Dynamic Phonon Modelling in Silica for Simulating Substrate Backgrounds in the BeEST Experiment

The BeEST experiment searches for physics beyond the standard model (BSM) in the neutrino sector by utilizing the electron capture (EC) decay of 7Be. The 7Be is embedded in superconducting tunnel junction (STJ) sensors such that the low-energy (eV-scale) decay products are detected with high energy resolution and efficiency. Modelling of low-energy backgrounds is crucial to understanding potential beyond standard model (BSM) physics, including low-energy phonon and quasiparticle generation within the superconductors. In this talk, modelling methods for phonon dynamics in cryogenic silica are described in detail for use with G4CMP. Phonon properties calculated include scattering parameters, isotopic scattering rate, anharmonic decay rate, fractional phonon density of states based on phonon modes, etc. Beyond the BeEST experiment, accurate modelling of phonon dynamics is of significant importance for simulating the impact of radiation on quantum sensing regimes.