Liquid argon light collection and veto modeling in GERDA Phase II

The ability to detect liquid argon scintillation light from within a denselypacked high-purity germanium detector array allowed the GERDA experiment toreach an exceptionally low background rate in the search for neutrinolessdouble beta decay of $^{76}$Ge. Proper modeling of the light propagationthroughout the experimental setup, from any origin in the liquid argon volumeto its eventual detection by the novel light read-out system, provides insightinto the rejection capability and is a necessary ingredient to obtain robustbackground predictions. In this paper, we present a model of the GERDA liquidargon veto, as obtained by Monte Carlo simulations and constrained bycalibration data, and highlight its application for background decomposition.