Limits on spin-independent WIMP-nucleon interactions from the two-tower run of the Cryogenic Dark Matter Search

Abstract

We report new results from the Cryogenic Dark Matter Search (CDMS II) at the Soudan Underground Laboratory. Two towers, each consisting of six detectors, were operated for 74.5 live days, giving spectrum-weighted exposures of 34 kg-d for germanium and 12 kg-d for silicon targets after cuts, averaged over recoil energies 10-100 keV for a WIMP mass of 60 GeV. A blind analysis was conducted, incorporating improved techniques for rejecting surface events. No WIMP signal exceeding expected backgrounds was observed. When combined with our previous results from Soudan, the 90% C.L. upper limit on the spin-independent WIMP-nucleon cross section is 1.6 x 10^{-43} cm^2 from Ge, and 3 x 10^{-42} cm^2 from Si, for a WIMP mass of 60 GeV. The combined limit from Ge (Si) is a factor of 2.5 (10) lower than our previous results, and constrains predictions of supersymmetric models.

Figures (4)

• Figure 1: Ionization yield versus timing parameter (see text) for calibration data in T2Z3(Ge), with recoil energies in the range 10--100 keV. Typical bulk-electron recoils from the $^{133}$Ba source of gamma rays are dots (red), with yield near unity. Low-yield $^{133}$Ba events (+, black), attributed to surface electron recoils, have small values of timing parameter, allowing discrimination from neutron-induced nuclear recoils from $^{252}$Cf ($\circ$, blue), which show a wide range of timing parameter values. The vertical dashed line shows the minimum timing parameter allowed for WIMP candidates, and the box shows the approximate signal region, which is in fact weakly energy dependent. (Color online.)
• Figure 2: Variables used to reject surface electron recoils (7-100 keV), for data from T2Z4 (Si/left) and T1Z2 (Ge/right), for the timing parameter (upper) and $\Delta\chi^2$ (lower). Light (red) lines show distributions of low-yield electron recoils from the $^{133}$Ba source, while dark (blue) lines show distributions of nuclear recoils from the $^{252}$Cf source. Dashed lines indicate the minimum values for acceptable WIMP candidates. A cut on the timing parameter is used for the Ge detectors, while a requirement on $\Delta\chi^2$ is used for the Si detectors. (Color online.)
• Figure 3: Ionization yield versus recoil energy for events in all Ge detectors (upper) and all Si detectors (lower) passing initial data selection cuts prior to applying the surface electron recoil rejection cut. The signal region consists of recoil energies exceeding 7 keV, shown with a vertical dashed line, and yields between the curved lines defined with recoils induced by the $^{252}$Cf source while WIMP-search data were still masked. Below 7 keV, separation between nuclear and electron recoils becomes poor. Events passing the surface electron recoil cut are a star (red) inside the signal region and dark filled circles (blue) outside. Bulk-electron recoils with yield near unity are above the vertical scale limits. (Color online.)
• Figure 4: WIMP-nucleon cross section upper limits (90% C.L.) versus WIMP mass. The upper CDMS Ge curve also uses data from the current run, while the lower Ge curve includes the previous run akerib04. Supersymmetric models allow the largest shaded (light-blue) region bottino, and the smaller shaded (green) region ellis05. The shaded region in the upper left (see text) is from DAMA gondolo, and experimental limits are from DAMA bernabei96, EDELWEISS sanglard05, and ZEPLIN zeplin. (Color online.)