. The BeEST combines cutting-edge STJ sensors developed at Lawrence Livermore National Laboratory implanted with pure, high-intensity, radioactive 7Be ion beams developed at TRIUMF. Successful proof-of-principle implantations already led to improved exclusion limits on SNs between 100 to 850 keV by an order of magnitude. However, there are current experimental limitations that hinder further progress in our research. This project aims to address these limitations through various objectives: (1) a new ion-implantation station design with highly optimized beam diagnostics and beam modification capabilities for reliable low-energy implantation, (2) assembly and commissioning of the new portable station including a functionality to install multiple STJ detector arrays, (3) implantation and subsequent data taking in an adiabatic demagnetization refrigerator, followed by (4) a highly involved data analysis. The short-term goals of this project will significantly enhance the BeEST's capabilities, allowing it to narrow down exclusion limits for keV-mass SN to an impressive sensitivity of 10-9, bringing us closer to solving the mystery of dark matter.