Search for inelastic dark matter with the CDMS II experiment

Results are presented from a reanalysis of the entire five-tower data set acquired with the Cryogenic Dark Matter Search (CDMS II) experiment at the Soudan Underground Laboratory, with an exposure of 969 kg-days. The analysis window was extended to a recoil energy of 150 keV, and an improved surface-event background-rejection cut was defined to increase the sensitivity of the experiment to the inelastic dark matter (iDM) model. Three dark matter candidates were found between 25 keV and 150 keV. The probability to observe three or more background events in this energy range is 11%. Because of the occurrence of these events, the constraints on the iDM parameter space are slightly less stringent than those from our previous analysis, which used an energy window of 10–100 keV.

Figure 1

Differential recoil spectra in a Ge target for a WIMP-mass splitting $\delta$ of 120 keV and a few representative WIMP masses $m_W$. For comparison the spectrum for a WIMP with a mass of $120\mathrm{GeV}/{\mathrm{c}}^2$ assuming elastic scattering ($\delta =0\mathrm{keV}$) is also shown (black/solid). The spectra are normalized to unity in the 10–150 keV recoil-energy range. The vertical lines denote the analysis threshold at 10 keV, the lower boundary for the setting of the surface-event rejection cut at 25 keV, and the upper analysis limit from our previous analysis at 100 keV [7]. See text for details.

Figure 2

Ionization yield versus recoil energy of ${}^{252}\mathrm{Cf}$ calibration data from a representative detector in one of the six data runs. The black/solid lines represent the electron-recoil band around a yield of one and the nuclear-recoil band around 0.3. The black/dashed line denotes the mean of the latter band, while the similar but blue/solid line is the corresponding prediction from Lindhard theory [22, 23].

Figure 3

Expected sensitivity of this analysis for a WIMP of mass $100\mathrm{GeV}/{\mathrm{c}}^2$ and mass splitting 120 keV for different predefined surface-event leakages at which the timing cut could be fixed. The cut corresponding to the minimum of 0.6 events was chosen as the final cut for this analysis before looking at the WIMP-search signal region.

Figure 4

Distribution of the surface-event leakage estimate in the 25–150 keV energy range. See text for details.

Figure 5

Comparison of the total nuclear-recoil efficiency obtained in this analysis (red/dashed) and from our previous analysis based only on runs 125–128 (black/solid) [7]. The latter is only defined up to 100 keV. Redefining the timing cut achieved an increase in efficiency by a factor of $\sim 1.5$. As in Fig. 1, the vertical lines denote the analysis threshold at 10 keV, the lower boundary for the setting of the surface-event rejection cut at 25 keV, and the upper analysis limit from our previous analysis at 100 keV.

Figure 6

Ionization yield versus recoil energy for all low-yield WIMP-search single scatters from the combined five-tower data set (runs 123–128) passing all constraints except for cuts on the ionization yield and timing parameter. Most events fail the timing cut (green dots), while most of the few dozen that pass the timing cut (×) fall outside the nuclear-recoil band (blue/solid lines). Eleven events pass all the selection criteria, with three occurring within the 25–150 keV range upon which the surface-event rejection cut was defined, and eight between the 10 keV recoil-energy threshold and 25 keV. The ionization-energy threshold is also shown (blue/dashed-dotted). This threshold and the shown nuclear-recoil band represent the exposure-weighted means over all runs and detectors.

Figure 7

Number of standard deviations each event is away from the mean of the nuclear-recoil band (normalized ionization yield) versus timing parameter relative to the timing-cut position (shifted timing parameter) for the three detectors with WIMP candidates above 25 keV. The detectors are (from top to bottom) T4Z6, T4Z2 and T1Z2 with candidate events at 37.3 keV, 73.3 keV and 129.5 keV, respectively. In detector T4Z6 three additional candidates occurred in the 10–25 keV range. The acceptance regions are indicated by the shaded boxes.

Figure 8

90% C.L. upper limits on the scalar WIMP-nucleon cross section for WIMP-mass splittings of 0 keV (left) and 120 keV (right) from this analysis (red/dashed) and from our previous analysis (black/solid) [7]. The red/dotted line in the right plot indicates the expected sensitivity for this analysis based on our estimate of the total background. The colored regions represent DAMA/LIBRA allowed regions at two different C.L.s (90, 99.9%) calculated following a ${\chi }^2$ goodness-of-fit technique [25]. The cross (×) marks the parameter space point which yields the minimum ${\chi }^2$ in the shown cross section versus WIMP-mass plane given the WIMP-mass splitting.

Figure 9

The blue/shaded regions represent WIMP masses and WIMP-mass splittings for which cross sections exist that are compatible with the modulation spectrum observed by DAMA/LIBRA at 90% C.L. The hatched regions show constraints on these parameters from the analysis presented in this paper (red/dashed) and from our previous analysis (black/solid) [7]. The black/dashed line represents the maximum reach of the CDMS II experiment.

Figure 10

Constraints from CDMS on the iDM parameter space allowed by DAMA/LIBRA. Same as Fig. 9 but with different velocity-distribution parameters, as given in the plots. All other parameters remain unchanged.