Results from the Final Exposure of the CDMS II Experiment

Abstract

We report results from a blind analysis of the final data taken with the Cryogenic Dark Matter Search experiment (CDMS II) at the Soudan Underground Laboratory, Minnesota, USA. A total raw exposure of 612 kg-days was analyzed for this work. We observed two events in the signal region; based on our background estimate, the probability of observing two or more background events is 23%. These data set an upper limit on the Weakly Interacting Massive Particle (WIMP)-nucleon elastic-scattering spin-independent cross-section of 7.0x10^{-44} cm^2 for a WIMP of mass 70 GeV/c^2 at the 90% confidence level. Combining this result with all previous CDMS II data gives an upper limit on the WIMP-nucleon spin-independent cross-section of 3.8x10^{-44} cm^2 for a WIMP of mass 70 GeV/c^2. We also exclude new parameter space in recently proposed inelastic dark matter models.

Figures (5)

• Figure 1: The power of the primary background discrimination parameters, ionization yield and phonon timing, is illustrated for a typical detector using in situ calibration sources. Shown are bulk electron recoils (red points), surface electron events (black crosses) and nuclear recoils (blue circles) with recoil energy between 10 and 100 keV. Top: Ionization yield versus recoil energy. The solid black lines define bands that are $2\sigma$ from the mean electron- and nuclear-recoil yields. The sloping magenta line indicates the ionization energy threshold while the vertical dashed line is the recoil energy analysis threshold. The region enclosed by the black dotted lines defines the sample of events that are used to develop surface-event cuts. Bottom: Normalized ionization yield (number of standard deviations from mean of nuclear recoil band) versus normalized timing parameter (timing relative to acceptance region) is shown for the same data. Events to the right of the vertical red dashed line pass the surface-event rejection cut for this detector. The solid red box is the WIMP signal region. (Color online.)
• Figure 2: Ionization yield versus recoil energy for events passing all cuts, excluding yield and timing. The top (bottom) plot shows events for detector T1Z5(T3Z4). The solid red lines indicate the $2\sigma$ electron and nuclear recoil bands. The vertical dashed line represents the recoil energy threshold and the sloping magenta dashed line is the ionization threshold. Events that pass the timing cut are shown with round markers. The candidate events are the round markers inside the nuclear-recoil bands. (Color online.)
• Figure 3: Normalized ionization yield (number of standard deviations from mean of nuclear recoil band) versus normalized timing parameter (timing relative to acceptance region) for events passing all cuts, excluding yield and timing.� The top (bottom) plot shows events for detector T1Z5(T3Z4). Events that pass the phonon timing cut are shown with round markers. The solid red box indicates the signal region for that detector. The candidate events are the round markers inside the signal regions. (Color online.)
• Figure 4: 90% C.L. upper limits on the WIMP-nucleon spin-independent cross section as a function of WIMP mass. The red (upper) solid line shows the limit obtained from the exposure analyzed in this work. The solid black line shows the combined limit for the full data set recorded at Soudan. The dotted line indicates the expected sensitivity for this exposure based on our estimated background combined with the observed sensitivity of past Soudan data. Prior results from CDMS Ahmed:2008eu, EDELWEISS II Armengaud:2009hc, XENON10 Aprile:2008rc, and ZEPLIN III Lebedenko:2008gb are shown for comparison. The shaded regions indicate allowed parameter space calculated from certain Minimal Supersymmetric Models Ellis:2005mbRoszkowski:2007fd (Color online.)
• Figure 5: The shaded green region represents WIMP masses and mass splittings for which there exists a cross section compatible with the DAMA/LIBRA Bernabei:2008yi modulation spectrum at 90% C. L. under the inelastic dark matter interpretation TuckerSmith:2001hy. Excluded regions for CDMS II (solid-black hatched) and XENON10 Collaboration:2009xb (red-dashed hatched) were calculated in this work using the Optimum Interval Method. (Color online.)