Eliminating Backgrounds in the Search for Dark Matter with the PICO-60 Bubble Chamber

摘要

Dark matter represents one of the most sought after discoveries in physics, and leading theories predict that extremely low-background detectors could be sensitive to nuclear recoils from dark matter interactions. The PICO collaboration uses bubble chambers to look for the energy deposition from such an interaction. However, unknown backgrounds have plagued the PICO detectors for many years. I work to uncover the source(s) of these mysterious backgrounds with the goal of eliminating them using, amongst other tools, various small-scale bubble chamber detectors and a xenon time projection chamber at Fermilab. Currently, all dark matter detection experiments treat electron recoils the same way, whether they come from neutrino or Compton scatters, beta-decay, or photoabsorption. My work indicates that this treatment is over-simplistic and results in a breakdown of existing calibration schemes. I have applied my knowledge of backgrounds to the PICO-60 detector as run coordinator. The resulting background-free month-long dark matter search represents the first background-free operation of a bubble chamber dark matter detector at the 40~liter scale, and is the most sensitive direct detection experiment to spin-dependent WIMP-proton dark matter interactions to date. Specifically, for 30 GeV c--2 dark matter, WIMP-proton spin-dependent cross sections larger than 3.4 x 10--41 cm2 are excluded at the 90% confidence level.