University of Minnesota
School of Physics & Astronomy

Physics and Astronomy Calendar

Thursday, December 18th 2014
09:30 am:
Thesis Defense in 120 PaN
Speaker: Scott Fallows, University of Minnesota
Subject: Measurement of Nuclear Recoils in the CDMS II Dark Matter Search
This is the public portion of Mr. Fallows Thesis Defense

The Cryogenic Dark Matter Search (CDMS) experiment is designed to
directly detect elastic scatters of weakly-interacting massive dark
matter particles (WIMPs), on target nuclei in semiconductor crystals
composed of Si and Ge. These scatters would occur very rarely, in an
overwhelming background composed primarily of electron recoils from
photons and electrons, as well as a smaller but non-negligible
background of WIMP-like nuclear recoils from neutrons, which must be
carefully limited. The CDMS II generation of detectors simultaneously
measure ionization and athermal phonon signals from each scatter, which
together allow effective discrimination against electron recoil
backgrounds.
Nuclear recoils have suppressed ionization signals relative to electron
recoils of the same recoil energy. The overall normalization and
linearity of the energy scale for electron recoils in CDMS II detectors
is clearly established by peaks of known gamma energy in the ionization
spectrum of 133Ba calibration data. This electron-equivalent (keVee)
energy scale enables relative calibration of the total phonon signal by
enforcing unity yield for electron recoils, in aggregate. Subtracting an
event's Luke phonon contribution from its calibrated total phonon
energy, as measured by the ionization signal, results in a valid measure
of the true recoil energy (keVr) for both electron and nuclear recoils.
Systematic uncertainties affecting this energy scale for nuclear recoils
are presented, along with several methods to constrain their magnitude.
The resulting adjusted WIMP limits from CDMS II are presented.

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