Attenuated Alpha Backgrounds in the DEAP-3600 Dark Matter Search Experiment

Abstract

DEAP-3600 is a dark matter experiment using 3.3 tonnes of liquid argon as a scintillation target to directly detect Weakly Interacting Massive Particles (WIMPs), a dark matter candidate. Mitigating background sources is crucial to dark matter searches. A large background model contribution comes from attenuated alphas originating from 210 Po decays within the acrylic vessel surfaces. Alphas from decays within the acrylic inner vessel and from the acrylic neck flowguide are analyzed. The activity of the inner vessel is separated into surface and bulk components, and determined to be 0.22 ± 0.02 mBq/m 2 and 3.68 ± 0.06 mBq. An event rate of 53.5 +30 −4.6 µHz is found for alphas originating from the neck flowguide. An optimized event selection is obtained, making use of machine-learning algorithms to reject neck flowguide alphas and maximize WIMP sensitivity. In 802 live-days of DEAP-3600 data, the expected upper limit on the spin-independent WIMP-nucleon interaction cross-section is 1.9×10 −45 cm 2 (90% C.L.) for a 100 GeV/c 2 WIMP mass. D&D nights and mid-day Catan sessions kept me alive during this entire process, and this thesis would likely sound like the words of a madman without all three of you.Most importantly, I want to thank my parents. I am incredibly lucky to have such supportive and caring parents, who were there for me through this whole process. You taught me how to put my full effort into my work, never give up, and be proud of my accomplishments. That knowledge is the most important lesson I have learned in my life. I will always be grateful for your unconditional love. And to Shawna. It is incredible to me that you can somehow manage your own Masters, a full time job, and all of my nonsense, but you do, and I am grateful every day that you are with me. You are the rock in my life, and the first person I go to when I am excited or nervous. Now that we're both through, I can't wait to see what our life has in store.