Physics
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Item Studies of Ionization Backgrounds in Noble Liquid Detectors For Dark Matter Searches(2024) Mizrachi, Eli; Xu, Jingke; Hall, Carter; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Dark matter is believed to make up almost 85% of the total mass of the universe, yet its identity remains unclear. Weakly Interacting Massive Particles (WIMPs) have historically been a favored dark matter candidate, and dual-phase noble liquid time projection chambers (TPCs) have set the strongest interaction limits to date on WIMPs with a mass greater than several GeV. However, because no definitive interactions have been observed, the parameter space for conventional WIMPs is highly constrained. This has sparked greater interest in new sub-GeV dark matter models. At this mass scale, dark matter interactions with xenon or argon target media may still produce detectable signals at or near the single electron limit. However, these signals are currently obscured by delayed ionization backgrounds (“electron trains”) which persist for seconds after an ionization event occurs. Electron trains have been observed in many different experiments and exhibit similar characteristics, but their cause is only partially understood. This work examines the nature of electron trains in various contexts, as well as possible strategies to mitigate them. First, a characterization of electron trains in the LZ experiment is presented, including new evidence of a dependence on detector conditions. The characterization also informed the development of an electron-train veto for LZ's first WIMP search, which set world-leading limits on the spin-independent and spin-dependent WIMP-nucleon cross-sections for medium and high-mass WIMPs. Next, to complement the analysis of LZ data, hardware upgrades were performed in XeNeu, a small xenon TPC at Lawrence Livermore National Lab. These included replacing plastics with low-outgassing metal and machinable ceramic components, as well as a replacement of XeNeu's photomultiplier tube array with silicon photomultipliers. The resulting reduction in the intensity of electron-trains and better position resolution from the respective upgrades will improve future studies of low energy interactions and phenomena. Concurrent with this work, XeNeu was used to perform a nuclear recoil calibration and a search for the Migdal effect, the latter of which can substantially enhance an experiment's low-mass dark matter sensitivity. Finally, the development of CoHerent Ionization Limits in Liquid Argon and Xenon (CHILLAX), is reported. CHILLAX is a new xenon-doped, dual-phase argon test stand that has the potential to have a higher sensitivity to low-mass dark matter interactions and lower backgrounds than current liquid xenon TPCs. The system is designed to handle high (percent level) xenon concentrations in liquid argon that can enable a range of ionization signal production and collection benefits. CHILLAX demonstrated the feasibility of such concepts by achieving a world record xenon doping concentration with stable operation.Item Novel Dark Matter Phenomenology At Colliders(2015) Wardlow, Kyle; Agashe, Kaustubh; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)While a suitable candidate particle for dark matter (DM) has yet to be discovered, it is possible one will be found by experiments currently investigating physics on the weak scale. If discovered on that energy scale, the dark matter will likely be producible in significant quantities at colliders like the LHC, allowing the properties of and underlying physical model characterizing the dark matter to be precisely determined. I assume that the dark matter will be produced as one of the decay products of a new massive resonance related to physics beyond the Standard Model, and using the energy distributions of the associated visible decay products, develop techniques for determining the symmetry protecting these potential dark matter candidates from decaying into lighter Standard Model (SM) particles and to simultaneously measure the masses of both the dark matter candidate and the particle from which it decays.Item Flavored Dark Matter(2012) Agrawal, Prateek; Chacko, Zackaria; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Astrophysical observations point to the fact that most of the matter density in the universe is in the form of a non-luminous “dark matter”. No particles detected so far satisfy the criteria to be dark matter candidates. If the present day dark matter density is set by thermal freeze out due to the expansion of the universe, then the dark matter particles likely have weak-scale interactions with ordinary matter. This opens up the possibility of detecting them in current experiments. We focus on a novel class of models — flavored dark matter — which contains multiple copies, or flavors, of dark matter particles, in analogy with the three copies of matter particles observed in nature. We classify such models and consider their implications for various dark matter searches. As an example, we choose one specific model with tau flavored dark matter, and consider its prospects for detection at the Large Hadron Collider.