Improved Limits on Scattering of Weakly Interacting Massive Particles from Reanalysis of 2013 LUX Data

We present constraints on weakly interacting massive particles (WIMP)-nucleus scattering from the 2013 data of the Large Underground Xenon dark matter experiment, including $1.4\times {10}^4\mathrm{kg}\text{day}$ of search exposure. This new analysis incorporates several advances: single-photon calibration at the scintillation wavelength, improved event-reconstruction algorithms, a revised background model including events originating on the detector walls in an enlarged fiducial volume, and new calibrations from decays of an injected tritium $\beta$ source and from kinematically constrained nuclear recoils down to 1.1 keV. Sensitivity, especially to low-mass WIMPs, is enhanced compared to our previous results which modeled the signal only above a 3 keV minimum energy. Under standard dark matter halo assumptions and in the mass range above $4\mathrm{GeV}{c}^{-2}$, these new results give the most stringent direct limits on the spin-independent WIMP-nucleon cross section. The 90% C.L. upper limit has a minimum of 0.6 zb at $33\mathrm{GeV}{c}^{-2}$ WIMP mass.

Figure 1

Top, middle Yields of electrons and photons, respectively, for nuclear recoils in LUX, measured in situ with D-D neutrons. Error bars are statistical. Bottom: efficiencies for NR event detection, averaged over the fiducial volume and estimated using luxsim with parameters tuned to D-D calibration. In descending order of efficiency—red: detection of an $\mathcal{S}2$ ($\ge 2$ electrons emitted); green: detection of an $\mathcal{S}1$ ($\ge 2$ PMTs detecting photons); blue: detection of both an $\mathcal{S}1$ and an $\mathcal{S}2$; black: detection passing thresholds in $S1$ and raw $\mathcal{S}2$ size. The $97.5\%\pm 1.7\%$ event-classification efficiency is applied as an additional, energy-independent scaling. The vertical line at 1.1 keV marks the low-energy cutoff applied in the signal model. All panels: solid lines show the best fit of the Lindhard parametrization; shaded regions span its 1- and $2\text{−}\sigma$ uncertainty used for the final result. Dashed lines show the best fit of the alternate, Bezrukov NR parametrization.

Figure 2

Observed events in the 2013 LUX exposure of 95 live days and 145 kg fiducial mass. Points at $<18\mathrm{cm}$ radius are black; those at 18–20 cm are gray. Distributions of uniform-in-energy electron recoils (blue) and an example $50\mathrm{GeV}{c}^{-2}$ WIMP signal (red) are indicated by 50th (solid), 10th, and 90th (dashed) percentiles of $S2$ at given $S1$. Gray lines, with ER scale of keVee at top and Lindhard-model NR scale of keVnr at bottom, are contours of the linear combined $S1$-and-$S2$ energy estimator [19].

Figure 3

Upper limits on the spin-independent elastic WIMP-nucleon cross section at 90% C.L. Observed limit in black, with the 1- and $2\text{−}\sigma$ ranges of background-only trials shaded green and yellow. Also shown are limits from the first LUX analysis [6] (gray), SuperCDMS [40] (green), CDMSlite [41] (light blue), XENON100 [42] (red), DarkSide-50 [43] (orange), and PandaX [44] (purple). The expected spectrum of coherent neutrino-nucleus scattering by ${}^8\mathrm{B}$ solar neutrinos can be fit by a WIMP model as in [45], plotted here as a black dot.