JINST Instrumentation Theses Archive

Ph.D. degree thesis
accepted by Case Western Reserve University, Cleveland, Ohio, USA, in 2005

Sharmila Kamat

Supervisor: Daniel S Akerib

Extending the Sensitivity to the Detection of WIMP Dark Matter with an Improved Understanding of the Limiting Neutron

 Keywords:

• Cryogenic detectors
• Detector modelling and simulations I (interaction of radiation with matter, interaction of photons with matter, interaction of hadrons with matter, etc)
• Interaction of radiation with matter

 Abstract:
The Cryogenic Dark Matter Search (CDMS) uses position-sensitive Germanium and Silicon crystals in the direct detection of Weakly Interacting Massive Particles (WIMPs) believed to constitute most of the dark matter in the Universe. WIMP interactions with matter being rare, identifying and eliminating known backgrounds is critical for detection. Event-by-event discrimination by the detectors rejects the predominant gamma and beta backgrounds while Monte Carlo simulations help estimate, and subtract, the contribution from the neutrons.

This thesis describes the effort to understand neutron backgrounds as seen in the two stages of the CDMS search for WIMPs. The first stage of the experiment was at a shallow site at the Stanford Underground Facility where the limiting background came from high-energy neutrons produced by cosmic-ray muon interactions in the rock surrounding the cavern.

Simulations of this background helped inform the analysis of data from an experimental run at this site and served as input for the background reduction techniques necessary to set new exclusion limits on the WIMP-nucleon cross-section, excluding new parameter space for WIMPs of masses 8-20 GeV/c2.

This thesis considers the simulation methods used as well as how various event populations in the data served as checks on the simulations to allow them to be used in the interpretation of the WIMP-search data. The studies also confirmed the presence of a limiting neutron background at the shallow site, necessitating the move to the 713-meter deep Soudan Underground Facility.

Similar computer-based studies helped quantify the neutron background seen at the deeper site and informed the analysis of the data emerging from the first physics run of the experiment at Soudan. In conjunction with the WIMP-search and calibration data, the simulations confirmed that increased depth considerably reduced the neutron backgrounds seen, greatly improving the sensitivity to WIMP detection.

The data run set an upper limit of 4 x 10−43 on the WIMP-nucleon cross section for a WIMP mass of 60 GeV/c2. Upper limits to the rate of background neutrons have also been determined.