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Japanese T2K experiment sees evidence of a new type of neutrino "flavor change"

An international collaboration reported first indications of the production of electron neutrinos from muon neutrinos, raising the prospect that it may be possible for future experiments to test for violation of the symmetry between matter and antimatter.

Where did all the matter in the universe come from? This is one of the biggest mysteries in fundamental physics, and exciting results released from the international T2K neutrino experiment in Japan could be an important step toward resolving this puzzle.

The intriguing results indicate a new property of the enigmatic particles known as neutrinos.

There are three types of neutrinos (called flavors): one paired by particle interactions with the familiar electron (called the electron neutrino); and two more paired with the electron’s heavier cousins, the mu and tau leptons. Previous experiments around the world have shown that these different flavors of neutrinos can spontaneously change into each other, a phenomenon called “neutrino oscillation."

Researchers have already observed two types of oscillations, but in its first full period of operation the T2K experiment has already seen evidence for a new type of oscillation (the appearance of electron neutrinos in a muon neutrino beam). This means that researchers have now observed that neutrinos can oscillate in every way possible.

This level of complexity opens the possibility that the oscillations of neutrinos and their antiparticles (called antineutrinos) could be different. And if the oscillations of neutrinos and antineutrinos are different, it would be an example of what physicists call CP violation. This could be the key to explaining why there is more matter than antimatter in the universe (an excess which could not happen within the known laws of physics).

The experiment ran from January 2010 until March 11 this year, when it was dramatically interrupted by the Japanese earthquake. Fortunately, the multinational T2K team were unharmed, and their highly sensitive detectors were largely undamaged. Six clean electron neutrino events are observed in the data from before the earthquake, while in the absence of oscillations there should only have been 1.5. Even though such an excess could only happen by chance about one time in a hundred, that is not good enough to confirm a new physics discovery, so this is called an “indication."

Professor Dave Wark of the Science and Technology Facilities Council in the United Kingdom and Imperial College London, who served for 4 years as the iInternational co-spokesperson of the experiment and is head of the UK group, explained, “People sometimes think that scientific discoveries are like light switches that click from ‘off’ to ‘on’, but in reality it goes from ‘maybe’ to ‘probably’ to ‘almost certainly’ as you get more data. Right now, we are somewhere between ‘probably’ and ‘almost certainly’."

Professor Christos Touramanis from Liverpool University is the project manager for the UK contributions to T2K: “We have examined the near detectors and turned some of them back on, and everything that we have tried works pretty well. So far it looks like our earthquake engineering was good enough, but we never wanted to see it tested so thoroughly."

Professor Takashi Kobayashi of the KEK Laboratory in Japan and spokesperson for the T2K experiment said, “It shows the power of our experimental design that with only 2 percent of our design data we are already the most sensitive experiment in the world for looking for this new type of oscillation."

Japan's T2K neutrino experiment
Japan's T2K neutrino experiment has seen evidence for a new type of oscillation (the appearance of electron neutrinos in a muon neutrino beam). This means that researchers have now observed that neutrinos can oscillate in every way possible.
University of Tokyo

Where did all the matter in the universe come from? This is one of the biggest mysteries in fundamental physics, and exciting results released from the international T2K neutrino experiment in Japan could be an important step toward resolving this puzzle.

The intriguing results indicate a new property of the enigmatic particles known as neutrinos.

There are three types of neutrinos (called flavors): one paired by particle interactions with the familiar electron (called the electron neutrino); and two more paired with the electron’s heavier cousins, the mu and tau leptons. Previous experiments around the world have shown that these different flavors of neutrinos can spontaneously change into each other, a phenomenon called “neutrino oscillation."

Researchers have already observed two types of oscillations, but in its first full period of operation the T2K experiment has already seen evidence for a new type of oscillation (the appearance of electron neutrinos in a muon neutrino beam). This means that researchers have now observed that neutrinos can oscillate in every way possible.

This level of complexity opens the possibility that the oscillations of neutrinos and their antiparticles (called antineutrinos) could be different. And if the oscillations of neutrinos and antineutrinos are different, it would be an example of what physicists call CP violation. This could be the key to explaining why there is more matter than antimatter in the universe (an excess which could not happen within the known laws of physics).

The experiment ran from January 2010 until March 11 this year, when it was dramatically interrupted by the Japanese earthquake. Fortunately, the multinational T2K team were unharmed, and their highly sensitive detectors were largely undamaged. Six clean electron neutrino events are observed in the data from before the earthquake, while in the absence of oscillations there should only have been 1.5. Even though such an excess could only happen by chance about one time in a hundred, that is not good enough to confirm a new physics discovery, so this is called an “indication."

Professor Dave Wark of the Science and Technology Facilities Council in the United Kingdom and Imperial College London, who served for 4 years as the iInternational co-spokesperson of the experiment and is head of the UK group, explained, “People sometimes think that scientific discoveries are like light switches that click from ‘off’ to ‘on’, but in reality it goes from ‘maybe’ to ‘probably’ to ‘almost certainly’ as you get more data. Right now, we are somewhere between ‘probably’ and ‘almost certainly’."

Professor Christos Touramanis from Liverpool University is the project manager for the UK contributions to T2K: “We have examined the near detectors and turned some of them back on, and everything that we have tried works pretty well. So far it looks like our earthquake engineering was good enough, but we never wanted to see it tested so thoroughly."

Professor Takashi Kobayashi of the KEK Laboratory in Japan and spokesperson for the T2K experiment said, “It shows the power of our experimental design that with only 2 percent of our design data we are already the most sensitive experiment in the world for looking for this new type of oscillation."

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