First 5 Tower WIMP-search Results from the Cryogenic Dark Matter Search with Improved Understanding of Neutron Backgrounds and Benchmarking

Non-baryonic dark matter makes one quarter of the energy density of the Universe and is concentrated in the halos of galaxies, including the Milky Way. The Weakly Interacting Massive Particle (WIMP) is a dark matter candidate with a scattering cross section with an atomic nucleus of the order of the weak interaction and a mass comparable to that of an atomic nucleus. The Cryogenic Dark Matter Search (CDMS-II) experiment, using Ge and Si cryogenic particle detectors at the Soudan Underground Laboratory, aims to directly detect nuclear recoils from WIMP interactions.

This thesis presents the first 5 tower WIMP-search results from CDMS-II, an estimate of the cosmogenic neutron backgrounds expected at the Soudan Underground Laboratory, and a proposal for a new measurement of high-energy neutrons underground to benchmark the Monte Carlo simulations.

Based on the non-observation of WIMPs and using standard assumptions about the galactic halo, the 90% C.L. upper limit of the spin-independent WIMP-nucleon cross section for the first 5 tower run is 6.6×10^-44cm² for a 60~GeV/c² WIMP mass.

A combined limit using all the data taken at Soudan results in an upper limit of 4.6×10^-44cm² at 90% C.L.for a 60~GeV/c² WIMP mass. This new limit corresponds to a factor of ∼3 improvement over any previous CDMS-II limit and a factor of ∼2 above 60~GeV/c² better than any other WIMP search to date.

This thesis presents an estimation, based on Monte Carlo simulations, of the nuclear recoils produced by cosmic-ray muons and their secondaries (at the Soudan site) for a 5 tower Ge and Si configuration as well as for a 7 supertower array. The results of the Monte Carlo are that CDMS-II should expect 0.06± 0.02^+0.18_-0.02/kg-year unvetoed single nuclear recoils in Ge for the 5 tower configuration, and 0.05± 0.01^+0.15_-0.02/kg-year for the 7 supertower configuration. The systematic error is based on the available underground neutron data (that we are aware of) relevant to the unvetoed neutron population. Therefore, for the first 5 tower run, a prediction of <0.2 events from cosmogenic neutrons was obtained.

Furthermore, this thesis describes a proposal for a new measurement of the absolute flux of high-energy neutrons (>60,MeV) deep underground. The cosmogenic neutron detector could measure, at a depth of 2000,meters of water equivalent, a rate of 70±8 (stat),events/year. Based on these studies, the benefits of using a neutron multiplicity meter as a component of active shielding in experiments with similar background concerns are described.