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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.

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.
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

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

"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

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.