Measurements and models of enhanced recombination following inner-shell vacancies in liquid xenon

Electron-capture decays of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mmultiscripts> <a:mrow> <a:mi>Xe</a:mi> </a:mrow> <a:mprescripts/> <a:none/> <a:mrow> <a:mn>125</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>Xe</c:mi> </c:mrow> <c:mprescripts/> <c:none/> <c:mrow> <c:mn>127</c:mn> </c:mrow> </c:mmultiscripts> </c:mrow> </c:math> , and double-electron-capture decays of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mrow> <e:mmultiscripts> <e:mrow> <e:mi>Xe</e:mi> </e:mrow> <e:mprescripts/> <e:none/> <e:mrow> <e:mn>124</e:mn> </e:mrow> </e:mmultiscripts> </e:mrow> </e:math> , are backgrounds in searches for weakly interacting massive particles (WIMPs) conducted by dual-phase xenon time projection chambers such as LUX-ZEPLIN (LZ). These decays produce signals with more light and less charge than equivalent-energy <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mi>β</g:mi> </g:math> decays and correspondingly overlap more with WIMP signals. We measure three electron-capture charge yields in LZ: the 1.1 keV M-shell, 5.2 keV L-shell, and 33.2 keV K-shell at drift fields of 193 and <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:mrow> <i:mn>96.5</i:mn> <i:mtext> </i:mtext> <i:mtext> </i:mtext> <i:mi mathvariant="normal">V</i:mi> <i:mo>/</i:mo> <i:mi>cm</i:mi> </i:mrow> </i:math> . The LL double-electron-capture decay of <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"> <l:mrow> <l:mmultiscripts> <l:mrow> <l:mi>Xe</l:mi> </l:mrow> <l:mprescripts/> <l:none/> <l:mrow> <l:mn>124</l:mn> </l:mrow> </l:mmultiscripts> </l:mrow> </l:math> exhibits even more pronounced shifts in charge and light. We provide a first model of double-electron-capture charge yields using the link between ionization density and electron-ion recombination, and identify a need for more accurate calculations. Finally, we discuss the implications of the reduced charge yield of these decays and other interactions creating inner-shell vacancies for future dark matter searches.