EXO-200, a time projection chamber utilizing 200 kg of liquid xenon enriched in isotope 136, has recently observed the never-before-seen 2-neutrino double-beta decay of Xe-136. This is a standard model process and the slowest process directly observed by humans (half life ~1021 years). The ultimate goal of EXO-200, however, is to set limits on or observe the neutrinoless double beta decay mode, which would indicate the neutrino is a Majorana particle (that is, its own antiparticle). Already, massive neutrinos have provided some of the first hints of physics beyond the standard model. A Majorana neutrino would be the first fundamental Majorana particle observed and evidence for lepton number violation, which could explain why the universe is made of matter and not antimatter. It would also provide a measurement of the still-unknown mass of the neutrinos. So, despite our sucess, there's lots more to do.
The EXO collaboration is relatively small but is currently taking data almost faster than our analysis teams can keep up. There are plenty of opportunities for graduate students to contribute significantly to the analysis of this data. Additionally, EXO combines many different fields of physics, from AMO (for barium tagging) to HEP (detector technology). It takes lots of cryogenics, vacuum systems, controls, electronics, and low radioactive background engineering techniques to bring it together, so students involved learn a lot that can be applied to any future research. And how many other experiments allow you to brag that your experiment is 2000 ft underground in a salt mine used for nuclear waste disposal?
EXO-200 is looking for a process with a half life orders of magnitude beyond the age of the universe. Our analysis will have to optimize energy resolution (to pick out the monoenergetic zero-neutrino mode decay) and get good background rejection. Right now, our analysis is manpower limited, and there are great opportunities for motivated grad students to make significant contributions. Some possible topics include detector simulations, signal/background separation, waveform analysis, and fitting.
The EXO-200 TPC utilizes 200 kg of liquid xenon to search for the double beta decay of 136-Xe. Because the process is rare, the detector is made of low background materials including an ultra-thin copper vessel. Additionally, xenon is only liquid at cryogenic temperatures and greater-than-atmospheric pressures. This poses unique engineering and hardware challenges. A slow controls system maintains the xenon at the correct temperature and minimizes the pressure differential between the interior and exterior of this thin vessel. The low background nature of the experiment also poses novel electronics problems. For example, solid-state Avalanche PhotoDiodes (APDs) are used instead of traditional photomultiplier tubes. Interested rotation students would join the SLAC group to work on both the slow controls (software and hardware) and on the TPC readout electronics that collect the physics data, including preparing for possible hardware upgrades and improvements to make EXO-200 more sensitive.
One important feature of the next-generation tonne-scale EXO experiment is the ability to retrieve and tag individual double beta decay Barium-136 daughter nuclei. As part of the R&D effort, the SLAC group is building an apparatus to study desorption and ionization processes of barium from a solid surface. Specific rotation projects include building and testing a time-of-flight spectrometer, building a laser desorption and electron gun heating systems, developing LabVIEW base data acquisition systems and studying single atom desorption and ionization processes.
EXO is already planning the successor to EXO-200: nEXO (next EXO). This experiment will use roughly 5 tonnes of enriched xenon (compared to EXO-200's 200 kg). The SLAC group is currently doing much of the design work for this experiment. Specific rotation projects include simulating radioactive backgrounds, doing simulations to optimize the design of the detector, and building and operating small-scale tests to make sure aspects of the detector are feasible.
If you're intersted in any of the above, some combination of the above, or just EXO and EXO-200, please contract us. Prof. Martin Breidenbach leads the SLAC group, and you can also talk to Dr. Peter Rowson or Dr. Ryan MacLellan. For a grad student's perspective, you can contact current Ph.D. student Steve Herrin.