New Results from the Cryogenic Dark Matter Search Experiment

Abstract

Using improved Ge and Si detectors, better neutron shielding, and increased counting time, the Cryogenic Dark Matter Search (CDMS) experiment has obtained stricter limits on the cross section of weakly interacting massive particles (WIMPs) elastically scattering from nuclei. Increased discrimination against electromagnetic backgrounds and reduction of neutron flux confirm WIMP-candidate events previously detected by CDMS were consistent with neutrons and give limits on spin-independent WIMP interactions which are >2X lower than previous CDMS results for high WIMP mass, and which exclude new parameter space for WIMPs with mass between 8-20 GeV/c^2.

Figures (4)

• Figure 1: Ionization yield (Y) vs. recoil energy for veto-anticoincident single scatters in the 4 Ge (black $\times$, diamond, square, and $+$) and 1 inner Si (grey triangle) detectors above the 5 keV analysis threshold (dashed vertical line). Twenty events (circled) lie within the mean nominal 95% nuclear-recoil acceptance region (dashed curves), above the ionization threshold (dot-dashed curve). Most of the events with $0.5<{\rm Y}<0.8$ are in the top Ge detector ($\times$), whose top face is unshielded, or in the bottom Ge detector ($+$), whose bottom disk region faces the contaminated Si detector.
• Figure 2: Histogram in 10-keV bins of the 20 veto-anticoincident, single-scatter nuclear-recoil candidates observed in the 4 Ge detectors of total mass 1 kg. The dashed curve is the shape of the expected recoil-energy spectrum due to incident neutrons, while the solid curve also takes into account the detection efficiency and is normalized to 20 events.
• Figure 3: Scatter plot of ionization yields for veto-anticoincident triple scatters (filled circles) and non-nearest-neighbor double scatters ($\times$'s) with all scatters between 5 and 100 keV and within the fiducial volume. For triple scatters, the size and color of the circle indicates the ionization yield in the third detector. Note both neutron triple-scatter candidates show low yield in all three detectors, while no other triple scatters have low yield in any detector. As expected for such events, there is clearly negligible contamination from surface events.
• Figure 4: Spin-independent $\sigma$ vs. $M$. The regions above the curves are excluded at 90% CL. Solid, thick black curve: limit from this analysis including statistical subtraction of the neutron background. Solid red curve: limit from this analysis without statistical subtraction of the neutron background. Dashed curves: CDMS expected sensitivity (median simulated limit) given the expected neutron background (as normalized based on this and previous work) of 3.3 multiple-scatters, 18 single scatters in Ge, and an expected background in Si of 0.8 electrons and 3.6 neutrons, with (black) or without (red) neutron subtraction. Blue dotted curve: previous CDMS upper limit r19prd. Green dot-dashed curve: DAMA limit using pulse-shape analysis DAMApsa. The DAMA 3$\sigma$ allowed region not including the DAMA limit DAMA2000 is shown as a shaded region. Above 30 GeV c$^{-2}$, the EDELWEISS edel2002 (purple dot-dashed curve) experiment provides more sensitive limits. All curves are normalized following lewin, using the Helm spin-independent form-factor, A$^2$ scaling, WIMP characteristic velocity $v_0 = 220$ km s$^{-1}$, mean Earth velocity $v_E = 232$ km s$^{-1}$, and $\rho = 0.3$ GeV c$^{-2}$ cm$^{-3}$.