The SuperCDMS experiment is designed to directly detect weakly interacting massive particles (WIMPs) that may constitute the dark matter in our Galaxy. During its operation at the Soudan Underground Laboratory, germanium detectors were run in the CDMSlite mode to gather data sets with sensitivity specifically for WIMPs with masses <10 GeV=c 2 . In this mode, a higher detector-bias voltage is applied to amplify the phonon signals produced by drifting charges. This paper presents studies of the experimental noise and its effect on the achievable energy threshold, which is demonstrated to be as low as 56 eV ee (electron equivalent energy). The detector-biasing configuration is described in detail, with analysis corrections for voltage variations to the level of a few percent. Detailed studies of the electric-field geometry, and the resulting successful development of a fiducial parameter, eliminate poorly measured events, yielding an energy resolution ranging from ∼9 eV ee at 0 keV to 101 eV ee at ∼10 keV ee . New results are derived for astrophysical uncertainties relevant to the WIMP-search limits, specifically examining how they are affected by variations in the most probable WIMP velocity and the Galactic escape velocity. These variations become more important for WIMP masses below 10 GeV=c 2 . Finally, new limits on spin-dependent low-mass WIMP-nucleon interactions are derived, with new parameter space excluded for WIMP masses ≲3 GeV=c 2 .
SuperCDMS is an experiment designed to directly detect weakly interacting massive particles (WIMPs), a favored candidate for dark matter ubiquitous in the Universe. In this Letter, we present WIMP-search results using a calorimetric technique we call CDMSlite, which relies on voltage-assisted Luke-Neganov amplification of the ionization energy deposited by particle interactions. The data were collected with a single 0.6 kg germanium detector running for ten live days at the Soudan Underground Laboratory. A low energy threshold of 170 eV ee (electron equivalent) was obtained, which allows us to constrain new WIMP-nucleon spin-independent parameter space for WIMP masses below 6 GeV=c Independent astrophysical surveys and cosmological studies confirm that dark matter constitutes 27% of the energy density of the Universe (reviewed in [1]). Weakly interacting massive particles (WIMPs) are one of the favored particle candidates for dark matter. Theoretical predictions for WIMP masses, and for WIMP-interaction cross sections on normal matter, both span many orders of magnitude. However, WIMPs may elastically scatter off nuclei with enough energy, and at a sufficient rate, to be detected by laboratory detectors [2]. Measurements of the nuclear-recoil energy spectrum by these experiments can constrain the properties of WIMP dark matter [3][4][5].
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