Results from a search for dark matter in the complete LUX exposure

TL;DR

This search yields no evidence of WIMP nuclear recoils and constraints on spin-independent weakly interacting massive particle (WIMP)-nucleon scattering using a 3.35×10^{4}  kg day exposure of the Large Underground Xenon experiment are reported.

Abstract

We report constraints on spin-independent weakly interacting massive particle (WIMP)-nucleon scattering using a 3.35e4 kg-day exposure of the Large Underground Xenon (LUX) experiment. A dual-phase xenon time projection chamber with 250 kg of active mass is operated at the Sanford Underground Research Facility under Lead, South Dakota (USA). With roughly fourfold improvement in sensitivity for high WIMP masses relative to our previous results, this search yields no evidence of WIMP nuclear recoils. At a WIMP mass of 50 GeV/c^2, WIMP-nucleon spin-independent cross sections above 2.2e-46 cm^2 are excluded at the 90% confidence level. When combined with the previously reported LUX exposure, this exclusion strengthens to 1.1e-46 cm^2 at 50 GeV/c^2.

Figures (4)

• Figure 1: WS2014--16 data passing all selection criteria. Fiducial events within 1 cm of the radial fiducial volume boundary are indicated as unfilled circles to convey their low WIMP-signal probability relative to background models (in particular the $^{206}$Pb wall background). Exposure-weighted average ER and NR bands are indicated in blue and red, respectively (mean, 10$\%$, and 90$\%$ contours indicated). Of the 16 models used, the scale of model variation is indicated by showing the extrema boundaries (the upper edge of the highest-S2 model and the lower edge of the lowest-S2 model) as fainter dashed lines for both ER and NR. Gray curves indicate a data selection boundary applied before application of the profile likelihood ratio method. Green curves indicate mean (exposure-weighted) energy contours in the ER interpretation (top labels) and NR interpretation (lower labels), with extrema models dashed.
• Figure 2: Efficiencies for NR event detection, estimated using simulation with parameters tuned to calibration data. In descending order of efficiency---red: detection of an S2 (and classification as such by analysis); green: detection of an S1 ($\geq$2 PMTs detecting photons); blue: detection of both an S1 and an S2; black: detection passing analysis selection criteria. Solid curves indicate exposure-weighted means of the 16 calibrated models. The scale of model variation is illustrated by including the efficiencies of the date and $z$ bins with highest and lowest total efficiency (black dashed curves). Below 1.1 keV nuclear recoil energy, the lowest energy for which light yield was measured in Akerib:2015:dd, efficiency is conservatively assumed to be zero.
• Figure 3: Upper limits on the spin-independent elastic WIMP-nucleon cross section at 90% C.L. The solid gray curves show the exclusion curves from LUX WS2013 (95 live days) Akerib:2015:run3 and LUX WS2014--16 (332 live days, this work). These two data sets are combined to give the full LUX exclusion curve in solid black ("LUX WS2013+WS2014--16"). The 1-- and 2--$\sigma$ ranges of background-only trials for this combined result are shown in green and yellow, respectively; the combined LUX WS2013+WS2014--16 limit curve is power constrained at the --1$\sigma$ level. Also shown are limits from XENON100 Aprile:2016swn (red), DarkSide-50 Darkside:2015 (orange), and PandaX-II Pandax:2016 (purple). The expected spectrum of coherent neutrino-nucleus scattering by $^{8}$B solar neutrinos can be fit by a WIMP model as in Billard:2013, plotted here as a black dot. Parameters favored by SUSY CMSSM Bagnaschi:2015 before this result are indicated as dark and light gray (1-- and 2--$\sigma$) filled regions.
• Figure 4: A comparison of the measured position of the detector wall and cathode to that predicted by the best-fit electrostatic field model. As the electrons are drifted upwards, they are pushed radially inwards; they therefore exit the liquid surface (where they are detected and their $x$-$y$ position is measured) at a radius that is less than the radius at which they originated. As a result, the measured shape of the detector wall, which is physically vertical, is warped in observed coordinates. Similarly, though the cathode is physically horizontal, the field-dependent drift velocity of electrons in liquid xenon causes its shape to appear as an inverted 'U' in measured coordinates. In each of the four axes, the blue contour is the measured shape of the detector wall from calibration data, while the green contour indicates the prediction of the wall shape from the best-fit field model. The width of each contour indicates the uncertainty in the wall position resulting from the histogram bin sizes used to construct the contours. Note that the radius of the wall in observed coordinates ("r$_{\mathrm{S2}}$") is not axially symmetric, and therefore the contours here represent an average over azimuthal angle (this is not the fit space; the fits are instead performed in 3-D). The background model for events from radon plate-out on the walls is constructed directly in measured coordinates entirely from side bands, and does not use these field maps. Horizontal gray-dashed lines, at 40 and 300 $\mu$s, indicate the drift-time extent of the fiducial volume used in WS2014--16.