Collaborative Research for Underground Science: Barium Tagging for EXO

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The observation of neutrinoless double beta decay will help determine the masses of neutrinos, discover lepton number violation and reveal if two-component Majorana fermions exist in Nature. EXO, a multi-tonne neutrinoless double beta decay experiment employing xenon enriched to ~80% in the isotope Xenon-136, is now taking data at the WIPP underground site in New Mexico. However, the next generation of such experiments requires advanced background reduction methods to reach Majorana neutrino mass sensitivities below 0.01 eV.EXO is developing a novel technique to tag the Barium (Ba) produced in the final state of the neutrinoless double beta decay using atomic spectroscopy. This technique has the promise of drastically reducing radioactive backgrounds, providing a clean and un-ambiguous measurement with unprecedented sensitivity. Recent breakthroughs have been obtained in the Ba-tagging front. These groups have produced, for the first time, Ba transport and tagging efficiencies above 1%. The R&D work funded with this award builds on such breakthroughs and aspires to provide the first demonstration of high efficiency tagging of individual Ba atoms in liquid xenon. Broader impacts: While the primary scientific goal of this proposal is in the field of neutrino physics, its broader impact is substantial. The techniques being developed span a range of topics, from nuclear and particle astrophysics, to AMO, surface physics and material science. This research, if successful, is likely to be applicable to other problems in science and technology, where the high efficiency transfer and identification of single atoms would be of interest. Examples include trace analysis for homeland security applications and the detection of rare phenomena. The team is also planning to continue the summer program integrating high school teachers into their labs.