Results of a Search for Sub-GeV Dark Matter Using 2013 LUX Data

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Results of a Search for Sub-GeV Dark Matter Using 2013 LUX Data

D. S. Akerib et al. (LUX Collaboration)

Phys. Rev. Lett. 122, 131301 – Published 1 April 2019

Abstract

The scattering of dark matter (DM) particles with sub-GeV masses off nuclei is difficult to detect using liquid xenon-based DM search instruments because the energy transfer during nuclear recoils is smaller than the typical detector threshold. However, the tree-level DM-nucleus scattering diagram can be accompanied by simultaneous emission of a bremsstrahlung photon or a so-called “Migdal” electron. These provide an electron recoil component to the experimental signature at higher energies than the corresponding nuclear recoil. The presence of this signature allows liquid xenon detectors to use both the scintillation and the ionization signals in the analysis where the nuclear recoil signal would not be otherwise visible. We report constraints on spin-independent DM-nucleon scattering for DM particles with masses of $0.4 - 5 GeV / c^{2}$ using $1.4 \times 10^{4} kg day$ of search exposure from the 2013 data from the Large Underground Xenon (LUX) experiment for four different classes of mediators. This analysis extends the reach of liquid xenon-based DM search instruments to lower DM masses than has been achieved previously.

Received 29 November 2018
Revised 12 February 2019

DOI:https://doi.org/10.1103/PhysRevLett.122.131301

Physics Subject Headings (PhySH)

Particle dark matter Weakly interacting massive particles

Dark matter detectors

Particles & Fields

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Vol. 122, Iss. 13 — 5 April 2019

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Images

Figure 1
The light (blue) and charge (green) yields of tritium ER events as a function of recoil energy as measured in situ by the LUX detector at $180 V / cm$ (solid lines) compared to nest v2.0 simulations (dashed pink line). The bands indicate the $1 − σ$ systematic uncertainties of the measurement. The dotted gray line shows the 1.24 keV energy threshold implemented in the analysis.
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Figure 2
Scattering rates in xenon for the bremsstrahlung (solid blue) and Migdal effects (dashed teal). The DM-nucleus scattering rates resulting in elastic NR in LUX are also shown (dash-dot pink). Also shown is a signal cutoff at 1.24 keV (dotted gray) applied in the analysis, corresponding to 50% efficiency of ER detection. Note that 50% efficiency for NR event detection occurs at 3.3 keV [6].
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Figure 3
Illustration of the DM-nucleus scattering event rate from the Migdal effect with a heavy scalar mediator (solid black line) for $m_{DM} = 1 GeV / c^{2}$ with a cross section per nucleus of $1 \times 10^{- 35} {cm}^{2}$ . The scattering event rate was calculated following Ref. [5]. Also shown is the efficiency from the in situ tritium measurements performed by the LUX detector (dashed teal line). The hatched blue area indicates the event rate considered for this analysis with tritium efficiency and a 1.24 keV energy threshold (dotted gray line) applied. Data quality cuts are not included.
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Figure 4
The expected signal from DM-nucleus interactions through the Migdal effect with a cross section per nucleus of $1 \times 10^{- 35} {cm}^{2}$ projected onto a two-dimensional space of $\log_{10} S 2$ vs $S 1$ . Assumptions are the same as in Fig. 3 with additional data quality cuts applied.
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Figure 5
Contours containing 95% of the expected DM signal from the bremsstrahlung and Migdal effects using nest package v2.0 [22]. The solid amber contour indicates a bremsstrahlung signal of $m_{DM} = 0.4 GeV / c^{2}$ assuming a heavy scalar mediator (7.9 events). The other two contours are for the Migdal effect: The dashed teal contour is for $m_{DM} = 1 GeV / c^{2}$ assuming a heavy scalar mediator (10.8 events), and the dash-dot light blue contour is for $m_{DM} = 5 GeV / c^{2}$ assuming a light vector mediator (11.5 events). The number in parentheses indicates the expected number of signal events within the contour for a given signal model with a cross section at the 90% C.L. upper limit. The contours are overlaid on 591 events observed in the region of interest from the 2013 LUX exposure of 95 live days and 145 kg fiducial mass (cf. Ref. [6]). Points at radius $< 18 cm$ are black; those at 18–20 cm are gray since they are more likely to be caused by radio contaminants near the detector walls. Distributions of uniform-in-energy electron recoils (blue) and an example signal from $m_{DM} = 50 GeV / c^{2}$ (red) are indicated by 50th (solid), 10th, and 90th (dashed) percentiles of $S 2$ at given $S 1$ . Gray lines, with an ER scale of keVee at the top and Lindhard-model NR scale of keVnr at the bottom, are contours of the linear-combined $S 1$ -and- $S 2$ energy estimator [26].
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Figure 6
Upper limits on the SI DM-nucleon cross section at 90% C.L. as calculated using the bremsstrahlung and Migdal effect signal models assuming a scalar mediator (coupling proportional to $A^{2}$ ). The 1- and $2 − σ$ ranges of background-only trials for this result are presented as green and yellow bands, respectively, with the median limit shown as a black dashed line. The top figure presents the limit for a light mediator with $q_{ref} = 1 MeV$ . Also shown is a limit from PandaX-II [10] (pink), but note that Ref. [10] uses a slightly different definition of $F_{med}$ in their signal model. The bottom figure shows limits for a heavy mediator along with limits from the SI analyses of LUX [1] (red), PandaX-II [2] (gray), XENON1T [28] (orange), XENON100 $S 2$ -only [29] (pink), CDEX-10 [30] (purple), CDMSlite [31] (teal), CRESST-II [32] (dark blue), CRESST-III [33] (light blue), CRESST-surface [34] (cyan), DarkSide-50 [35] (green), NEWS-G [36] (brown), and XMASS [37] (lavender).
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