Results from a Low-Energy Analysis of the CDMS II Germanium Data

We report results from a reanalysis of data from the Cryogenic Dark Matter Search (CDMS II) experiment at the Soudan Underground Laboratory. Data taken between October 2006 and September 2008 using eight germanium detectors are reanalyzed with a lowered, 2 keV recoil-energy threshold, to give increased sensitivity to interactions from weakly interacting massive particles (WIMPs) with masses below $\sim 10\mathrm{GeV}/c^2$. This analysis provides stronger constraints than previous CDMS II results for WIMP masses below $9\mathrm{GeV}/c^2$ and excludes parameter space associated with possible low-mass WIMP signals from the DAMA/LIBRA and CoGeNT experiments.

Figure 1

Comparison of the energy spectra for the candidate events and background estimates, co-added over the 8 detectors used in this analysis. The observed event rate (error bars) agrees well with the electron-recoil background estimate (solid line), which is a sum of the contributions from zero-charge events (dashed line), surface events ($+$), bulk events (dash-dotted line), and the 1.3 keV line (dotted line). The selection efficiencies have been applied to the background estimates for direct comparison with the observed rate, which does not include a correction for the nuclear-recoil acceptance. The inset shows the measured nuclear-recoil acceptance efficiency, averaged over all detectors.

Figure 2

Events in the ionization-yield versus recoil-energy plane for T1Z5. Events within the $(+1.25,-0.5)\sigma$ nuclear-recoil band (solid) are WIMP candidates (large dots). Events outside these bands (small, dark dots) pass all selection criteria except the ionization-energy requirement. The widths of the band edges denote variations between data runs. Events from the ${}^{252}\mathrm{Cf}$ calibration data are also shown (small, light dots). The recoil-energy scale assumes the ionization signal is consistent with a nuclear recoil, causing electron recoils to be shifted to higher recoil energies and lower yields.

Figure 3

Top: comparison of the spin-independent (SI) exclusion limits from these data (solid line) to previous results in the same mass range (all at 90% C.L.). Limits from a low-threshold analysis of the CDMS shallow-site data [16] (dashed line), CDMS II Ge results with a 10 keV threshold [12] (dash-dotted line), recalculated for lower WIMP masses, and XENON100 with constant ($+$) or decreasing (□) scintillation-efficiency extrapolations at low-energy [20] are also shown. The filled regions indicate possible signal regions from DAMA/LIBRA [6, 8] (dark line), CoGeNT (light) [7, 8], and a combined fit to the DAMA/LIBRA and CoGeNT data [8] (hatched). Bottom: comparison of the WIMP-neutron spin-dependent (SD) exclusion limits from these data (solid line), CDMS II Ge results with a 10 keV threshold (dash-dotted line), XENON10 [25] (▵), and CRESST [27] (○). The filled region denotes the 99.7% C.L. DAMA/LIBRA allowed region for neutron-only scattering [28]. An escape velocity of $544\mathrm{km}/\mathrm{s}$ was used for the CDMS and XENON100 exclusion limits, whereas the other results assume an escape velocity from $600–650\mathrm{km}/\mathrm{s}$. Using the same halo parameters as assumed for the allowed regions would lead to slightly stronger limits (dotted line).