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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mmultiscripts><a:mrow><a:mi>Rn</a:mi></a:mrow><a:mprescripts/><a:none/><a:mrow><a:mn>222</a:mn></a:mrow></a:mmultiscripts></a:mrow></a:math> and <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:mmultiscripts><c:mrow><c:mi>Po</c:mi></c:mrow><c:mprescripts/><c:none/><c:mrow><c:mn>218</c:mn></c:mrow></c:mmultiscripts></c:mrow></c:math> events, and the rms convection speed was measured to be <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:mn>0.30</e:mn><e:mo>±</e:mo><e:mn>0.01</e:mn><e:mtext> </e:mtext><e:mtext> </e:mtext><e:mi>cm</e:mi><e:mo>/</e:mo><e:mi mathvariant="normal">s</e:mi></e:mrow></e:math>. Given this velocity field, <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:mrow><h:mmultiscripts><h:mrow><h:mi>Pb</h:mi></h:mrow><h:mprescripts/><h:none/><h:mrow><h:mn>214</h:mn></h:mrow></h:mmultiscripts></h:mrow></h:math> background events can be tagged when they are followed by <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"><j:mrow><j:mmultiscripts><j:mrow><j:mi>Bi</j:mi></j:mrow><j:mprescripts/><j:none/><j:mrow><j:mn>214</j:mn></j:mrow></j:mmultiscripts></j:mrow></j:math> and <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"><l:mrow><l:mmultiscripts><l:mrow><l:mi>Po</l:mi></l:mrow><l:mprescripts/><l:none/><l:mrow><l:mn>214</l:mn></l:mrow></l:mmultiscripts></l:mrow></l:math> decays, or preceded by <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"><n:mrow><n:mmultiscripts><n:mrow><n:mi>Po</n:mi></n:mrow><n:mprescripts/><n:none/><n:mrow><n:mn>218</n:mn></n:mrow></n:mmultiscripts></n:mrow></n:math> decays. This was achieved by evolving a point cloud in the direction of a measured convection velocity field, and searching for <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline"><p:mrow><p:mmultiscripts><p:mrow><p:mi>Bi</p:mi></p:mrow><p:mprescripts/><p:none/><p:mrow><p:mn>214</p:mn></p:mrow></p:mmultiscripts></p:mrow></p:math> and <r:math xmlns:r="http://www.w3.org/1998/Math/MathML" display="inline"><r:mrow><r:mmultiscripts><r:mrow><r:mi>Po</r:mi></r:mrow><r:mprescripts/><r:none/><r:mrow><r:mn>214</r:mn></r:mrow></r:mmultiscripts></r:mrow></r:math> decays or <t:math xmlns:t="http://www.w3.org/1998/Math/MathML" display="inline"><t:mrow><t:mmultiscripts><t:mrow><t:mi>Po</t:mi></t:mrow><t:mprescripts/><t:none/><t:mrow><t:mn>218</t:mn></t:mrow></t:mmultiscripts></t:mrow></t:math> decays within a volume defined by the point cloud. In XENON1T, this tagging system achieved a <v:math xmlns:v="http://www.w3.org/1998/Math/MathML" display="inline"><v:mrow><v:mmultiscripts><v:mrow><v:mi>Pb</v:mi></v:mrow><v:mprescripts/><v:none/><v:mrow><v:mn>214</v:mn></v:mrow></v:mmultiscripts></v:mrow></v:math> background reduction of <x:math xmlns:x="http://www.w3.org/1998/Math/MathML" display="inline"><x:mrow><x:msubsup><x:mrow><x:mn>6.2</x:mn></x:mrow><x:mrow><x:mo>−</x:mo><x:mn>0.9</x:mn></x:mrow><x:mrow><x:mo>+</x:mo><x:mn>0.4</x:mn></x:mrow></x:msubsup><x:mo>%</x:mo></x:mrow></x:math> with an exposure loss of <z:math xmlns:z="http://www.w3.org/1998/Math/MathML" display="inline"><z:mn>1.8</z:mn><z:mo>±</z:mo><z:mn>0.2</z:mn><z:mo>%</z:mo></z:math>, 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 <bb:math xmlns:bb="http://www.w3.org/1998/Math/MathML" display="inline"><bb:mrow><bb:mmultiscripts><bb:mrow><bb:mi>Xe</bb:mi></bb:mrow><bb:mprescripts/><bb:none/><bb:mrow><bb:mn>137</bb:mn></bb:mrow></bb:mmultiscripts></bb:mrow></bb:math> background, which is relevant to the search for neutrinoless double-beta decay.