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  • Open Access

Background determination for the LUX-ZEPLIN dark matter experiment

J. Aalbers et al. (The LUX-ZEPLIN Collaboration)
Phys. Rev. D 108, 012010 – Published 17 July 2023
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  • INTRODUCTION
  • BACKGROUND SIMULATIONS
  • GLOBAL TPC BACKGROUND DETERMINATION
  • BACKGROUNDS COUNTED IN THE SR1 EXPOSURE
  • BACKGROUND IMPACT ON THE SR1 WIMP SEARCH
  • CONCLUSIONS
  • ACKNOWLEDGMENTS
    • Supplemental Material
    References

    Abstract

    The LUX-ZEPLIN experiment recently reported limits on WIMP-nucleus interactions from its initial science run, down to 9.2×10−48  cm2 for the spin-independent interaction of a 36  GeV/c2 WIMP at 90% confidence level. In this paper, we present a comprehensive analysis of the backgrounds important for this result and for other upcoming physics analyses, including neutrinoless double-beta decay searches and effective field theory interpretations of LUX-ZEPLIN data. We confirm that the in-situ determinations of bulk and fixed radioactive backgrounds are consistent with expectations from the ex-situ assays. The observed background rate after WIMP search criteria were applied was (6.3±0.5)×10−5  events/keVee/kg/day in the low-energy region, approximately 60 times lower than the equivalent rate reported by the LUX experiment.

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    • Received 22 December 2022
    • Accepted 30 May 2023

    DOI:https://doi.org/10.1103/PhysRevD.108.012010

    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 matterWeakly interacting massive particles
    1. Techniques
    Dark matter detectors
    Particles & Fields

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    Supplemental Material

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    Issue

    Vol. 108, Iss. 1 — 1 July 2023

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    • Figure 1
      Figure 1

      Fitted radon alpha spectra in S1-linear calibrated energy. Top: All alpha events in the TPC are shown with only TBA cuts applied to remove excess grid and below-cathode alpha events. The Rn222, Po218, and Po214 were modeled as double Gaussians, Po212 as a single Gaussian, and Po210 as a modified Crystal Ball function. Bottom: Alpha spectra for SS events in the SR1 fiducial volume (see Sec. 5a), where position information from the S2 pulse was used to correct the spectra, resulting in improved resolution. The sum of five Gaussians was used to model the Rn222, Po218, Po216, Po214, and Po212 peaks.

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    • Figure 2
      Figure 2

      The observed and simulated distributions of selected Rn222 daughters. The Po218 distribution in data was found from SS events and is shown in panel (a); the Rn222 distribution can be considered to be nearly identical to that of Po218. The measured Po214 distribution is shown in panel (b), for which the S1-S2 pair of Po214 was extracted from each Bi-Po event to reconstruct its position analogously to a SS event. Simulated Pb214 and Po214 distributions in panels (c) and (d), respectively. The robustness of the toy model was validated from the agreement between the simulated and observed Po214 distributions. The FV boundary is shown with a dashed gray line on all panels, whereas for the observed distributions, two additional dotted lines further separate the FV into upper, lower, and outer regions. These volumes are used in Table 1 to quantify the nonuniformity of the observed alphas.

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    • Figure 3
      Figure 3

      Rates of activated Xe127, Xe131m, Xe129m, and Xe133 as a function of time in 1.5 day bins during the SR1 exposure beginning 23 December 2021. The two hiatuses are due to the mid-SR1 DD calibration, which resulted in additional neutron activation of the active xenon, and a circulation interruption that led to a decrease in purity, which affected energy reconstruction. Exponential trends for the measured half-lives are superposed.

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    • Figure 4
      Figure 4

      Fit results for the SR1 exposure for the inner one-tonne region of the TPC. Data are shown in black and the summed background model in purple. Left: Pre-DD calibrations Right: Post-DD calibrations.

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    • Figure 5
      Figure 5

      Fit of the cavern gamma spectra to data taken during technical commissioning when the TPC was filled with gaseous xenon and the water tanks and OD were empty. The reconstructed energy scale is based on the observed S1 pulses, using a linear energy calibration derived with the gamma-ray photopeak signals.

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    • Figure 6
      Figure 6

      The simulated position distribution of all single scatter ER events from detector components and cavern gamma rays. Overlaid are the three subvolumes (upper, lateral, lower) and the SR1 FV in which fits were performed. The inner one-tonne volume, used for the fitting of internal sources in Sec. 3c, is also shown.

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    • Figure 7
      Figure 7

      Fitted detector component spectra in the SR1 FV, following the prescribed sequence of subvolume fits. The fit results are consistent with an average residual of approximately zero, with fluctuations at certain peaks arising from imperfections in energy resolution matching between data and simulations. The contribution from internals is fixed here according to the outputs of the central subvolume fit. Gray shading is used to obscure data <80  keVee to avoid inferences in the regions of interest for future EFT and Xe124 2νDEC searches.

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    • Figure 8
      Figure 8

      Total pulse size spectra in the OD for both SR1 background data and an AmLi calibration of duration 7.4 hours.

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    • Figure 9
      Figure 9

      Locations of MS neutron events identified in the SR1 dataset, correlated across all three detectors. Top: Distribution of the 10 identified neutron events in log10(S2c)−S1c space overlaid with the MS NR band, as well as their averaged positions in the TPC. White crosses denote three example events displayed in detail on the second row. Bottom: Chains of reconstructed scatters demonstrating interdetector coincidences in tagging neutron events. Working outwards: the red outline indicates the SR1 FV; the gray curve highlights the TPC wall boundary in reconstructed space; the black box indicates the physical edges of the active xenon volume; the teal profile denotes the liquid xenon Skin; the outermost green region represents the OD acrylic tanks containing the GdLS. As the exact chronology of the event could not be determined, interactions were ordered by drift time. Black circles denote the locations of the scatters with shortest drift time in the given neutron MS chain, with empty circles showing the positions of other interactions in the TPC. Scatters in the Skin and OD are shaded in blue and green respectively. Neutron captures in the OD are marked as a *, and resulting gamma-ray splashes observed in the Skin are labeled with a pink cross. OD points are randomly assigned radial positions as XY reconstruction there is often biased towards the centre, a correction for which is under development.

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    • Figure 10
      Figure 10

      Dependence of the radial position resolution of the wall events on S2 pulse size. Events below the ER band and in the WS sideband region (80<S1c<500  phd) were selected to determine the radial position resolution of wall events, and were observed to depend on the S2 pulse size based on the equation: σΔr(S2)=aS2+b, where the fitting parameters a and b were 21.04±0.65 and 0.22±0.02, respectively.

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    • Figure 11
      Figure 11

      Time dependence of events in the 2σ contour for Ar37 in log10(S2c)−S1c space (see Fig. 15). Black points show the data. The orange band shows the best fit to a rate constant in time, including systematic uncertainty from the fit, and the blue band shows the best fit to a rate constant in time plus an exponential decay with a 35 day half-life. The data and predictions are corrected for the live time in each bin.

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    • Figure 12
      Figure 12

      The accidentals PDF normalized to events/tonne/yr. The 10% and 90% quantiles (dashed) as well as the median (solid) of the ER and NR bands, as reported in Ref. [7], are shown in blue and red, respectively. SR1 WS events remaining after all data selections are also shown (black dots). The regions outside the WS ROI are marked with a shaded gray area. The number of predicted accidental events in the entire ROI is 1.2 (0.2 inside the NR band). The top and right panels show the projections on each axis of the ACS events surviving all analysis cuts, where the distribution falls off at small log10(S2c) due to the applied 600 phd lower bound in raw S2 pulse size. The orange lines represent the functions that were used to build the two-dimensional PDF. Regions with larger data fluctuations, starting from the dashed gray lines, were smoothed out.

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    • Figure 13
      Figure 13

      Events that passed all WS cuts, but were tagged by the OD (i.e. fail the WS OD veto cut) are shown in log10(S2c)−S1c space. Each data point is represented as a pie chart, with sectors representing the likelihood it originated from the given background. 1σ and 2σ contours for background and signal (neutrons in this case) are overlaid.

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    • Figure 14
      Figure 14

      Background model before fitting to the SR1 data (except for the Ar37 component, for which the post-fit normalization is used). The total model is shown in dark blue, and SR1 data after all WS cuts have been applied are denoted by the black points. This represents a background event rate of (6.3±0.5)×10−5  events/keVee/kg/day.

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    • Figure 15
      Figure 15

      Left: Low-energy data after all data quality and physics cuts in log10(S2c)−S1c space. Contours enclose 1σ and 2σ of the best-fit background model (shaded gray), the Ar37 component (orange ellipses), a 30  GeV/c2 WIMP (purple dashed lines), and B8 solar neutrinos (shaded green regions). The solid red line shows the NR median, and the red dotted lines indicate the 10% and 90% quantiles. Right: Corrected z and r2 positions of the same data. Dashed black lines outline the active liquid xenon volume and dashed gray lines represent the fiducial volume. In both figures, events falling below the 2σ contour of the best-fit background model are shown as pie charts for which the size of each wedge is determined by the relative weight of each of the background components in the fit.

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