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Extending light WIMP searches to single scintillation photons in LUX

D. S. Akerib et al. (LUX Collaboration)
Phys. Rev. D 101, 042001 – Published 10 February 2020
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Abstract
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Article Text
  • INTRODUCTION
  • PHOTOMULTIPLIER CALIBRATION
  • EXTENDING THE TRITIUM ER CALIBRATION
  • WIMP SEARCH
  • DISCUSSION
  • CONCLUSION
  • ACKNOWLEDGMENTS
    • References

    Abstract

    We present a novel analysis technique for liquid xenon time projection chambers that allows for a lower threshold by relying on events with a prompt scintillation signal consisting of single detected photons. The energy threshold of the LUX dark matter experiment is primarily determined by the smallest scintillation response detectable, which previously required a twofold coincidence signal in its photomultiplier arrays, enforced in data analysis. The technique presented here exploits the double photoelectron emission effect observed in some photomultiplier models at vacuum ultraviolet wavelengths. We demonstrate this analysis using an electron recoil calibration dataset and place new constraints on the spin-independent scattering cross section of weakly interacting massive particles (WIMPs) down to 2.5  GeV/c2 WIMP mass using the 2013 LUX dataset. This new technique is promising to enhance light WIMP and astrophysical neutrino searches in next-generation liquid xenon experiments.

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    • Received 23 July 2019
    • Accepted 24 December 2019

    DOI:https://doi.org/10.1103/PhysRevD.101.042001

    © 2020 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Dark matterParticle dark matterWeakly interacting massive particles
    1. Techniques
    Dark matter detectors
    Particles & FieldsGravitation, Cosmology & Astrophysics

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    Vol. 101, Iss. 4 — 15 February 2020

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    Images

    • Figure 1
      Figure 1

      A single S1-photon event from the December 2013 LUX tritium dataset. The S2 pulse (∼35 extracted electrons) was preceded by a pulse integrating to 2.4 phe—likely due to DPE emission in response to a real S1 detected photon (the relevant PMT waveform is shown in the inset). In the standard S1+S2 analysis this would have been classified as an “S2-only” event, as the S1 candidate has failed the twofold requirement [13].

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    • Figure 2
      Figure 2

      Pulse area distributions (in phe) of SPE-like pulses summed across the top (left) and bottom (right) arrays. These were identified in the quiet waveforms preceding tritium S1 pulses (in red), or between the tritium S1 and S2 pulses (in blue). These distributions do not include contributions from 10 channels that were removed for this analysis.

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    • Figure 3
      Figure 3

      Pulse area distributions (in phe) for the single photoelectron response (left) and the single VUV photon response (right) of an example PMT, along with the fit parameters. These include the mean and standard deviation obtained from the dark count population (μDC,σDC), and the mean and standard deviation of both the SPE (μ1,σ1) and DPE responses (μ2,σ2) obtained from the single scintillation photon response. The DPE probability for this PMT is R=(20.8±3.0)%. Grey vertical lines, in the right panel plot, indicate the signal region for this channel.

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    • Figure 4
      Figure 4

      DPE signal region for each active channel. Blue bars illustrate the [μ1+3σ1, μ2+2σ2] region for each PMT, while missing bars indicate where channels were not used for this analysis. The dark blue bars indicate channels located in the top PMT array, while the lighter blue bars show bottom array channels. Black lines show the mean of the DPE response (μ2) for each PMT.

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    • Figure 5
      Figure 5

      S2 pulse area spectrum for tritium dataset events in which the S1 consisted of a single detected VUV photon with pulse area within the DPE signal region, along with nest v2.0.0 prediction added to the background expectation from the DC+S2-only coincidence events. The error bars on the nest v2.0.0+Background line represent the uncertainty on the background expectation. The shaded blue regions represent the systematic uncertainty due to the g1 and DPE cut acceptance error, while the shaded grey regions represent the yield uncertainties (produced by incorporating appropriate yield variations in the nest model, shown in Fig. 6). The mean energy as predicted by nest v2.0.0 for the range of S2s in each bin is indicated on the top x axis.

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    • Figure 6
      Figure 6

      The ER (left) and NR (right) ionization (red) and scintillation (grey) yields as a function of recoil energy given by nest v2.0.0. The bands around the ER yields indicate model uncertainties and correspond to those presented in Fig. 5, while red and black points indicate LUX measurements from the tritium (left) [13] and D-D neutron (right) [29] calibrations. Additional data are shown in blue and green for the ER ionization yield [30, 31]. Recent NR ionization yield measurements, performed at the Lawrence Livermore National Laboratory (LLNL), that have not yet been incorporated in the nest v2.0.0 models but are used here to motivate the low-energy threshold are presented in blue [32]. The nest v2.0.1 NR model yields that became available after journal submission incorporating the new data are shown using dotted lines. The thresholds adopted for the new analyses correspond to the rightmost part of the shaded regions.

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    • Figure 7
      Figure 7

      Events observed in the 2013 LUX WIMP search exposure of 95 live days and 118 kg fiducial mass (11, 210 kgdays). Solid black markers represent events meeting the S1 twofold coincidence requirement, while the six hollow markers indicate those with an S1 of 1 phd which are the focus of this analysis. Distribution contours for an ER beta spectrum (grey) and an example 50  GeV/c2 WIMP signal (red) are indicated at the 50th (solid), 10th, and 90th (dashed) percentiles of S2 at given S1. These percentiles are shown separately at 1 phd, with the S2 threshold lowered to the 100 phd value (uncorrected) adopted in this analysis. The color histogram illustrates the expected WIMP signal for a mass of 4  GeV/c2 at 1 phd only, for an exposure of 106 kgdays and cross section of 10−40  cm2; the integrated number of events for this model is ∼8,000.

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    • Figure 8
      Figure 8

      Top: Efficiencies for NR event detection as presented in the original analysis of the LUX 2013 data [9] (dashed gray) and as estimated using nest v2.0.0 for the single-photon search presented here (black). These efficiencies include detection of single scatter events passing the S1 and S2 thresholds. In the original analysis the S1 threshold required at least two PMTs detecting photons and a minimum uncorrected S2 of 165 phd. The single photon analysis includes events with a single S1 photon producing double-photoelectron emission and an uncorrected S2 threshold of 100 phd. Bottom: Overall WIMP signal acceptance for the single photon analysis simulated with nest v2.0.0 after all cuts (black), along with background expectations (green) for the 2013 WIMP search exposure calculated for the appropriate acceptance region for each mass. Shaded green and black regions represent corresponding uncertainties.

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    • Figure 9
      Figure 9

      The 90% C.L. upper limits on the spin-independent WIMP-nucleon cross section obtained using the single-photon population producing double-photoelectron emission in the LUX 2013 WIMP search. The observed limit with a 0.3 keV NR energy cutoff is shown in solid black, with 1σ and 2σ bands of background-only trials shown in green and yellow. The dashed black line is derived from the same analysis but with a model cutoff at 1.1 keV. Both of these results correspond to the nest v2.0.0 model shown in Fig. 6. The upper limit using a 0.3 keV NR energy cutoff with the newer nest v2.0.1 model is shown using a dotted black line. Also shown are the previous results from the LUX 2013 search [9] (gray), the LUX complete exposure result [10] (red), as well as from the DarkSide-50 [17] (green), PandaX-II [36] (blue), PICO60 [37] (lilac), and CDMSLite [38] (purple).

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