CU-Boulder part of international team to discover neutrinos can change 'flavors'

Published: June 15, 2011

An international research team led by Japan and including the University of Colorado Boulder may have taken a significant step in discovering why matter trumped antimatter at the time of the Big Bang, helping to create virtually all of the galaxies and stars in the universe.

The experiment, known as the Tokai to Kamioka experiment, or T2K, included shooting a beam of neutrinos underground from the Japan Proton Accelerator Research Complex, or J-PARC, on the country's east coast to a detector near Japan's west coast, a distance of about 185 miles. Elementary particles that are fundamental building blocks of nature, neutrinos generally travel at the speed of light and can pass through ordinary matter, like Earth's crust, with ease. Neutrinos come in three types: muon, electron and tau.

The T2K team discovered that muon neutrinos can spontaneously change their "flavor" to electron neutrinos, a finding that may help explain why the universe is made up mostly of matter rather than antimatter, said CU-Boulder Assistant Professor Alysia Marino of the physics department, who is part of a university contingent that participated in the experiment. Scientists had previously measured the change of muon neutrinos to tau neutrinos and electron neutrinos to muon neutrinos or tau neutrinos, she said.

The shift of muon neutrinos to electron neutrinos detected in the new experiment is a new type of neutron oscillation that opens the way for new studies of a matter-antimatter symmetry called charge-parity, or CP violation, said Marino. "This CP violation phenomenon has not yet been observed in a neutrino, but may be the reason that our universe today is made up mostly of matter and not antimatter," she said.

Scientists believe matter and antimatter were present in nearly equal proportions at the onset of the Big Bang. Since matter and antimatter particles cancel each other out, it has been proposed that there must have been CP violation in the early universe that produced slightly more matter than antimatter, which accounts for all the stars, galaxies, planets and life present today.

The T2K project is a collaboration of roughly 500 scientists from 12 nations. Other participating U.S. institutions include Boston University, Brookhaven National Laboratory, the University of California-Irvine, Colorado State University, Duke University, Louisiana State University, Stony Brook University, the University of Pittsburgh, the University of Rochester and the University of Washington. The United States contingent is funded by the U.S. Department of Energy.

The CU-Boulder group includes Marino, physics Associate Professor Eric D. Zimmerman, postdoctoral researchers Stephen Coleman and Robert Johnson, graduate students Andrew Missert and Tianlu Yuan, and former undergraduates Christopher Vanek, Bryan Kaufman, Eric Hansen, Zhon Butcher and Joshua Spitz.

The CU-Boulder team designed and built one of three magnetic horns used to generate neutrino beams. The horns are large aluminum conductors that use very high electrical currents to produce a magnetic field. The magnetic field focuses on short-lived neutrino-producing particles called pions and kaons, enhancing the intensity of the neutrino beam, said Zimmerman.

The CU-Boulder researchers also developed a device to monitor the position of the proton beam that creates the neutrinos. In addition, they contributed to the installation and operation of a T2K detector at the J-PARC site 60 miles northeast of Tokyo that measures the neutrinos right after they are produced, Marino said.

Zimmerman said more data will be required to confirm the new results. The J-PARC accelerator is being repaired following damage from the earthquake that hit Japan on March 11. The accelerator and experiment are expected to be operational again by the end of the year, said Zimmerman.

The neutrino detector used in the T2K experiments surrounds 50,000 gallons of super pure water with 11,200 light-detecting photomultiplier tubes. (Image courtesy of the Science and Technology Facilities Council of the United Kingdom)