Background Discrimination and the Search for Dark Matter with the LUX-ZEPLIN Experiment
Thesis defense:
Jack Bargemann, Tuesday, Oct 1, 11 am, Broida 5233, https://ucsb.zoom.us/my/jackbargemann
Title:
Background Discrimination and the Search for Dark Matter with the LUX-ZEPLIN Experiment
Abstract:
There is abundant astrophysical evidence for the existence of dark matter, and a growing body of evidence for what it is not, but we know relatively little about its nature. Weakly Interacting Massive Particles (WIMPS) are a theoretical class of particles which are a leading candidate for the composition of dark matter. The LUX-ZEPLIN experiment, a large dual-phase xenon time projection chamber (LXe-TPC), was designed to search for WIMPS and has set world-leading exclusion limits on WIMP - nucleus cross sections.
A primary challenge in searching for rare events such as WIMP dark matter is reducing and rejecting the non WIMP dark matter backgrounds, such as radioactive decays inside the detector or energetic particles passing through the detector. To reduce the rate of these backgrounds, LUX-ZEPLIN was constructed exclusively of radiopure materials, and located 4,580 feet below ground to shield from cosmogenic radiation. Yet to further increase sensitivity, experiments must be able to distinguish between their remaining background events, and the WIMP dark matter signals they are looking for.
Dark matter searches using LXe-TPCs, such as LUX-ZEPLIN, use the ratio of charge signal to light signals of an interaction to discriminate between electron recoils (most backgrounds) and nuclear recoils (the expected signal channel of WIMP dark matter). The charge and light yields of electron recoils are often calibrated with 𝛽-decays, such as 3H and 212Pb. 𝛽-decays, however, are not the only type of electron recoil background - other backgrounds, such as neutrino-electron recoils and electron-capture decays, produce signals similar yet not identical to the standard 𝛽-decay calibrations. As the sensitivities of dark matter experiments increase, these backgrounds must be understood, modeled, and when possible, directly calibrated. This work investigates discrimination and modeling of electron recoils in LUX-ZEPLIN, and the in-situ calibrations of electron capture charge and light yields using 127Xe.
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