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Steve Chaplin
IU Communications

Last modified: Tuesday, June 5, 2012

IU joins international team in unveiling first results of underground search for neutrino properties

Evidence shows that neutrinos, which have traveled the universe for 13 billion years, behave like other particles

June 5, 2012

BLOOMINGTON, Ind. -- An Indiana University team led by physicist Lisa J. Kaufman is announcing in collaboration with the Enriched Xenon Observatory 200 experiment the best result to date showing that neutrinos behave like other elementary particles at the quantum level.

WIPP Facility

EXO-200 is a neutrino experiment involving IU that is housed 2,150 feet below ground in a salt basin in New Mexico. The subterranean location isolates the experiment from cosmic rays and other sources of natural radioactivity.

Print-Quality Photo

Scientists want to understand the properties of neutrinos because the energy-carrying subatomic particles have traveled the universe for 13 billion years, acting as a central character in the formation of the universe and playing a role in phenomena such as supernova explosions, where they carry away 99 percent of the energy generated from those events.

The purpose of EXO-200 is to try to detect, for the first time, neutrino-less double beta decay. Beta decay, a special case of nuclear decay, occurs when a neutron in an unstable nucleus becomes a proton and in the process emits an electron and an antineutrino. Double beta decay occurs when two electrons and two antineutrinos are ejected from the nucleus when two neutrons become protons. Neutrino-less double beta decay, as predicted, would cause no neutrinos to be emitted from the nucleus, meaning the neutrino must be its own antiparticle and cancel each other out.

In the experiment conducted a half-mile below New Mexico's Chihuahuan Desert, scientists filled a time projection chamber with 200 kilograms of liquid xenon, enriched to 80 percent of the double beta decaying isotope xenon-136. When a decay event occurs, the energetic electrons produced interact with the xenon to create scintillation light that can then be detected through specialized photodiodes. The electrons also ionize some of the xenon, and the ionized electrons drift to charge collection wires at the end of the detector in an electric field. The time between the light pulse and the electrons reaching the wires tells physicists how far in the detector the event occurred since the drift time can be calculated.

After 120 days of data collection, between September 2011 and April 2012, the detector recorded only one event in the region where double-beta decay was expected to occur, providing scientists with the strongest evidence yet that no decay occurred and that neutrinos do not have a different quantum structure than other elementary particles. The half-lives of double beta decay isotopes like xenon are extremely long, more than a billion times longer than the age of the universe, and the EXO-200 is capable of detecting decays that happen only once every 1025 years, or one quadrillion times the age of the universe.

Exo Detector

The large copper cylindrical vessel is EXO-200's time projection chamber, the part of the detector that contains the liquid xenon, isotopically enriched in xenon-136. The photo shows the chamber being inserted into the cryostat, which keeps the experiment at extremely low temperatures.

Print-Quality Photo

"We did not observe this decay, which constitutes the strongest evidence yet that neutrinos behave like other particles," Kaufman said. And just as important to Kaufman and her team, the powerful measurement was made possible by the exquisite performance of EXO-200, the first of a new breed of detectors for this type of beta decay. The energy resolution of EXO-200 at the double beta decay endpoint was 1.67 percent, very close to the design value of 1.60 percent. Kaufman's group was responsible for monitoring light collection in the detector, an important part of the analysis used to achieve energy resolution as close to the design value as possible.

If the decay had been observed it would signal that neutrinos have a different quantum structure than other elementary particles.

"We have also been responsible for the xenon purity measurement, or electron lifetime measurement, in the EXO-200 detector using the calibration data," she said. "And we consistently measured lifetimes of about three milliseconds, which is important for good charge collection in the detector."

Good charge collection in the detector means very good energy resolution, which enabled the international collaboration to make precise measurements in the region of interest for the decay.

In addition to finding no evidence for neutrino-less double beta decay, the experiment also provided for the first time the best limit ever for the half life of xenon-136 at about 1015 times the age of the universe, a figure that is close to the best for any other isotope. The result also refutes the claim made about 10 years ago of the observance of neutrino-less double beta decay by scientists in Japan; provides evidence of the power and accuracy of a new type of advanced particle detector; and narrows 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 electron has a mass of roughly 500,000 electronvolts.

"EXO-200 is working incredibly well, and we've already reached our goals for energy resolution, and we have very low background in the region of interest," Kaufman noted. "So we'll be doing excellent physics for a long time and maybe we'll get lucky in a couple of years and see some new physics."

Kaufman, an assistant professor in the IU Bloomington College of Arts and Science's Department of Physics, was supported in her work by physics graduate student Tessa Johnson and undergraduate student Keith Scott. She came to IU in 2010 and is also a faculty member of IU's Center for Exploration of Energy and Matter.

EXO-200 is a collaboration that involves scientists from the U.S. Department of Energy's SLAC National Accelerator Laboratory, 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 National Science Foundation in the United States, the National Sciences and Engineering Research Council of Canada, the Swiss National Science Foundation, and the Russian Foundation for Basic Research. This research used resources of the National Energy Research Scientific Computing Center.

For more information or to speak with Kaufman, please contact Steve Chaplin, IU Communications, at 812-856-1896 or Tweeting IU science news, @IndianaScience, blogging IU science at Science at Work.