Low-energy GEANT4 simulations of electron capture decay in superconducting tunnel junctions

Abstract

Despite its broad success describing fundamental particles, the Standard Model (SM) is incomplete in its description of neutrinos, as it does not include the possibility for non-zero masses which are empirically observed. Extensions to the SM can be built on the existence of additional neutrino mass eigenstates, that may help to understand the nature of the observed dark matter in our Universe. The BeEST (pronounced "beast") experiment is a search for such additional neutrino masses in the recoil energy spectrum for electron capture decay of $^7$Be. Here, detailed GEANT4 simulations are constructed to model the low-energy physics response for detection of this decay using Superconducting Tunnel Junctions (STJs) to reconstruct the neutrino mass eigenstates. This Thesis provides a proof-of-concept that GEANT4 can be used for sub-eV-scale simulations of detector physics and can recreate the working principle of STJ detectors. Furthermore, this work provides a foundation for further advances in ultra-low-energy general physics simulations in GEANT4 by implementing changes to allow sub-eV particle tracking and fix approximations in GEANT4 that break down at eV-scales.