UC Berkeley

Many compelling pieces of indirect evidence pointing to the existence of dark matter. While a confirmed and direct signature of dark matter has yet to be observed, many theoretical models have been developed in an attempt to explain the indirect evidence and to provide phenomenological models that can be tested with targeted experiments. The WIMP is a well-motivated dark matter candidate currently being sought for by several experiments. A variety of detector technologies are utilized, including liquid noble detectors, to look for WIMP scattering as a direct signature of dark matter.

The CLEAN experiment is a proposed single-phase, monolithic, large-scale liquid argon experiment designed to look for high-mass WIMPs. A liquid neon target could be exchanged with the argon target to study solar neutrinos and to test the A2 dependence of a possible dark matter signal. Before scaling up to the multi-tonne scale of the full CLEAN detector, the design philosophy and background rejection capabilities required for the next-generation project are being tested using the MiniCLEAN prototype. As of mid-2018, MiniCLEAN has been constructed at SNOLAB and is currently being filled with natural liquid argon for a dark matter run. Following a short dark matter run, MiniCLEAN will be spiked with elevated levels of 39Ar to test the scaling limits of pulse shape discrimination, the primary method for electronic background rejection. These results will inform existing experiments and the next-generation of large-scale liquid argon detectors.

A good understanding of light propagation is critical for optical experiments such as CLEAN, whose event reconstruction and background rejection relies primarily on scintillation light collection. This work presents two classes of complementary results which are expected to improve the modeling of scintillation light collection in current and future neutrino and dark matter detectors. These are, first, the dependence of the scintillation light time structure (triplet lifetime) and relative light yield of gaseous argon as a function of impurity level and, second, the measurement of several parameters critical to constructing a microphysically-motivated model of tetraphenyl butadiene (TPB) wavelength shifting thin films - a technology which is commonly used in many existing and proposed liquid noble gas experiments.