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

J. Aalbers, D. S. Akerib, A. K. Al Musalhi, F. Alder, C. S. Amarasinghe, A. Ames, T. J. Anderson, N. Angelides, H. M. Araújo, J. E. Armstrong, M. Arthurs, A. Baker, S. Balashov, J. Bang, J. W. Bargemann, E. E. Barillier, D. Bauer, K. Beattie, T. Benson, A. Bhatti, A. Biekert, T. P. Biesiadzinski, H. J. Birch, E. Bishop, G. M. Blockinger, B. Boxer, C. A. J. Brew, P. Brás, S. Burdin, M. Buuck, M. C. Carmona-Benitez, M. Carter, A. Chawla, H. Chen, J. J. Cherwinka, Y. T. Chin, N. I. Chott, M. V. Converse, R. Coronel, A. Cottle, G. Cox, D. Curran, C. E. Dahl, I. Darlington, S. Dave, A. David, J. Delgaudio, S. Dey, L. de Viveiros, L. Di Felice, C. Ding, J. E. Y. Dobson, E. Druszkiewicz, S. Dubey, S. R. Eriksen, A. Fan, S. Fayer, N. M. Fearon, N. Fieldhouse, S. Fiorucci, H. Flaecher, E. D. Fraser, T. M. A. Fruth, R. J. Gaitskell, A. Geffre, J. Genovesi, C. Ghag, A. Ghosh, R. Gibbons, S. Gokhale, J. Green, M. G. D. van der Grinten, J. J. Haiston, C. R. Hall, T. J. Hall, S. Han, E. Hartigan-O'Connor, S. J. Haselschwardt, M. A. Hernandez, S. A. Hertel, G. Heuermann, G. J. Homenides, M. Horn, D. Q. Huang, D. Hunt, E. Jacquet, R. S. James, J. Johnson, A. C. Kaboth, A. C. Kamaha, Meghna K. K., D. Khaitan, A. Khazov, I. Khurana, J. Kim, Y. D. Kim, J. Kingston, R. Kirk, D. Kodroff, L. Korley, E. V. Korolkova, H. Kraus, S. Kravitz, L. Kreczko, V. A. Kudryavtsev, C. Lawes, D. S. Leonard, K. T. Lesko, C. Levy, J. Lin, A. Lindote, W. H. Lippincott, M. I. Lopes, W. Lorenzon, C. Lu, S. Luitz, P. A. Majewski, A. Manalaysay, R. L. Mannino, C. Maupin, M. E. McCarthy, G. McDowell, D. N. McKinsey, J. McLaughlin, J. B. McLaughlin, R. McMonigle, E. Mizrachi, A. Monte, M. E. Monzani, J. D. Morales Mendoza, E. Morrison, B. J. Mount, M. Murdy, A. St. J. Murphy, A. Naylor, H. N. Nelson, F. Neves, A. Nguyen, C. L. O'Brien, I. Olcina, K. C. Oliver-Mallory, J. Orpwood, K. Y Oyulmaz, K. J. Palladino, J. Palmer, N. J. Pannifer, N. Parveen, S. J. Patton, B. Penning, G. Pereira, E. Perry, T. Pershing, A. Piepke, Y. Qie, J. Reichenbacher, C. A. Rhyne, A. Richards, Q. Riffard, G. R. C. Rischbieter, E. Ritchey, H. S. Riyat, R. Rosero, T. Rushton, D. Rynders, D. Santone, A. B. M. R. Sazzad, R. W. Schnee, G. Sehr, B. Shafer, S. Shaw, T. Shutt, J. J. Silk, C. Silva, G. Sinev, J. Siniscalco, R. Smith, V. N. Solovov, P. Sorensen, J. Soria, I. Stancu, A. Stevens, K. Stifter, B. Suerfu, T. J. Sumner, M. Szydagis, D. R. Tiedt, M. Timalsina, Z. Tong, D. R. Tovey, J. Tranter, M. Trask, M. Tripathi, A. Usón, A. Vacheret, A. C. Vaitkus, O. Valentino, V. Velan, A. Wang, J. J. Wang, Y. Wang, J. R. Watson, L. Weeldreyer, T. J. Whitis, K. Wild, M. Williams, W. J. Wisniewski, L. Wolf, F. L. H. Wolfs, S. Woodford, D. Woodward, C. J. Wright, Q. Xia, J. Xu, Y. Xu, M. Yeh, D. Yeum, W. Zha, E. A. Zweig

TL;DR

This paper analyzes inner-shell vacancy effects on recombination in liquid xenon by measuring electron-capture charge yields for $^{125}$Xe and $^{127}$Xe and double-electron-capture charge yields for $^{124}$Xe in the LZ detector. Using multi-site (MS) and skin-tagged single-site (SS) selections, the authors quantify $Q_y$ relative to beta decays across two drift fields, finding EC decays to have reduced charge yields and LL DEC decays to show even stronger suppression. A first Thomas-Imel box-model interpretation links enhanced recombination to increased ionization density in ISV cascades, yielding estimates like $Q_y^{LL}/Q_y^{\beta}=0.70\pm0.04$ that align with WS2024 fits, though first-principles predictions remain challenging. The results imply DEC backgrounds are non-negligible for WIMP searches and must be accurately modeled, yet the overall impact on LZ sensitivity remains modest (around 10%), while highlighting the need for improved track-structure and recombination modeling in future xenon TPC experiments.

Abstract

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

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

TL;DR

This paper analyzes inner-shell vacancy effects on recombination in liquid xenon by measuring electron-capture charge yields for Xe and Xe and double-electron-capture charge yields for Xe in the LZ detector. Using multi-site (MS) and skin-tagged single-site (SS) selections, the authors quantify relative to beta decays across two drift fields, finding EC decays to have reduced charge yields and LL DEC decays to show even stronger suppression. A first Thomas-Imel box-model interpretation links enhanced recombination to increased ionization density in ISV cascades, yielding estimates like that align with WS2024 fits, though first-principles predictions remain challenging. The results imply DEC backgrounds are non-negligible for WIMP searches and must be accurately modeled, yet the overall impact on LZ sensitivity remains modest (around 10%), while highlighting the need for improved track-structure and recombination modeling in future xenon TPC experiments.

Abstract

Electron-capture decays of Xe and Xe, and double-electron-capture decays of 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 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~V/cm. The LL double-electron-capture decay of 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.

Paper Structure

This paper contains 17 sections, 7 equations, 11 figures, 6 tables.

Table of Contents

  1. Introduction
  2. Signal production in liquid xenon
  3. Methods
  4. The LZ experiment
  5. Description of EC and DEC sources
  6. EC on $^{125}\mathrm{Xe}$ and $^{127}\mathrm{Xe}$
  7. DEC on $^{124}\mathrm{Xe}$
  8. EC event selections
  9. MS selection of K-, L-, and M-captures
  10. SS selection of K- and L-captures
  11. Results
  12. Discussion
  13. The Thomas-Imel box model
  14. Impact on dark matter searches
  15. Conclusion
  16. ...and 2 more sections

Figures (11)

  • Figure 1: Simplified schematic of xenon decaying to iodine via electron capture (left), and the ISV relaxing (right) by emitting Auger-Meitner electrons (yellow) and X-rays. A virtual photon is shown in red to illustrate the Auger-Meitner process, as well as an X-ray in green.
  • Figure 2: Nuclear decay scheme of $^{127}$Xe (a), $^{125}$Xe (b) and $^{124}$Xe (c) showing only states with a branching ratio $>1\%$branchingratios. The number above each transition is the $\gamma$-ray energy in keV, while that to the side is the percentage of parent decays that involve the transition. Relaxation through IC emission is not included in the indicated $\gamma$ intensities. The bold horizontal lines represent the ground states of the respective nuclei, while the finer ones mark excited states of the iodine isotopes, with their energy in keV on the right.
  • Figure 3: Schematics of EC decay events with either an MS (a) or Skin-tagged SS (b) topology, along with waveforms. The MS signature has one merged S1 and two S2s: one from the decay site and one from the vertically displaced $\gamma$-ray site. The SS case has an S1--S2 pair solely from the atomic cascade, with the outgoing $\gamma$-ray interaction generating a pulse in the Skin coincident with the TPC S1.
  • Figure 4: Selection of $^{127}$Xe EC decays in WS2022 ($^{125}$Xe is subdominant in this run). The corrected area of the second S2 to reach the liquid surface is plotted against that of the first. S2 areas are in units of electrons; the single electron size in WS2022 is 58.5 phd aalbers2023first. The horizontal arm (above the diagonal line) identifies events in which the S2 of the atomic cascade arrives first, i.e. the where the atomic cascade occurs above the nuclear $\gamma$ energy deposit. Events where the S2 of the $\gamma$ deposit arrives first (below the diagonal line) are not used in this analysis since the smaller, second S2 from the atomic cascade is obscured by the tail of the first. The continuum of events adjacent to the diagonal is formed by the two-step decay from the 203 keV state, where the S2 from the 58 keV $\gamma$ ray merges with the EC.
  • Figure 5: Skin-tagged events in WS2022 data within the expanded volume optimized for the SS EC analysis. Orange curves show the 1 and 2$\sigma$ contours of a Gaussian fit to the L-capture peak. The fit is based on the population of solid points, defined by an energy selection (tan shading) and a loose 4$\sigma$ cut around the $\beta$-decay background region (1 and 2$\sigma$ regions indicated by gray bands). The hollow points outside the fit window include a population of low-S2 events that are absent in the WIMP search fiducial volume and are expected from field non-uniformities and charge loss near the TPC wall. There is a distinct shift in the L-capture population down from the $\beta$-decay band, encroaching on the WIMP region of interest, shown by the dashed purple lines (1 and 2$\sigma$ contours for a 30 GeV/c$^2$ WIMP) and red lines (centroid and 1$\sigma$ contours for a flat-in-energy NR signal).
  • ...and 6 more figures