Offline tagging of radon-induced backgrounds in XENON1T and applicability to other liquid xenon time projection chambers

This paper details the first application of a software tagging algorithm to reduce radon-induced backgrounds in liquid noble element time projection chambers, such as XENON1T and XENONnT. The convection velocity field in XENON1T was mapped out using Rn-222 and Po-218 events, and the rms convection speed was measured to be 0.30 +/- 0.01 cm/s. Given this velocity field, Pb-214 background events can be tagged when they are followed by Bi-214 and Po-214 decays, or preceded by Po-218 decays. This was achieved by evolving a point cloud in the direction of a measured convection velocity field, and searching for Bi-214 and Po-214 decays or Po-218 decays within a volume defined by the point cloud. InXENON1T, this tagging system achieved a Pb-214 background reduction of 6.2(-0.9)(+0.4) % with an exposure loss of 1.8 +/- 0.2%, despite the timescales of convection being smaller than the relevant decay times. We show that the performance can be improved in XENONnT, and that the performance of such a software-tagging approach can be expected to be further improved in a diffusion-limited scenario. Finally, a similar method might be useful to tag the cosmogenic Xe-137 background, which is relevant to the search for neutrinoless double-beta decay.