Measurement of charge and light yields for 127Xe L-shell electron captures in liquid xenon

Dark matter searches using dual-phase xenon time-projection chambers (LXe-TPCs) rely on their ability to reject background electron recoils (ERs) while searching for signal-like nuclear recoils (NRs). ER response is typically calibrated using beta-decay sources, such as tritium, but these calibrations do not characterize events accompanied by an atomic vacancy, as in solar neutrino scatters off inner-shell electrons. Such events lead to emission of x rays and Auger electrons, resulting in higher electron-ion recombination and thus a more NR-like response than inferred from beta-decay calibration. We present a cross-calibration of tritium beta-decays and Xe-127 electron-capture decays (which produce inner-shell vacancies) in a small-scale LXe-TPC and give the most precise measurements to date of light and charge yields for the Xe-127 L-shell electron-capture in liquid xenon. We observe a 6.9 sigma (9.2 sigma) discrepancy in the L-shell capture response relative to tritium beta decays, measured at a drift field of 363 +/- 14 V/cm (258 +/- 13 V/cm), when compared to simulations tuned to reproduce the correct beta-decay response. In dark matter searches, use of a background model that neglects this effect leads to overcoverage (higher limits) for background-only multi-kiloton-year exposures, but at a level much less than the 1-sigma experiment-to-experiment variation of the 90% C.L. upper limit on the interaction rate of a 50 GeV/c(2) dark matter particle.