Detector Simulation and WIMP Search Analysis for the Cryogenic Dark Matter Search Experiment

Abstract

Astrophysical and cosmological measurements on the scales of galaxies, galaxy clusters, and the universe indicate that ∼85% of the matter in the universe is composed of dark matter, made up of non-baryonic particles that interact with cross-sections on the weak scale or lower. Hypothetical Weakly Interacting Massive Particles, or WIMPs, represent a potential solution to the dark matter problem, and naturally arise in certain Standard Model extensions.The Cryogenic Dark Matter Search (CDMS) collaboration aims to detect the scattering of WIMP particles from nuclei in terrestrial detectors. Germanium and silicon particle detectors are deployed in the Soudan Underground Laboratory in Minnesota. These detectors are instrumented with phonon and ionization sensors, which allows for discrimination against electromagnetic backgrounds, which strike the detector at rates orders of magnitude higher than the expected WIMP signal.This dissertation presents the development of numerical models of the physics of the CDMS detectors, implemented in a computational package collectively known as the CDMS Detector Monte Carlo (DMC). After substantial validation of the models against data, the DMC is used to investigate potential backgrounds to the next iteration of the CDMS experiment, known as SuperCDMS. Finally, an investigation of using the DMC in a reverse Monte Carlo analysis of WIMP search data is presented.140.23 kg-days of WIMP search data from the silicon detectors in the CDMSII experiment is also analyzed. The resulting upper limits on the WIMP-nucleon crosssection are higher than those published by other experiments at all WIMP masses, and the lowest limit on the WIMP-nucleon cross-section is 1.07*10 −42 cm 2 at a mass 3 of 60 GeV/c 2 . These results do provide new and interesting constraints at WIMP masses ≤40 GeV/c 2 and cross sections from 10 −42 -10 −39 cm 2 , a region in which some WIMP search experiments have claimed evidence for a WIMP signal, which other experiments claim to have ruled out.

A Search for Light Weakly-Interacting Massive Particles with SuperCDMS and Applications to Neutrino Physics

“…First, before starting this analysis, SuperCDMS did not have a fully validated detector simulation. While extensive development had gone into the ambitious SuperCDMS detector Monte Carlo (DMC) [64,65], which simulates the physics of iZIPs from first principles, it was not integrated into the analysis pipeline and validated on the kinds of backgrounds most important to this analysis. Given the aggressive timescale required to release this analysis before the G2 funding downselect, it was believed that using the full DMC was too ambitious, requiring us to develop simpler but still robust methods.…”

“…Low-energy analyses in CDMS II did not use any background model, while analyses of higher-energy data simply normalized ' 33 Ba calibration data and multiple-scatter events in WIMP-search data to the sideband data above the nuclear recoil band, extrapolating the event rate into the signal region. Although a variety of careful attempts were made to correct for the systematic bias between single-and multiple-scatter surface events, it difficult to validate this modeling approach, which leaves much room for criticism [65,72]. Criticism aside, we will see that low-energy backgrounds in SuperCDMS contain a large contribution from 21 0 Pb daughter surface events that are completely absent in the multiple-scatter and ' 33 Ba calibrations, rendering the calibration and WIMP-search multiple-scatter samples useless.…”

“…Cosmogenic production of the long-lived isotopes "Ge and 65 Zn is well-known from simulations [901, while the short-lived isotope 68 Ga is a product of 'Ge decay. Because these isotopes are produced by neutrons and cosmic rays, which have a long range in Ge, the activation in the detectors is spatially uniform to an excellent approximation.…”

“…As such, intense 5 1Cr may be less ideal for this investigation, but still worth considering given the advantages in producing the required activity. With it's high energy neutrino emission, 65 Zn is also an attractive source for consideration [218]. However, its 1.1 MeV gamma emission complicates the shielding, so this source is not considered further.…”

Development of CDMS-II Surface Event Rejection Techniques and Their Extensions to Lower Energy Thresholds

“…The example shown is for the Ge neural network analysis. later [112], and utilized a modified version of the 5D χ 2 method using only two variables, and thus it was called the "2D χ 2 " analysis. The remainder of this section describes the high threshold, zero leakage analyses performed with these four timing cuts.…”