Argon Background Impact on Beyond Standard Model Physics Searches in LEGEND-200

The LEGEND-200 experiment combines technologies from the MAJORANA DEMONSTRATOR and GERDA experiments in the search for neutrinoless double beta decay (0νββ) in the ⁷⁶Ge isotope. A key technology is the use of liquid argon (LAr), in which bare ⁷⁶Ge-enriched detectors are submerged, as a shield against external radioactivity. A drawback to using natural LAr is its intrinsic low-energy radioactivity from cosmogenically activated ³⁹Ar, a long-lived (half-life of 269.2y) beta-emitter. With a Q-value of 565 keV, betas from ³⁹Ar decays populate the low-energy ROI for searches of physics beyond the standard model, such as bosonic dark matter. ⁴²Ar (half-life of 32.9y) and its progeny ⁴²K also emit betas that become backgrounds in both the low-energy and 0νββ ROIs. Argon from underground sources has lower activity; specifically, the ³⁹Ar concentration is reduced by a factor of 1400. Monte Carlo simulations show that using radioisotope-depleted argon in the LEGEND-200 experiment would allow the experiment to search for dark matter and other BSM physics.