First search for a dark matter annual modulation signal with NaI(Tl) in the Southern Hemisphere by DM-Ice17

E. Barbosa de Souza et al. (DM-Ice Collaboration)

Phys. Rev. D 95, 032006 – Published 28 February 2017

Abstract

We present the first search for a dark matter annual modulation signal in the Southern Hemisphere conducted with NaI(Tl) detectors, performed by the DM-Ice17 experiment. Nuclear recoils from dark matter interactions are expected to yield an annually modulated signal independent of location within the Earth’s hemispheres. DM-Ice17, the first step in the DM-Ice experimental program, consists of 17 kg of NaI(Tl) located at the South Pole under 2200 m.w.e. overburden of Antarctic glacial ice. Taken over 3.6 years for a total exposure of 60.8 kg yr, DM-Ice17 data are consistent with no modulation in the energy range of 4–20 keV, providing the strongest limits on weakly interacting massive particle dark matter from a direct detection experiment located in the Southern Hemisphere. The successful deployment and stable long-term operation of DM-Ice17 establishes the South Pole ice as a viable location for future dark matter searches and in particular for a high-sensitivity NaI(Tl) dark matter experiment to directly test the DAMA/LIBRA claim of the observation of dark matter.

Received 24 February 2016

DOI:https://doi.org/10.1103/PhysRevD.95.032006

Physics Subject Headings (PhySH)

Particle dark matter

Weakly interacting massive particles

Dark matter detectors Scintillators

Gravitation, Cosmology & AstrophysicsParticles & Fields

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Vol. 95, Iss. 3 — 1 February 2017

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Images

Figure 1
Energy spectrum of DM-Ice17 from 0–30 keV (top) and signal acceptance fraction (bottom). Shown is the spectrum from Det-1 after noise cuts (solid black) and with the addition of a cut efficiency correction (dashed black). The full simulation (red) is also shown, along with the contributions from ${}^{40}K$ in the crystal (blue), ${}^{238}U$ surface contamination on the copper encapsulation of the crystals (violet), and the sum total of backgrounds from other sources (orange). The signal acceptance fraction for the noise cuts is shown below the spectra, with the cut efficiency data (black dots) fit with a functional form (cyan) used in calculations. The green line indicates the 4 keV analysis threshold and the shaded area represents the region of interest (4–20 keV) in this analysis. Det-2 data are similar, though its lower gain leads to less efficient noise cuts and higher analysis threshold (6 keV).
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Figure 2
Event rate vs time for Det-1 before subtracting the linear rate component (top) and residual event rate vs time after the subtraction (bottom). Rates are provided in half-month intervals for 4–6 keV (black), 6–8 keV (red), 8–10 keV (green), and 10–20 keV (blue). The horizontal error bars represent the half-month bin width and the vertical error bars represent $\pm 1 σ$ error due to statistical and uptime uncertainties. In each of these energy bins, the data are consistent with the no modulation. Det-2 data are similarly consistent with no modulation and exhibit a comparable linear rate component.
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Figure 3
Allowed regions in amplitude ( $counts / day / keV / kg$ ) vs phase for annual modulation fits to the Det-1 4–6 keV data, with contours at (inner to outer) 68%, 95%, and 99% C.L (red). The DAMA/LIBRA 2–4 keV 99% C.L. (blue) is also shown for comparison. Phase of 0 corresponds to January 1st. The predicted phase of a dark matter modulation signal under generic halo models (June 2nd) is indicated by the green line.
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Figure 4
Amplitude of modulation vs energy showing maximum likelihood fits for DAMA [9] (blue) and DM-Ice17 rates [Det-1 (red) and Det-2 (green)]. The linear backgrounds underlying the event rate and the modulation amplitude are free parameters in these fits, with period and phase forced to that of an expected dark matter signal (1 year and 152.5 days, respectively). Horizontal error bars represent the width of the energy bins used for the analysis. Vertical error bars are $\pm 1 σ$ error on the binned modulation fit amplitudes.
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Figure 5
WIMP exclusion limits at 90% C.L. from the 60.8 kg yr DM-Ice17 physics data set (red), with DAMA preferred $5 σ$ C.L. contour (blue) for comparison. As a reference, the projected sensitivity (median 90% C.L.) for a planned future NaI(Tl)-based detector with $2 counts / day / keV / kg$ backgrounds in the ROI after 500 kg yr and a 2 keV analysis threshold is shown in the dashed black line [47].
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