Exclusion limits on the WIMP-nucleon cross-section from the Cryogenic Dark Matter Search

TL;DR

CDMS uses cryogenic Ge and Si detectors to search for WIMP-nucleon elastic scattering by exploiting simultaneous ionization and phonon measurements to distinguish nuclear recoils from backgrounds. A meticulous calibration program and a robust background-treatment framework, including a detailed neutron background estimate from muon-induced processes, underpin a likelihood-based exclusion of spin-independent WIMP-nucleon cross sections in the $10$–$70$ GeV/$c^2$ mass range, without a detected WIMP signal. The analysis yields competitive 90% CL limits that exclude portions of parameter space relevant to SUSY models and DAMA's annual modulation interpretation under standard halo assumptions, though not the entire DAMA-allowed region. The work emphasizes background control, multi-detector consistency checks, and transparent treatment of uncertainties, establishing CDMS as a leading direct-detection experiment in the intermediate WIMP-mass regime.

Abstract

The Cryogenic Dark Matter Search (CDMS) employs low-temperature Ge and Si detectors to search for Weakly Interacting Massive Particles (WIMPs) via their elastic-scattering interactions with nuclei while discriminating against interactions of background particles. For recoil energies above 10 keV, events due to background photons are rejected with >99.9% efficiency, and surface events are rejected with >95% efficiency. The estimate of the background due to neutrons is based primarily on the observation of multiple-scatter events that should all be neutrons. Data selection is determined primarily by examining calibration data and vetoed events. Resulting efficiencies should be accurate to about 10%. Results of CDMS data from 1998 and 1999 with a relaxed fiducial-volume cut (resulting in 15.8 kg-days exposure on Ge) are consistent with an earlier analysis with a more restrictive fiducial-volume cut. Twenty-three WIMP candidate events are observed, but these events are consistent with a background from neutrons in all ways tested. Resulting limits on the spin-independent WIMP-nucleon elastic-scattering cross-section exclude unexplored parameter space for WIMPs with masses between 10-70 GeV c^{-2}. These limits border, but do not exclude, parameter space allowed by supersymmetry models and accelerator constraints. Results are compatible with some regions reported as allowed at 3-sigma by the annual-modulation measurement of the DAMA collaboration. However, under the assumptions of standard WIMP interactions and a standard halo, the results are incompatible with the DAMA most likely value at >99.9% CL, and are incompatible with the model-independent annual-modulation signal of DAMA at 99.99% CL in the asymptotic limit.

Figures (49)

• Figure 1: A BLIP detector. The ionization-electrode breaks are indicated. The NTD thermistors are not shown in the side view; they are 0.26-cm high.
• Figure 2: A diagram of the phonon sensors for the 100-g Si ZIP detector run in 1998. The central item depicts the basic layout with each phonon sensor occupying a detector quadrant. Each sensor is divided into 37 units each 5 mm square (magnified to the right) which themselves contain 12 individual transition-edge-sensor (TES) elements (far right) connected in parallel. Aluminum quasiparticle-collector fins cover 82 $\%$ of the top surface of the Si and also provide the ground electrode for the ionization measurement. On the far left is shown the W outer ionization electrode that is patterned (10 % area coverage) to minimize athermal-phonon absorption.
• Figure 3: Ionization yield $Y$ versus recoil energy $E_{\mathrm{R}}$ for 1334 electron-recoil events due to photons from an external $^{60}$Co source ($\times$'s) and for 616 nuclear-recoil events due to neutrons from a separate calibration with an external $^{252}$Cf source (grey dots) for a Ge BLIP detector. These in situ external-source calibrations are described below in Sec. \ref{['sect:cal']}. The dashed curve (at $E_{\mathrm{Q}}=1.1$ keV) indicates the ionization-search threshold (described below in Sec. \ref{['sect:thresh']}) for the neutron-calibration data.
• Figure 4: Schematic illustration of bulk-Ge/$\alpha$-Si interface, indicating qualitative misalignment suggested by data from test devices. Mid-gap states that may serve to define the alignment are schematically indicated.
• Figure 5: Ionization-readout circuit used for both BLIPs and ZIPs, together with the BLIP phonon-readout circuit. The ionization amplifier connects to the biased side via a coupling capacitor with $C_{\mathrm{c}}\approx 330$ pF. The detector capacitance $C_{\mathrm{d}} \approx 40$ pF. The ionization-bias resistor $R_{\mathrm{b}} = 40\,\mathrm{M}\Omega$. The parasitic capacitance $C_{\mathrm{p}} \approx 50$ pF is dominated by FET capacitance. Figure taken from dasilvathesis.