Underground search for neutrino properties unveils first results

Scientists studying neutrinos have found with the highest degree of sensitivity yet that these mysterious particles behave like other elementary particles at the quantum level. The results shed light on the mass and other properties of the neutrino and prove the effectiveness of a new instrument that will yield even greater discoveries in this area.

The Enriched Xenon Observatory 200 (EXO-200), a sixteen-institution international collaboration, led by Stanford University and the U.S. Department of Energy’s (DOE) SLAC National Accelerator Laboratory and including researchers from the Department of Physics at the University of Illinois at Urbana-Champaign, has begun one of the most sensitive searches ever for a mysterious mechanism called "neutrinoless double-beta decay" in which two neutrinos, acting as particle and antiparticle, do not emerge from the nucleus.

If this decay were observed, it would signal that neutrinos have a different quantum structure than other elementary particles. EXO-200, which is capable of detecting decays that happen, on average, only once every 1025 years (1 quadrillion times the age of the universe), did not observe this decay, which constitutes the strongest evidence yet that neutrinos behave like other particles.

Members of the University of Illinois EXO team include Assistant Professor of Physics Liang Yang, who has been a member of the collaboration since 2007, Professor of Physics Douglas H. Beck, and graduate student Josiah Walton. 

Before joining the physics department in January 2012, Yang played a leading role in the construction of the ultralow background detector and the commissioning of the high-voltage and front-end electronics systems as a research associate at SLAC. The Illinois group is currently involved in the data analysis efforts of EXO-200 and R&D of the future tonne-scale experiment. 

“Neutrinoless double beta decay is the most sensitive probe to the question of whether or not the neutrino is its own antiparticle,” said Yang. “With only few months of data, EXO-200 has already achieved one of the best limits for such a search.”

“The result could only have been more exciting if we'd been hit by a stroke of luck and detected neutrinoless double-beta decay,” said Giorgio Gratta, a professor of physics at Stanford University and spokesperson for EXO-200. “In the region where neutrinoless double-beta decay was expected, the detector recorded only one event. That means the background activity is very low and the detector is very sensitive. It’s great news to say that we see nothing!”

EXO-200 has been able to all but rule out a previous, highly controversial result claiming to have detected the decay, and they've also been able to narrow down the mass of the neutrino to less than 140- to 380- thousandths of an electronvolt (the unit of mass used in particle physics). For comparison, the miniscule electron has a mass of roughly 500,000 electronvolts.

At the heart of EXO-200 is a thin-walled cylinder made of extremely pure copper full of about 200 kilograms of liquid xenon buried 700 meters deep at the DOE’s Waste Isolation Pilot Plant (WIPP), a New Mexico salt bed where low-level radioactive waste is stored. The xenon—in particular the isotope xenon-136, which makes up the lion's share of the xenon in EXO-200—is one of the few substances that can theoretically undergo the decay. Constructing the experiment of exceedingly pure materials and locating it underground ensured that all other traces of radioactivity and cosmic radiation are eliminated or kept at a minimum.

EXO-200 will take data for a few more years, and the team hopes in the future to expand the technique to a several-ton version that would be even more sensitive to observing the nearly imperceptible physical processes that have been theorized.

“The performance of the EXO-200 detector exceeded the expectation of many of us,” said Yang.  “At Illinois, we are working on analysis methods to further improve the physics results we can extract from the low background data.  We are also excited about the new techniques under development in our lab that can significantly improve performance of next generation detectors.”

EXO is a collaboration that involves scientists from SLAC, Stanford, the University of Alabama, Universität Bern, Caltech, Carleton University, Colorado State University, University of Illinois Urbana-Champaign, Indiana University, UC Irvine, ITEP (Moscow), Laurentian University, the University of Maryland, the University of Massachusetts – Amherst, the University of Seoul and the Technische Universität München. This research was supported by DOE and NSF in the United States, NSERC in Canada, SNF in Switzerland and RFBR in Russia. This research used resources of the National Energy Research Scientific Computing Center (NERSC).

SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the U.S. Department of Energy Office of Science. To learn more, please visit www.slac.stanford.edu.

DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov
____________________

Contact: Liang Yang, Department of Physics, University of Illinois at Urbana-Champaign, 217/244-2792.

Andy Freeberg, Media Relations Manager, SLAC, 650/926-4359.

If you have any questions about the College of Engineering, or other story ideas, contact Rick Kubetz,, editor, Engineering Communications Office, University of Illinois at Urbana-Champaign, 217/244-7716.