Limits on Spin-Dependent WIMP-Nucleon Cross Section Obtained from the Complete LUX Exposure

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Limits on Spin-Dependent WIMP-Nucleon Cross Section Obtained from the Complete LUX Exposure

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

Phys. Rev. Lett. 118, 251302 – Published 23 June 2017

Abstract

We present experimental constraints on the spin-dependent WIMP-nucleon elastic cross sections from the total $129.5 kg yr$ exposure acquired by the Large Underground Xenon experiment (LUX), operating at the Sanford Underground Research Facility in Lead, South Dakota (USA). A profile likelihood ratio analysis allows 90% C.L. upper limits to be set on the WIMP-neutron (WIMP-proton) cross section of $σ_{n} = 1.6 \times 10^{- 41} {cm}^{2}$ ( $σ_{p} = 5 \times 10^{- 40} {cm}^{2}$ ) at $35 GeV c^{- 2}$ , almost a sixfold improvement over the previous LUX spin-dependent results. The spin-dependent WIMP-neutron limit is the most sensitive constraint to date.

Received 8 May 2017

DOI:https://doi.org/10.1103/PhysRevLett.118.251302

Physics Subject Headings (PhySH)

Particle dark matter

Weakly interacting massive particles

Dark matter detectors

Gravitation, Cosmology & Astrophysics

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Dark Matter Search Results from the $PICO − 60 C_{3} F_{8}$ Bubble Chamber

C. Amole et al. (PICO Collaboration)

Phys. Rev. Lett. 118, 251301 (2017)

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Vol. 118, Iss. 25 — 23 June 2017

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Figure 1
LUX total efficiency (black curve), averaged over the entire exposure. NR model uncertainties are illustrated by the gray band, indicating $\pm 1 σ$ variation. The vertical dashed line represents the analysis threshold of 1.1 keV (the lowest DD calibration point), below which we conservatively take the LUX efficiency to be zero. Sample WIMP spectra are plotted in color, with values corresponding to the right-hand $y$ axis. dru is the differential rate unit (events ${kg}^{- 1} d^{- 1} ke V^{- 1}$ ), and the spectra have each been calculated with WIMP-nucleon cross $section = 1 pb$ . Spectra for WIMP-proton and -neutron scattering are given by the red and blue curves, respectively, for three WIMP masses: 5 (dotted curves), 50 (dash-dotted curves), and $1000 GeV c^{- 2}$ (dashed curves). Structure functions from Ref. [14] are used for this calculation.
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Figure 2
90% C.L. upper limits on the WIMP-neutron (top) and WIMP-proton (bottom) cross section. Results from this analysis are shown in thick black (“LUX WS2013+WS2014–16”), with the range of expected sensitivity indicated by the green ( $1 σ$ ) and yellow ( $2 σ$ ) bands. Solid gray curves show the previously published LUX WS2013 limits [13]. Constraints from other LXe TPC experiments are also shown, including XENON100 [28] and PandaX-II [29]. In the top panel, model-dependent (axial-vector mediator with indicated couplings) LHC search results are represented by dashed lines, with CMS [30] in light blue and ATLAS [31] in dark blue. As calculated by a new profile likelihood scan of the MSSM7 [32], favored parameter space is shown as dark ( $1 σ$ ) and light ( $2 σ$ ) peach regions; an earlier calculation using the MSSM-15 [33] is shown in gray, with analogous shading of confidence levels. In the bottom panel, the DAMA allowed region (as interpreted in Ref. [34]) is shown in pink (the analogous neutron-only region is above the bounds of the plot). Such an interpretation is in severe tension with this result, as well as the PICO-2L [35] and PICO-60 [36] constraints. Selected limits from indirect searches at neutrino observatories (Super-Kamiokande [37] and IceCube [38]) are plotted as dashed lines.
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Figure 3
90% C.L. exclusions on coupling parameters $a_{n}$ and $a_{p}$ for 50 and $1000 GeV c^{- 2}$ WIMPs. Ellipse boundaries are colored as in Fig. 2: this result (thick black), LUX WS2013 (gray), PandaX-II (purple), and PICO-60 (blue). Geometrically, Eq. (4) describes a rotated ellipse when the sum is performed over multiple isotopes with distinct $σ_{p}^{A} / σ_{n}^{A}$ , as is the case for LXe experiments. PICO-60 considers only ${}^{19}F$ (for which $⟨ S_{n} ⟩ \sim 0$ ) and thus sets limits only on $a_{p}$ . The innermost region (bounded by LUX and PICO-60) represents parameter space not in tension with experimental data. The model dependency of the LHC results is apparent in this plane, as the CMS excluded region (shown as a green band) is restricted to the $a_{n} = a_{p}$ line (see the main text for an important caveat). This line is absent from the lower panel, since, in this treatment, CMS is insensitive to WIMPs at the TeV mass scale. MSSM7 favored regions from the GAMBIT scan are also shown, with a red contour at the $2 σ$ level for visibility. The degeneracies assumed in the MSSM7 Lagrangian lead to the tight correlation between $a_{n}$ and $a_{p}$ . This scan includes a range of possible WIMP masses (unlike the mass-specific experimental exclusions) and thus appears identically in each panel, noting the change in the axis scale. Additionally, the scans include models with subdominant relic densities, for which experimental limits are rescaled accordingly.
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