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 $^{ 222}\text{ Rn}$ and $^{ 218}\text{ Po}$ events, and the root-mean-square convection speed was measured to be $0.30 \pm 0.01$ cm/s. Given this velocity field, $^{ 214}\text{ Pb}$ background events can be tagged when they are followed by $^{ 214}\text{ Bi}$ and $^{ 214}\text{ Po}$ decays, or preceded by $^{ 218}\text{ Po}$ decays. This was achieved by evolving a point cloud in the direction of a measured convection velocity field, and searching for $^{ 214}\text{ Bi}$ and $^{ 214}\text{ Po}$ decays or $^{ 218}\text{ Po}$ decays within a volume defined by the point cloud. In XENON1T, this tagging system achieved a $^{ 214}\text{ Pb}$ background reduction of $6.2^{ +0.4}_{ -0.9}\%$ with an exposure loss of $1.8\pm 0.2 \%$, 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 $^{ 137}\text{ Xe}$ background, which is relevant to the search for neutrinoless double-beta decay.
Categories: hep-ex physics.ins-det