Title : The Active Veto of the Large Underground Xenon Detector and the Study of Muons Deep Underground
Speaker: Douglas Tiedt, Universtiy of Maryland
Abstract: Cosmological observations of the universe find a stark mismatch between the expected levels of visible matter and the state of the universe. Rotational velocities of galaxies, interactions of galaxy clusters, and the cosmic microwave background (CMB) all point to the existence of some kind of ``dark matter" that primarily interacts gravitationally. One theory for the nature of this matter is the Weakly Interacting Massive Particle (WIMP), whose existence is predicted both by extensions to the particle physics standard model, and by straightforward thermodynamic arguments following the expansion and cooling of the early universe. The search for dark matter in our labs has become critical in order to understand its true nature. The Large Underground Xenon (LUX) dark matter detector operated at the Sanford Underground Research Facility (SURF) for several years until it was decommissioned in 2016. It was a two-phase time projection chamber (TPC) that used 370 kg of xenon to search for the nuclear recoil signature of WIMPs, producing world-leading results into 2017. The success of LUX relies on effective background controls. Several steps were taken to shield LUX from external backgrounds caused by high energy cosmic rays or from radioactive decays. First, its placement in the Davis Cavern at SURF provided $\approx$ 4200 m.w.e of overburden to eliminate most secondary particles produced by cosmic rays and reduce the atmospheric muon flux. LUX was also installed inside of a 20' by 25' water tank, with low background steel shielding underneath, to provide extensive passive protection from radioactive decays in the local environment. Muon-induced neutrons were of special concern, as they could be produced by muons passing through the water tank near the detector. In order to veto these events the water tank was instrumented with twenty 10" Hamamatsu photomultiplier tubes (PMTs) that monitor relativistic charged particles entering the water volume. This allowed the water tank to serve as an active water cherenkov muon veto during the final LUX WIMP-search (WS) run. This talk discusses the analysis of this active veto data, focusing on the measured muon flux and events coincident between LUX and the veto in the WIMP-search data. The overall muon flux is found to be consistent with previous measurements. No excess of muon-induced events were found in the WS dataset. Numerous detector issues, mostly related to the electronics, hampered the analysis work, requiring careful characterization and handling. This work is of particular interest as the successor to LUX, the LUX-ZEPLIN (LZ) experiment, will utilize the same water tank and encounter the same external backgrounds as LUX.