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Neutrinos Change Flavors While Crossing Japan

Neutrinos Change Flavors While Crossing Japan

Physicists say they have recorded a change in sub-atomic particles that could help explain why the universe is made of matter and not anti-matter.

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Neutrino flavor change
After traveling 295 km underneath Japan, a neutrino interacted with the giant Super-K detector and was recorded by its light detectors. Courtesy of Chris Walter, Duke.

Durham, NC -- By shooting a beam of neutrinos through a small slice of the earth under Japan, physicists say they've caught the particles changing their stripes in new ways.

These observations may one day help explain why the universe is made of matter rather than anti-matter.

The T2K experiment has been using the Japan Proton Accelerator Research Complex, or J-PARC, located on the east coast, to shoot a beam of muon neutrinos 185 miles, or 295 kilometers, underground toward the Super-Kamiokande, or Super-K, detector in Kamioka, near Japan's west coast.

The goal of the experiment, which is part of a new generation of neutrino-tracking facilities, is to observe the particles change "flavors" from muon neutrinos to electron neutrinos on this brief journey.

Neutrinos are elementary particles that come in three flavors -- muon, electron and tau. In past experiments, physicists have measured the change of muon neutrinos to tau neutrinos and electron neutrinos to muon neutrinos or tau neutrinos. 

"But no one had seen muon neutrinos turn into electron neutrinos," said Chris Walter, a physicist at Duke who is part of the T2K collaboration, along with Duke physicist Kate Scholberg.

The T2K collaboration, a team of physicists from around the world, began observing the neutrinos for their transformations in January 2010 when the experiment became fully operational. The group measured the neutrinos, determining their flavor near the accelerator and then again at Super-K. So far, the scientists caught 88 neutrinos with their detector. Six of these likely began their lives as muon neutrinos and turned into electron neutrinos on their way to Super-K.

"As it stands, this result is extremely interesting, but we are just getting started," Walter said. He explained that the T2K team has taken a little less than two percent of the planned neutrino measurements, partly due to the East Japan earthquake that struck on March 11, 2011 and forced the shutdown of T2K.

The preliminary findings were submitted to Physical Review Letters and announced at a press conference Wednesday in Japan.

"We could see as many electron neutrino candidates as we saw by chance, something, like one out of every 150 times," Walter said. "This is why the title of our paper includes the word 'indications' as opposed to observation or measurement."

If the "indications" become "measurements," these T2K results will be the first to measure a muon-electron neutrino change. Scientists want this measurement to study a fundamental parameter of physics called theta-13, which controls the muon-electron neutrino switch.

Walter said there is more than one way to measure theta-13 and that several experiments are currently competing to be the first.

"It's good news that we have evidence of a relatively large theta-13, since there are even more interesting measurements that can be done if it is big enough," he said.

If theta-13 is large, it will allow scientists to measure the difference between changes in neutrinos and changes in anti-neutrinos. Walter explained that in the early universe, "something caused there to be slightly more matter than anti-matter. When the matter and anti-matter annihilated each other, only that little bit of matter was left over. That matter is everything we see around us today.  But no one understands how this happened."

The difference between "neutrino and anti-neutrino properties that we might measure in future experiments might give clues to how the excess matter was generated," Walter said.

Of course that all depends on how quickly T2K can come back online after being shut down from the earthquake. Currently, the experiment is slated to re-start at the end of 2011.