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  • Open Access

Search for new physics in low-energy electron recoils from the first LZ exposure

J. Aalbers et al. (LZ Collaboration)
Phys. Rev. D 108, 072006 – Published 9 October 2023
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  • INTRODUCTION
  • DETECTOR, DATA AND SELECTION TREATMENT
  • SIGNAL MODELS
  • STATISTICAL ANALYSIS
  • SEARCH RESULTS
  • CONCLUSIONS
  • ACKNOWLEDGMENTS
  • APPENDICES
    • References

    Abstract

    The LUX-ZEPLIN (LZ) experiment is a dark matter detector centered on a dual-phase xenon time projection chamber. We report searches for new physics appearing through few-keV-scale electron recoils, using the experiment’s first exposure of 60 live days and a fiducial mass of 5.5 t. The data are found to be consistent with a background-only hypothesis, and limits are set on models for new physics including solar axion electron coupling, solar neutrino magnetic moment and millicharge, and electron couplings to galactic axionlike particles and hidden photons. Similar limits are set on weakly interacting massive particle (WIMP) dark matter producing signals through ionized atomic states from the Migdal effect.

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    • Received 28 July 2023
    • Accepted 12 September 2023

    DOI:https://doi.org/10.1103/PhysRevD.108.072006

    Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

    Published by the American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Particle dark matter
    1. Physical Systems
    Axion-like particlesAxionsNeutrinosWeakly interacting massive particles
    1. Techniques
    Dark matter detectorsTime-projection chambers
    Particles & Fields

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    Issue

    Vol. 108, Iss. 7 — 1 October 2023

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    • Figure 1
      Figure 1

      The SR1 search data (black points) is plotted in the {S1c,log10S2c} space, after all cuts and selections are applied. For illustration purposes, the exposure is shown separated into two periods of equal live time (top panel is the first half of SR1, bottom panel is the second half). In both panels, the 1σ and 2σ regions are indicated for various background model components: Ar37 (orange contours), Xe127 (green contours), B8 (filled green), and the broad-spectrum ER background encompassing Pb212, Pb214, Kr85, and external gammas (filled gray). The solid red line indicates the median, the dashed the 10% and 90%, quantiles of a flat NR background. Thin gray lines indicate contours of constant ER energy, with a spacing of 2 keVee. A reduction in Ar37 rate is the dominant change between the two time periods.

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    • Figure 2
      Figure 2

      Signal efficiency as a function of simulated true ER energy for the trigger (blue), the ≥ threefold coincidence and >3  phd threshold on S1c (orange), single-scatter (SS) reconstruction and analysis cuts (green), and the search ROI in S1 and S2 (black). The inset shows the low-energy behavior, with the dotted line at 1.56 keV marking 50% efficiency. The data quality acceptance error band (gray) is assessed using AmLi and tritium data. The nest model uncertainties (purple) are discussed in the Appendix.

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    • Figure 3
      Figure 3

      Reconstructed energy (keVee) spectra for the background-only model (gray) and representative signal models: solar axion (orange), solar neutrino magnetic moment (green), solar neutrino millicharge (red), spin independent WIMPs undergoing the Migdal effect (blue), and axionlike particles (ALPs) (purple). To produce these spectra, the detector S1 and S2 response is simulated and then all data selections, thresholds, and data quality acceptances are applied. This standard reconstructed energy quantity can be improved by applying an unequal weighting to the sum of S1c and S2c (see text for details), and we illustrate the advantage of an unequal weighting by showing an alternative reconstruction of the example ALP signal.

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    • Figure 4
      Figure 4

      Time dependence of the data and the best-fit model. Statistical tests are performed on observed and expected counts, while in this plot counts are converted to rates by normalizing by the live time fraction of each ten-day bin. Data is shown in black. The blue line shows total summed background. The darker blue band shows the 1σ model uncertainty and the lighter blue band the combined model and statistical uncertainty. Background components are shown in colors as given in the legend. All background components are included in the fit, but those appearing exclusively below the y axis bound are not listed in the legend.

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    • Figure 5
      Figure 5

      The 90% C.L. upper limit on effective neutrino magnetic moment (left) and neutrino effective millicharge (right). Selected limits from other experiments are also shown [4, 59, 60, 61, 62, 63, 64] and astrophysical observations [65, 66].

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    • Figure 6
      Figure 6

      The 90% C.L. upper limit (black line) on solar axion gae coupling constant. Selected limits from other experiments and astrophysical observations are also shown [4, 67, 68, 69, 70, 71, 72, 73]. The shaded orange region corresponds to predicted values from the benchmark QCD axion models DFSZ [74, 75] and KSVZ [76, 77]. The LZ median sensitivity is not displayed due to close overlap with the observed limit.

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    • Figure 7
      Figure 7

      The 90% C.L. upper limit (black line) on ALP coupling constant, gae (left panel), and the HP coupling constant squared, κ2 (right panel). Selected limits from other experiments are also shown [4, 59, 69, 71, 73, 78, 79, 80, 81] along with astrophysical bounds in Ref. [82].

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    • Figure 8
      Figure 8

      The 90% C.L. upper limit (black line) for the spin-independent (SI) WIMP cross section vs WIMP mass, using the Migdal signal pathway. Also shown are limits from other experiments [1, 2, 3, 40, 41, 42, 43, 44, 78, 83, 84, 85, 86, 87, 88, 89, 90, 91]. Labeling indicates limits employing a threshold in the S2-only regime (“S2”), a Migdal recoil process (“M”), and/or a conservative signal Migdal model including only the ER component (“ER”). The DarkSide-50 ER+NR Migdal result employs the “QF” liquid Ar NR response model described in [90, 92], which assumes a Lindhard electronic partition, binomial quenching fluctuations, and no recombination enhancement due to the proximity of the ER and NR components.

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    • Figure 9
      Figure 9

      The 90% C.L. upper limit (black line) for the spin-dependent (SD) WIMP cross section versus WIMP mass for coupling on neutrons (left) and protons (right) undergoing the Migdal effect. The SD proton and neutron modes use the mean nuclear structure functions from [93]. Also shown are limits from other experiments [41, 42, 78, 84, 85, 94, 95, 96].

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    • Figure 10
      Figure 10

      Top panel: the baseline (global fit) LXe response model and the alternative (LZ tritium-fitted) response models, as referred to in this section, both illustrated as light yields Ly. The fit uncertainty on the alternative model is also shown. Lower panel: reconstructed energy spectra of the tritium calibration source is shown (black) after all cuts are applied, along with the baseline model (blue) as described in [6]. The dark blue band represents systematic uncertainty from data quality acceptances and the light blue band is the Poissonian statistical uncertainty (added to systematic uncertainty). In red is the alternative response model. The red error bars illustrate the uncertainties in the model fit, corresponding to the shaded red region in the upper panel. Note that the same systematic and statistical uncertainties on the baseline model would also apply to the alternative model.

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