Search for Majorana Neutrinos in EXO-200 and nEXO
Our group is involved in the quest for one of the great unknowns of particle physics: are the neutrino and antineutrino the same particle (Majorana neutrinos) or are they distinct particles (Dirac neutrinos). The only known way to test this with currently conceivable technology is Neutrinoless Double Beta Decay (0nbb). If and when 0nbb decay is observed, it will demonstrate that neutrinos are Majorana particles, that the law of Conservation of Lepton Number is violated and give initial evidence for physics beyond the Standard model of elementary particle physics.
Neutrinoless Double Beta Decay
To date, 0nbb decay has not yet been discovered. Since 2005, our group has been part of the EXO-200 experiment, which has set a stringent limit on the half-life of 0nbb decay of 3.5×1025 years . The EXO-200 0nbb detector uses 200 kg of isotopically enriched liquid Xe-136. We also work on the next generation EXO experiment, called nEXO, that expected to use about 5 tons of enriched liquid Xe-136 and to run for ten years. Its projected half-life sensitivity is greater than 1028 years .
Single Barium Daughter Atom Detection
Current 0nbb decay searches detect the total energy of the two energetic electrons emitted. While very powerful at distinguishing potential 0nbb events from similar-appearing background events, typically a small number of indistinguishable events limit the achieved sensitivity. If there were a way to detect the existence of a Ba-136 daughter atom or ion or not at the decay site of a potential Xe-136 0nbb decay event, complete distinction between 0nbb decays and background events, that don’t produce a Ba-136 daughter, could be obtained, enhancing the 0nbb search sensitivity significantly.  This is called Barium Tagging. Our group has been pioneering Barium tagging for more than two decades  and is the only group in the world that has achieved published images of single atoms to date. . Images from our work are shown below. The ability to image and count single atoms or ions in solid noble gases has attracted interest from other groups for quantum computing  and a measurement of a very weak nuclear reaction that is a crucial bottleneck in the creation of the elements in the universe .
Moving a focused laser beam in a raster scan across a solid sample with a few Ba atoms deposited in the scan region.
As the laser moves from left to right, the signal increases abruptly when the laser is positioned at the location of the barium atom.
A “composite image of total Ba fluorescence vs. laser position. Two Ba atoms found in this scan of the sample can be easily counted. When we then parked the laser on the first peak, the signal suddenly turned off to “no barium” background (red points) after about 20 s. We have also seen the fluorescence “blink” off for a few image frames and then turn back on again.
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G Adhikari, et al., “nEXO: Neutrinoless double beta decay search beyond 1028 year half-life sensitivity”, J. Phys. G: Nucl. Part. Phys. 49, 015104 (2022). [arXiv:2106.16243]