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Study of few-electron backgrounds in the LUX-ZEPLIN detector

D. S. Akerib, A. K. Al Musalhi, F. Alder, J. Almquist, 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, K. Beattie, T. Benson, A. Bhatti, T. P. Biesiadzinski, H. J. Birch, E. Bishop, G. M. Blockinger, B. Boxer, C. A. J. Brew, P. Brás, S. Burdin, M. C. Carmona-Benitez, M. Carter, A. Chawla, H. Chen, Y. T. Chin, N. I. Chott, S. C. Contreras, 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, C. L. Dunbar, S. R. Eriksen, A. Fan, 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. Hall, E. Hartigan-O'Connor, S. J. Haselschwardt, M. A. Hernandez, S. A. Hertel, G. J. Homenides, M. Horn, D. Q. Huang, D. Hunt, E. Jacquet, R. S. James, K. Jenkins, A. C. Kaboth, A. C. Kamaha, M. K. Kannichankandy, D. Khaitan, A. Khazov, J. Kim, Y. D. Kim, J. Kingston, D. Kodroff, 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, J. Long, 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, B. Mitra, E. Mizrachi, M. E. Monzani, E. Morrison, B. J. Mount, M. Murdy, A. St. J. Murphy, 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, N. J. Pannifer, N. Parveen, S. J. Patton, B. Penning, G. Pereira, E. Perry, T. Pershing, A. Piepke, S. S. Poudel, Y. Qie, J. Reichenbacher, C. A. Rhyne, G. R. C. Rischbieter, E. Ritchey, H. S. Riyat, R. Rosero, T. Rushton, D. Rynders, S. Saltão, D. Santone, A. B. M. R. Sazzad, R. W. Schnee, G. Sehr, B. Shafer, S. Shaw, K. Shi, T. Shutt, C. Silva, G. Sinev, J. Siniscalco, A. M. Slivar, R. Smith, V. N. Solovov, P. Sorensen, J. Soria, A. Stevens, T. J. Sumner, A. Swain, M. Szydagis, D. R. Tiedt, M. Timalsina, Z. Tong, D. R. Tovey, J. Tranter, M. Trask, M. Tripathi, A. Usón, A. C. Vaitkus, O. Valentino, V. Velan, A. Wang, J. J. Wang, Y. Wang, L. Weeldreyer, T. J. Whitis, K. Wild, 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, H. Zhang, T. Zhang

TL;DR

This work investigates few-electron backgrounds in the LUX-ZEPLIN detector, focusing on delayed electron emission from liquid xenon impurities and spontaneous grid emission. It shows that drift electrons captured by impurities primarily drive delayed emission, with the rate following a non-integer power law in delay $\Delta t$ and an amplitude that scales with the progenitor S2 size; the field strength has only a modest effect on the exponent $\beta$. The study of grid emission reveals localized hot spots on the gate wire, with emission often accompanied by photons; a photon-tagging strategy is demonstrated to identify and significantly suppress grid-induced backgrounds in the few-electron regime, enabling improved sensitivity for ionization-only dark matter searches. The findings provide practical mitigation techniques and guide future optimizations to reduce backgrounds and extend reach for low-mass DM in LZ and similar detectors. Overall, the work advances background understanding and introduces a concrete background rejection tool based on electron-photon coincidences.

Abstract

The LUX-ZEPLIN (LZ) experiment aims to detect rare interactions between dark matter particles and xenon. Although the detector is designed to be the most sensitive to GeV/$c^2$--TeV/$c^2$ Weakly Interacting Massive Particles (WIMPs), it is also capable of measuring low-energy ionization signals down to a single electron that may be produced by scatters of sub-GeV/$c^2$ dark matter. The major challenge in exploiting this sensitivity is to understand and suppress the ionization background in the few-electron regime. We report a characterization of the delayed electron backgrounds following energy depositions in the LZ detector under different detector conditions. In addition, we quantify the probability for photons to be emitted in coincidence with electron emission from the high voltage grids. We then demonstrate that spontaneous grid electron emission can be identified and rejected with a high efficiency using a coincident photon tag, which provides a tool to improve the sensitivity of future dark matter searches.

Study of few-electron backgrounds in the LUX-ZEPLIN detector

TL;DR

This work investigates few-electron backgrounds in the LUX-ZEPLIN detector, focusing on delayed electron emission from liquid xenon impurities and spontaneous grid emission. It shows that drift electrons captured by impurities primarily drive delayed emission, with the rate following a non-integer power law in delay and an amplitude that scales with the progenitor S2 size; the field strength has only a modest effect on the exponent . The study of grid emission reveals localized hot spots on the gate wire, with emission often accompanied by photons; a photon-tagging strategy is demonstrated to identify and significantly suppress grid-induced backgrounds in the few-electron regime, enabling improved sensitivity for ionization-only dark matter searches. The findings provide practical mitigation techniques and guide future optimizations to reduce backgrounds and extend reach for low-mass DM in LZ and similar detectors. Overall, the work advances background understanding and introduces a concrete background rejection tool based on electron-photon coincidences.

Abstract

The LUX-ZEPLIN (LZ) experiment aims to detect rare interactions between dark matter particles and xenon. Although the detector is designed to be the most sensitive to GeV/--TeV/ Weakly Interacting Massive Particles (WIMPs), it is also capable of measuring low-energy ionization signals down to a single electron that may be produced by scatters of sub-GeV/ dark matter. The major challenge in exploiting this sensitivity is to understand and suppress the ionization background in the few-electron regime. We report a characterization of the delayed electron backgrounds following energy depositions in the LZ detector under different detector conditions. In addition, we quantify the probability for photons to be emitted in coincidence with electron emission from the high voltage grids. We then demonstrate that spontaneous grid electron emission can be identified and rejected with a high efficiency using a coincident photon tag, which provides a tool to improve the sensitivity of future dark matter searches.

Paper Structure

This paper contains 9 sections, 9 figures, 2 tables.

Table of Contents

  1. Introduction
  2. The LZ detector
  3. Delayed electron emission
  4. Impurity and drift time dependence
  5. Electric field dependence
  6. Electron emission from grids
  7. Photons accompanying grid electron emission
  8. Implications for dark matter searches
  9. Summary

Figures (9)

  • Figure 1: Rates of SE pulses at different radial distances relative to their progenitors. These rates were obtained from randomly triggered commissioning datasets. Each curve corresponds to a different time delay window $\Delta t$ following a progenitor. Pulse counts in this plot are normalized by the area of the annular ring subtended by a 3 cm wide bin in $\Delta r$. This bin width corresponds to the nominal spatial resolution for SE pulses in WS2022. An additional correction (described in the text) is applied to rates at values of $\Delta r$ that would be partially outside of the TPC.
  • Figure 2: Rates of position-correlated (blue) and position-uncorrelated (orange) SE pulses at time $\Delta t$ after progenitor S2 pulses which are at least 200 $e_R$ in size. These rates were obtained from randomly triggered commissioning datasets. The rates are normalized using the method explained in the text, and the position-uncorrelated rates have been subtracted from the position-correlated rates.
  • Figure 3: Evolution of observed position-correlated electron background rates for electron lifetimes ranging from 2 ms to 8 ms (left column) and for progenitor drift times ranging from $50\,\mu$s to $950\,\mu$s (right column). Rates in the top two plots are normalized by $e_I$ while rates in the bottom two plots are normalized by $e_L$; these quantities are defined in Tab. \ref{['tab:norm']}. Rates in all plots have their corresponding position-uncorrelated rates subtracted. These rates were obtained from the WS2022 dataset.
  • Figure 4: Power law fit parameters for $e_I$-normalized rates of position-correlated SEs at different progenitor drift times. The power law amplitude linearly depends on the progenitor drift time, whereas the exponent does not exhibit any significant dependence. Error bars shown here are statistical only and do not include a 10% systematic uncertainty on values of $\beta$ which is described in the text.
  • Figure 5: Evolution of electron background rates following progenitors which originate in the liquid above the gate grid in LZ, with 3.4 (left) and 3.9 kV/cm (right) electric fields. These rates were obtained from a combination of the commissioning and grid tuning datasets. The top two plots show both the position-correlated and position-uncorrelated rates. The bottom two plots show power law fits to the remaining position-correlated rates after subtracting the position-uncorrelated rates. While power law fits at each field setting appear consistent with previously observed results, they are subject to large errors. These errors result from a lack of statistics and fit windows that are restricted by un-modeled backgrounds such as the photoionization cascade.
  • ...and 4 more figures