Evaluation of Bremsstrahlung Models for nEXO Simulations

The nEXO experiment will search for neutrinoless double beta decay (0νββ) in ¹³⁶Xe, improving on the success of EXO-200. Double β-decays are primarily single-site events in liquid xenon detectors, but as βs deposit energy, they sometimes emit a photon due to bremsstrahlung radiation, making these events multi-site. More energetic electrons, like those expected from 0νββ, are more likely to produce bremsstrahlung. Multi-site events are mostly background, but a neural network-based discriminator identifies signal-like multi-site events with bremsstrahlung, increasing the detection efficiency of 0νββ events by about 10%. Signal-like multi-site events can be used to first verify the bremsstrahlung model and later verify the 2νββ energy spectrum. Both EXO-200 and nEXO use Geant4-based simulations, but they differ in the low-energy electromagnetic model they use. This study simulates β-decay and double β-decay events for both experiments to determine if the different models produce measurable differences in bremsstrahlung events. We aim to benchmark these models using EXO-200 data and to evaluate the impact of the model on nEXO sensitivity.