First Dark Matter Limits from the COUPP 4kg Bubble Chamber at a Deep Underground Site

Abstract

The COUPP 4 kg bubble chamber employs 4.0 kg of CF$_3$I as a WIMP scattering target for use as a dark matter direct detection search. This thesis reports the first experimental results from operating this bubble chamber at the deep underground site (6000 m.w.e.) of SNOLAB, near Sudbury, Ontario. Twenty dark matter candidate events were observed during an effective exposure of 553.0 kg-days, when operating the bubble chamber at three different bubble nucleation thresholds. These data are consistent with a neutron background internal to the detector. Characterization of this neutron background has led to the recommendation to replace two detector components to maximize dark matter signal sensitivity in a future run with this bubble chamber. A measurement of the gamma-ray flux has confirmed that this detector should not be sensitive to a gamma-induced background for more than three orders of magnitude below current sensitivity. The dark matter search data presented here set a new world-leading limit on the spin-dependent WIMP-proton scattering cross section and demonstrate significant sensitivity to spin-independent WIMP-nucleon scattering.