Hans Kraus

Abstract:

We report results of a search for nuclear recoils induced by weakly interacting massive particle (WIMP) dark matter using the LUX-ZEPLIN (LZ) two-phase xenon time projection chamber. This analysis uses a total exposure of $4.2\pm 0.1$ tonne-years from 280 live days of LZ operation, of which $3.3\pm 0.1$ tonne-years and 220 live days are new. A technique to actively tag background electronic recoils from ${}^{214}\mathrm{Pb}$ $\beta$ decays is featured for the first time. Enhanced electron-ion recombination is observed in two-neutrino double electron capture decays of ${}^{124}\mathrm{Xe}$ , representing a noteworthy new background. After removal of artificial signal-like events injected into the dataset to mitigate analyzer bias, we find no evidence for an excess over expected backgrounds. World-leading constraints are placed on spin-independent (SI) and spin-dependent WIMP-nucleon cross sections for masses $\ge 9\mathrm{GeV}/c^2$ . The strongest SI exclusion set is $2.2\times {10}^{-48}{\mathrm{cm}}^2$ at the 90% confidence level and the best SI median sensitivity achieved is $5.1\times {10}^{-48}{\mathrm{cm}}^2$ , both for a mass of $40\mathrm{GeV}/c^2$ . Published by the American Physical Society 2025

Abstract:

Electron-capture decays of ${}^{125}\mathrm{Xe}$ and ${}^{127}\mathrm{Xe}$ , and double-electron-capture decays of ${}^{124}\mathrm{Xe}$ , 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 $\beta$ 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 $96.5\mathrm{V}/\mathrm{cm}$ . The LL double-electron-capture decay of ${}^{124}\mathrm{Xe}$ 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.