New Results from the Search for Low-Mass Weakly Interacting Massive Particles with the CDMS Low Ionization Threshold Experiment

The CDMS low ionization threshold experiment (CDMSlite) uses cryogenic germanium detectors operated at a relatively high bias voltage to amplify the phonon signal in the search for weakly interacting massive particles (WIMPs). Results are presented from the second CDMSlite run with an exposure of 70 kg day, which reached an energy threshold for electron recoils as low as 56 eV. A fiducialization cut reduces backgrounds below those previously reported by CDMSlite. New parameter space for the WIMP-nucleon spin-independent cross section is excluded for WIMP masses between 1.6 and $5.5\mathrm{GeV}/c^2$.

Figure 1

Binned total signal efficiency after sequential application of selection criteria: single-scatter and muon-veto (orange dot dashed curve), pulse-shape (blue dotted curve), hardware-trigger (green dashed curve), and radial fiducial-volume (black solid curve) criteria. The gray band around the final curve shows the combined uncertainty on the overall efficiency at $1\sigma$ confidence.

Figure 2

Radial parameter as a function of energy for the first (top) and second (bottom) periods. The dashed lines indicate the radial cut. The densely populated band at larger values corresponds to events near the edge of the detector. The vertical clusters are the ${}^{71}\mathrm{Ge}$ capture peaks. The clear separation between outer and inner events decreases at high energy due to signal saturation in the outer phonon channel. The slight downward shift in the distribution after the maintenance period (caused by a small change in the operating point of the phonon sensors) together with the appearance of an unexplained localized background below $\sim 250{\mathrm{eV}}_{\mathrm{ee}}$ near the edge of the detector motivated a tighter radial cut in period 2.

Figure 3

Spectrum of events passing all selection criteria, corrected for all efficiencies except the trigger efficiency. Dashed lines indicate the prominent features of the ${}^{71}\mathrm{Ge}$ electron-capture decay with peaks at 10.37 keV ($K$ shell), 1.30 keV ($L$ shell), and 0.16 keV ($M$ shell). Inset: enlargement of the lowest energies that determine the low-mass WIMP sensitivity, including the $L$- and $M$-shell activation peaks. Solid vertical lines show the 50% trigger-efficiency points for the two periods.

Figure 4

Median (90% C.L.) and 95% interval of the WIMP limit from this analysis (black thick solid curve surrounded by salmon-shaded band) compared to other selected results. Other 90% upper limits shown are from the first CDMSlite run (red thin solid curve) [23], SuperCDMS (red thin dashed curve) [24], EDELWEISS-II (red thin dotted curve) [25], LUX (dark-yellow thick dot dashed curve) [5], CRESST (magenta thick dashed curve) [27], and DAMIC (purple thick dotted curve) [28]. Closed regions are CDMS II Si 90% C.L. (blue dashed shaded region) [17], and CoGeNT 90% C.L. (dark-green shaded region) [19].