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T2K discovery puts neutrino oscillation beyond doubt

The SuperKamiokande detector lies 1 km underground in the Mozumi mine in the city of Hida. (Courtesy: Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo)

The SuperKamiokande detector lies 1 km underground in the Mozumi mine in the city of Hida. (Courtesy: Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo)

By Hamish Johnston

Physicists working on the Tokai to Kamiokande (T2K) experiment have confirmed what many have suspected for nearly three decades – over time, a neutrino of one flavour will change into a neutrino of another flavour in a process called neutrino oscillation.

The team did this by firing a beam of muon neutrinos 295 km through the ground under Japan to the SuperKamiokande experiment. There, they detected the presence of electron neutrinos in the beam.

Several experiments have already shown that the number of neutrinos of a certain flavour in a beam decreases as it travels a long distance. However, this is the first time that the appearance of a different flavour in a beam has been seen with a statistical significance greater than 5σ – the gold standard for a discovery in particle physics.

The team fire the muon-neutrino beam from the J-PARC lab in Tokai. The beam passes through two detectors – the first is a “near detector” a few hundred metres from the source that characterizes the muon-neutrino flux and the second is SuperKamiokande. After running the experiment for two years, Super-Kamiokande registered 22 electron neutrinos whereas only 6 should have been detected if neutrino oscillation was not occurring. The result has a statistical significance of 7.5σ.

The discovery was presented today at the High Energy Physics 2013 conference in Stockholm by Michael Wilking of TRIUMF. His talk is entitled “The latest results from T2K on the neutrino oscillation and interactions“.

Despite this breakthrough, there is much more work to be done unravelling the mysteries of the neutrino. You can find out much more about neutrinos in the Physics World feature-length article “Neutrinos: ghosts of matter” by Dave Wark of Oxford University.

T2K is an international collaboration and Wark leads the UK contingent. “It’s a joy to see T2K deliver the science we designed it for,” says Wark. “I have been working on this for more than a decade, and what these results tell us is that we have more than another decade of work ahead of us.”

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  1. M. Asghar

    The detection of the 22 electron neutrinos by the T2K compared to 6 expected from the “background”, resulting from the flavour oscillation of the accelerator produced muon neutrinos, is a huge step forwards. Upto now the results of flavour oscillations were limited only to the disappearance of these resulting neutrinos as was the case for the solar or the nuclear reactor electron neutrinos.

  2. Hui Zhang

    The experiments to prove “the God does play dice” can’t meet the requirements that the tool of measurement doesn’t influence the quantum particle and records its time and position with no error. So we can say, “Even the God play dice, its result would obey his will”.

  3. Ismail

    Can neutrinos oscillate into other particles instead of just changing their flavour?

  4. Emmanuel

    The simplest phenomenological explanation of the new T2K neutrino oscillation results with 90% allowed region for the theta_13 angel is a semi-empirical geometric (Pythagorean) symmetry relation for the three neutrino mixing angles:
    cos^2(2θ12)+ cos^2(2θ23)+ cos^2(2θ13) = 1, sin^2(2θ13)= cos^2(2θ12)+cos^2(2θ23).
    With e. g. values of θ12 = 34grad and θ23 = 45grad: θ13 = 11grad — in very good agreement with new T2K data. See arxiv:1202.5043.
    Emmanuel Lipmanov

  5. Emmanuel

    The Pythagorean equation for neutrino mixing angles answers two basic questions related to the mixing angle theta_13, 1)it is relatively large mainly because the solar neutrino mixing angle θ12 is definitely not very close to maximum 45grad, and 2)it is nearly complementary to the solar angle, θ12 + θ13 = ~ 45grad, because the atmospheric angle θ23 is very close to maximum.

  6. Hui Zhang

    Even if Maldacena is right, some information may be lost when a quantum is measured by space and time which may be emerged from it.


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