Exclusion Limits on the WIMP-Nucleon Cross-Section from the First Run of the Cryogenic Dark Matter Search in the Soudan Underground Lab

TL;DR

The study presents first results from the CDMS-II Soudan run using Ge and Si ZIP detectors to search for WIMP dark matter via elastic WIMP-nucleon scattering. By leveraging simultaneous ionization and phonon measurements, the experiment achieves strong discrimination between electron and nuclear recoils, with surface events mitigated through phonon timing analyses. An extensive blind analysis yields one WIMP-like event consistent with a background leakage, leading to world-leading limits on both spin-independent and spin-dependent WIMP-nucleon interactions under standard halo assumptions. Specifically, the spin-independent cross-section is constrained to a minimum of $4x10^{-43}$ cm$^2$ at $m_chi=60$ GeV/c$^2$, while the spin-dependent WIMP-neutron cross-section minimum is $2x10^{-37}$ cm$^2$ at $m_chi=50$ GeV/c$^2$.

Abstract

The Cryogenic Dark Matter Search (CDMS-II) employs low-temperature Ge and Si detectors to seek Weakly Interacting Massive Particles (WIMPs) via their elastic scattering interactions with nuclei. Simultaneous measurements of both ionization and phonon energy provide discrimination against interactions of background particles. For recoil energies above 10 keV, events due to background photons are rejected with >99.99% efficiency. Electromagnetic events very near the detector surface can mimic nuclear recoils because of reduced charge collection, but these surface events are rejected with >96% efficiency by using additional information from the phonon pulse shape. Efficient use of active and passive shielding, combined with the the 2090 m.w.e. overburden at the experimental site in the Soudan mine, makes the background from neutrons negligible for this first exposure. All cuts are determined in a blind manner from in situ calibrations with external radioactive sources without any prior knowledge of the event distribution in the signal region. Resulting efficiencies are known to ~10%. A single event with a recoil of 64 keV passes all of the cuts and is consistent with the expected misidentification rate of surface-electron recoils. Under the assumptions for a standard dark matter halo, these data exclude previously unexplored parameter space for both spin-independent and spin-dependent WIMP-nucleon elastic scattering. The resulting limit on the spin-independent WIMP-nucleon elastic-scattering cross-section has a minimum of 4x10^-43 cm^2 at a WIMP mass of 60 GeV/c^2. The minimum of the limit for the spin-dependent WIMP-neutron elastic-scattering cross-section is 2x10^-37 cm^2 at a WIMP mass of 50 GeV/c^2.

Figures (42)

• Figure 1: Top view and side view of the CDMS-II shielding and veto. The detector volume is referred to as the "icebox." As shown, the stem to the right of the detector volume is the "cold stem" and connects the detectors and the copper cans to the cryostat. The stem to the left of the detector volume is the "electronics stem" and contains the wiring that connects the cold electronics to the room temperature electronics.
• Figure 2: Ionization energy versus recoil energy for detector Z5 (Ge). Black dots correspond to calibration events from a $^{133}$Ba source (emits gammas only) and gray dots correspond to calibration events from a $^{252}$Cf source (emits gammas and neutrons).
• Figure 3: Ionization yield versus recoil for detector Z5 (Ge). Black dots correspond to calibration events from a $^{133}$Ba source (emits gammas only) and gray dots correspond to calibration events from a $^{252}$Cf source (emits gammas and neutrons). The upper distribution of events are bulk electron recoils which define the "electron-recoil band." The lower distribution of events are nuclear recoils which define the "nuclear-recoil band."
• Figure 4: Diagram of a CDMS ZIP detector. a) The "ionization side" of the detector with a large inner electrode and an outer guard ring electrode. b) The "phonon side," divided into four quadrants labeled A, B, C, and D, each consisting of 37 dies of 28 QETs. The convention for the x-y axes is shown. The area outside the cells consists of a passive Al/W grid that is patterned sparsely (10 % area coverage) to minimize athermal-phonon absorption while maintaining field uniformity for the ionization measurement. c) One of the 37 dies constituting a single phonon channel; each die contains 28 QETs. d) One of the QETs consisting of a 1-$\mu$m-wide tungsten strip connected to 8 aluminum fins.
• Figure 5: Peak phonon delay versus ionization yield for detector Z5. Events from a $^{252}$Cf source (emits both gammas and neutrons) are shown as gray dots. Events from a $^{133}$Ba source (emits gammas only) are shown as black dots. Surface-electron recoils from the $^{133}$Ba calibration appear as a low-yield tail and having smaller peak phonon delays. Lines for a typical cut excluding events with high yield or low peak phonon delay are shown.