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Weighing Neutrinos (and the Universe) with Noble Liquid Time Projection Chambers

Andrea Pocar, University of Massachusetts, Amherst
Tuesday, May 2, 2017 - 4:00pm to 5:00pm
PAT C-520

​As the Standard Model of particle physics garners continued experimental success, most recently with the discovery of the Higgs boson at the LHC, big questions in fundamental physics and cosmology remain unanswered. Diverse astrophysical observations tell us that ‘regular’ matter, described with exquisite precision by the Standard Model, constitutes only about 5% of the energy budget of the Universe. The balance is made for two thirds of dark energy, possibly a cosmological constant allowed by Einstein’s general relativity, and one third of dark matter.

Neutrinos have been unique tools for our understanding of the weak interaction and for shaping the Standard Model. Recently, they have been shown to transmute between lepton flavors, a property that requires them to have tiny masses. While light, massive neutrinos cannot, alone, account for the dark matter, they could nonetheless be messengers of new physics. Of utmost interest would be the observation of neutrino- less double beta (0νββ) decay, the disintegration of a nucleus accompanied by the emission of two electrons but no neutrinos. This process would establish neutrinos as Majorana fermions, i.e. indistinguishable from their antiparticles, and yield a key to understanding their minute mass. In addition, the observation of lepton number non-conservation would provide a powerful ingredient to explain the abundance of matter over antimatter in the Universe.

The nEXO Collaboration is designing a five-tonne, liquid xenon Time Projection Chamber (TPC) to search for 0νββ decay of Xe-136. Given current experimental limits on the half-life of this process, tonne-scale experiments are needed for the next generation experiments. Within the context of the big questions above, I will illustrate the motivations for nEXO, its strengths and physics reach, current R&D activities to enable the key technologies it requires, and present results obtained with the EXO-200 predecessor currently running in New Mexico. If time allows, I will introduce the DarkSide-20k (DS-20k) experiment, a very large liquid argon TPC with significant technical overlap with nEXO. In the near future, DS-20k will search for heavy Weakly Interacting Massive Particles (WIMPs), a plausible candidate for dark matter, in our galactic halo with unprecedented sensitivity.

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