First Results from the Cryogenic Dark Matter Search in the Soudan Underground Lab

TL;DR

First results from a search for weakly interacting massive particles (WIMPs) in the Cryogenic Dark Matter Search experiment at the Soudan Underground Laboratory set the world's lowest exclusion limits on the coherent WIMP-nucleon scalar cross section for all W IMP masses above 15 GeV/c2, ruling out a significant range of neutralino supersymmetric models.

Abstract

We report the first results from a search for weakly interacting massive particles (WIMPs) in the Cryogenic Dark Matter Search (CDMS) experiment at the Soudan Underground Laboratory. Four Ge and two Si detectors were operated for 52.6 live days, providing 19.4 kg-d of Ge net exposure after cuts for recoil energies between 10--100 keV. A blind analysis was performed using only calibration data to define the energy threshold and selection criteria for nuclear-recoil candidates. Using the standard dark-matter halo and nuclear-physics WIMP model, these data set the world's lowest exclusion limits on the coherent WIMP-nucleon scalar cross-section for all WIMP masses above 15 GeV, ruling out a significant range of neutralino supersymmetric models. The minimum of this limit curve at the 90% C.L. is 4 x 10^{-43} cm^2 at a WIMP mass of 60 GeV.

Figures (5)

• Figure 1: Ionization yield versus recoil energy for calibration data with a $^{252}$Cf gamma and neutron source for detectors Z2, Z3 and Z5 in Tower 1 showing the $\pm 2 \sigma$ gamma band (solid curves) and the $\pm 2 \sigma$ nuclear-recoil band (dashed curves) for Z5, the detector with the worst noise of these three. Events with ionization yield $< 0.75$ (grey) are shown only if they pass the phonon-timing cuts. The vertical line is the 10 keV analysis threshold for these three detectors.
• Figure 2: Phonon start time versus ionization yield for $^{133}$Ba gamma-calibration events (diamonds) and $^{252}$Cf neutron-calibration events (dots) in the energy range 20--40 keV in detector Z5 in Tower 1. Lines indicate typical timing and ionization-yield cuts, resulting in high nuclear-recoil efficiency and a low rate of misidentified surface events.
• Figure 3: Efficiency of the combined cuts as a function of recoil energy, both for the blind analysis (solid) and for the second, non-blind analysis (dashed). The step at 20 keV is due to Z1's 20-keV analysis threshold.
• Figure 4: Ionization yield versus recoil energy for WIMP-search data from Z2 (triangle), Z3, and Z5 ($+$) in Tower 1, using the same yield-dependent cuts and showing the same curves as in Fig. \ref{['fig:Z235_Cf_tcutlowy']}. Above an ionization yield of 0.75, the events from all three detectors are drawn as identical points in order to show the 10.4 keV Ga line from neutron activation of Ge.
• Figure 5: New limit on the WIMP-nucleon scalar cross section from CDMS II at Soudan with no candidate events in 19.4 kg-d effective Ge exposure (solid curve). Parameter space above the curve is excluded at the 90% C.L. These limits constrain supersymmetry models, for example Baltz01 (dark grey) and Bottino03 (light grey). The DAMA (1-4) 3$\sigma$ signal region DAMA2000 is shown as a closed contour. Also shown are limits from CDMS at SUF akerib03 (dots), EDELWEISS edel0203 ($\times$'s), and the second, non-blind analysis of CDMS II at Soudan with 1 nuclear-recoil candidate event (dashes). All curves limitplotter are normalized following lewin, using the Helm spin-independent form-factor, $A^2$ scaling, WIMP characteristic velocity $v_0 = 220 {\rm \; km \; s^{-1}}$, mean Earth velocity $v_E = 232 {\rm \; km \; s^{-1}}$, and $\rho = 0.3 {\rm \; GeV \; c^{-2} \; cm^{-3}}$.