Indication of Electron Neutrino Appearance in the T2K Experiment


June 15th, 2011: The T2K experiment in Japan announced  indications that muon neutrinos are transforming into electron neutrinos.  Using a powerful accelerator north of Tokyo, a neutrino beam was directed to the Super-Kamiokande  detector 183 miles away under the Japanese Alps in western Japan.  The neutrinos were measured near where they were born and then again at Super-K. An analysis shows that a tiny number of the neutrinos that started north of Tokyo as muon neutrinos transformed into electron neutrinos before being detected in Super-K.  Eventually it is hoped that this research will lead to an understanding of why there is more matter than anti-matter in the universe.

A graphical display of one of the electron neutrino candidate events observed in the analysis reported here is 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.

The next physics quest for Super-K is the search for  the 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 JPARC 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.

For the latest results, the accelerator made a beam pulse of neutrinos over two million times.   Six events consistent with an electron neutrino were observed, although only 1.5 events would have been seen if muon neutrinos don’t oscillate into electron neutrinos.  The probability that the observation is just due to a chance fluctuation is less than 1%. 

Duke physicists played 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 almost 15 years. Graduate student Josh Albert’s Ph.D. thesis will document the oscillation analysis published in the physics paper submitted about this result.  Graduate student Taritree Wongjirad is an expert on the outer part of the Super-K detector used to make the sample pure, and postdoc Roger Wendell is based in Japan and plays a key role in the experimental operations of Super-K.  Both Professors Scholberg and Walter act as organizers of the analysis and running of the experiment.

The T2K collaboration consists of more than 500 physicists from 62 institutes in 12 countries (Japan, South Korea, Canada, the United States, the United Kingdom, France, Spain, Italy, Switzerland, Germany, Poland, and Russia).  The experiment consists of a new 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 almost 15 years on the Super-K or T2K experiments. Current Duke University participants in the experiment also include postdoc Roger Wendell, and graduate students Josh Albert and 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.