New results from T2K conclusively show muon neutrinos transform to electron neutrinos


July 19th, 2013: Today at the European Physical Society meeting in Stockholm, the international T2K collaboration announced definitive observation of muon neutrino to electron neutrino transformation. In 2011, the collaboration announced the first indication of this process, a new type of neutrino oscillation.  At that time, there was less than a 1% chance that the result could have been due to a statistical fluctuation. Today, with 3.5 times more data, this transformation is firmly established. The probability that a random statistical fluctuations alone would produce the observed excess of electron neutrinos is less than one in a trillion.  This T2K observation is the first of its kind in that an explicit appearance of a unique flavor of neutrino at a detection point is unequivocally observed from a different flavor of neutrino at its production point.  The T2K experiment expects to collect 10 times more data in the near future, including data with antineutrino beam for studies of CP violation in neutrinos.

A graphical display of some of the electron neutrino candidate events observed in the analysis reported here are shown at the bottom of the page.

You can read the official T2K press release here

                                    The T2K Experiment

The Super-Kamiokande experiment is a giant underground water Cherenkov experiment in Mozumi, Japan, designed to capture neutrinos from the Sun and sky: the 11,000 inner detector photomultiplier tubes (PMTs) record photons from the charged products of neutrino interactions in the ultra-pure water.  In 1998, Super-K showed that muon neutrinos produced by cosmic ray collisions in the Earth's atmosphere "disappear" by changing to almost-invisible tau flavor: the neutrinos "oscillate" from one flavor to another by interference of mass states.  Such flavor change is only possible if neutrinos have mass.  Neutrino masses and the parameters which govern neutrino flavor oscillation are deeply connected to both fundamental particle physics and cosmology.

Over the next few years, the Super-K atmospheric neutrino result was confirmed by other experiments.  The beam neutrinos "went missing" in exactly the numbers expected, and with exactly the expected energy dependence predicted by the oscillation hypothesis. Next began the search for the then unknown neutrino oscillation parameter, "θ13" as part of the T2K experiment.  The signature of non-zero θ13 is a tiny amount of electron neutrino appearance in a beam of muon neutrinos.  The T2K experiment is designed to measure this parameter by looking for muon neutrinos produced in an accelerator at the J-PARC center north of Tokyo to transform into electron neutrinos after they travel 295 km across Japan.  Since this is a very small effect, a powerful beam is needed to create just a few of these events. This type of transformation had never before been observed.

In 2011, T2K reported that six events consistent with an electron neutrino were observed, although only 1.5 events would have been seen if muon neutrinos didn’t oscillate into electron neutrinos.  The probability that the observation was due to chance fluctuation was less than 1%.  Today’s result uses 3.5 times more data and refined analysis techniques to definitively state that these transformations are taking place. 28 events were observed on an expected background of 4.6 events.  The chances that this is due to a statistical fluctuation is less than one in a trillion.  Stated in language of statistics, a statistical fluctuation is ruled out at the 7.5 sigma level; this is above the 5 sigma threshold particle physicists use to claim discoveries.

Duke physicists play important roles in the running and analysis of the T2K experiment.  Duke faculty Kate Scholberg and Chris Walter lead a team of postdocs and graduate students on the project.  Their work is based at the Super-K detector and they have worked on maintaining, building and running it for over 15 years. Former graduate student Josh Albert’s Ph.D. thesis documented the oscillation analysis published in the physics paper describing the 2011 results.  Graduate student Taritree Wongjirad is an expert on the outer part of the Super-K detector used to make the sample pure, and postdocs Alex Himmel and Tarek Akiri played important parts in the production and analysis of the data and simulations used in the analysis from the Super-K detector. Former postdoc Roger Wendell is now a professor at the University of Tokyo and is based at the Super-K experiment.  Both Professors Scholberg and Walter act as organizers of the analysis and running of the experiment.

The T2K collaboration consists of more than 400 physicists from 59 institutes in 11  countries (Japan,  Canada, the United States, the United Kingdom, France, Spain, Italy, Switzerland, Germany, Poland, and Russia).  The experiment consists of a neutrino beam-line using the recently constructed 30 GeV synchrotron at the J-PARC laboratory in Tokai, Japan, a set of near detectors constructed 280 m from the neutrino production target, and the Super-Kamiokande detector in western Japan.

After traveling 295 km underneath Japan, a neutrino interacted with the giant Super-K detector, and was recorded by its light detectors.

Duke personnel are seen suspended in Super-K, surrounded by photo-multiplier tubes which detect the light produced by particles traveling through the detector when full of water.

A candidate electron-neutrino event in the Super-Kamiokande detector.

The picture on the left shows a candidate electron-neutrino event in the Super-Kamiokande detector.  This display is an unrolled view of the  detector with each one of the colored dots representing a electronic detector which has been hit by the light created by the electron.

Duke Professors Kate Scholberg and  Chris Walter have worked for over 15 years on the Super-K or T2K experiments. Current Duke University participants in the T2K experiment also include postdocs Alex Himmel and Tarek Akiri, along with graduate student Taritree Wongjirad.

Duke postdocs and graduate students working on Super-K during the upgrade for T2K.

Research funding for the Duke neutrino group is supplied by the US Department of Energy Office of Science, and the National Science Foundation.

A 3D view of a  second candidate electron-neutrino event in the Super-Kamiokande detector.

The picture on the left shows a candidate electron-neutrino event in the Super-Kamiokande detector.  This display is an 3-D view of the  detector with each one of the colored dots representing a electronic detector which has been hit by the light created by the electron.This event was the first electron neutrino candidate seen after T2K restarted operations following the great Japan Earthquake.