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A Low-threshold Analysis of Data from the Cryogenic Dark Matter Search Experiment

Bunker, Raymond Adelbert
Degree Grantor:
University of California, Santa Barbara.Physics
Degree Supervisor:
Nelson Harry
Place of Publication:
[Santa Barbara, Calif.]
University of California, Santa Barbara
Creation Date:
Issued Date:
Astrophysics and Particle physics
direct detection
particle astrophysics
dark matter

Although dark matter appears to constitute over 80% of the matter in the Universe, its composition is a mystery. Astrophysical observations suggest that the luminous portions of the Galaxy are embedded in a halo of dark-matter particles. Weakly Interacting Massive Particles (WIMPs) are the most studied class of dark-matter candidates and arise naturally within the context of many weak-scale supersymmetric theories. Direct-detection experiments like the Cryogenic Dark Matter Search (CDMS) strive to discern the kinetic energy of recoiling nuclei resulting from WIMP interactions with terrestrial matter. This is a considerable challenge in which the low (expected) rate of WIMP interactions must be distinguished from an overwhelming rate due to known types of radiation.

An incontrovertible positive detection has remained elusive. However, a few experiments have recorded data that appear consistent with a low-mass WIMP. This thesis describes an attempt to probe the favored parameter space. To increase sensitivity to low-mass WIMPs, a low-threshold technique with improved sensitivity to small energy depositions is applied to CDMS shallow-site data. Four germanium and two silicon detectors were operated between December 2001 and June 2002, yielding 118 days of exposure. By sacrificing some of the CDMS detectors' ability to discriminate signal from background, energy thresholds of 1 and 2 keV were achieved for three of the germanium and both silicon detectors, respectively. A large number of WIMP candidate events are observed, most of which can be accounted for by misidentification of background sources. No conclusive evidence for a low-mass WIMP signal is found. The observed event rates are used to set upper limits on the WIMP-nucleon scattering cross section as a function of WIMP mass. Interesting parameter space is excluded for WIMPs with masses below 9 GeV/c^2. Under standard assumptions, the parameter space favored by interpretations of other experiments' data as low-mass WIMP signals is partially excluded, and new parameter space is excluded for WIMP masses between 3 and 4 GeV/c^2.

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