First T2K neutrino event observed at Super-Kamiokande

UK particle physicists working on the multinational T2K project, which is designed to detect some of the least understood particles in the universe, have helped track their first neutrino which has travelled 185 miles (295km) under Japan. The detection of the neutrino as it passed from the East to the West of the country means the study of the mysterious phenomenon of neutrino oscillations, which it is hoped will shed more light on the role of the neutrino in the early universe, can now begin. It could even help answer questions about why there is more matter than anti-matter in the universe.

T2K (Tokai-to-Kamioka), an international experiment led by Japan and part-funded by the UK’s Science and Technology Facilities Council (STFC), was built to help us understand with unprecedented precision more about the strange properties of the puzzling neutrino.

"Neutrinos are the elusive ghosts of particle physics, T2K spokesperson Takashi Kobayashi said."They come in three types, called electron neutrinos, muon neutrinos, and tau neutrinos, which used to be thought to be unchanging. This is a big step forward, we've been working hard for more than 10 years to make this happen."

T2K’s newly constructed neutrino beamline at the J-PARC facility in Tokai village (north of Tokyo) will now start to try to take measurements of the so-far unobserved neutrino oscillation which would cause a small fraction of the muon neutrinos produced there to become electron neutrinos by the time they reach the giant Super-Kamiokande underground detector on the other side of Japan.

“Observing the new type of oscillation would open up the prospect of comparing the oscillations of neutrinos and anti-neutrinos, which many theorists believe may be related to one of the great mysteries in fundamental physics - why is there more matter than anti-matter in the universe?” Said Professor Dave Wark of Imperial College London and STFC’s Rutherford Appleton Laboratory, who is the International Co-Spokesperson of the T2K experiment: “The observation of this first neutrino means that the hunt has just begun!”

Interacting only weakly with matter, neutrinos can traverse the entire earth with vastly less loss of intensity than light passing through a window. The very weakness of their interactions allows physicists to make what should be very accurate predictions of their behavior.

“The first measurements of the flux of neutrinos coming from the thermonuclear reactions which power our sun came as something of a shock because they were far lower than predicted”, said Professor Wark.

A second anomaly was then clearly demonstrated by Super-Kamiokande, when it showed that the flux of different types of neutrino generated within our atmosphere by cosmic ray interactions was different depending on whether the neutrinos were coming from above or below (which should not have been possible given our understanding of particle physics). Other experiments, such as KamLAND (also performed at Kamioka), the Canadian-American-UK SNO experiment, and the STFC-supported MINOS experiment, have conclusively demonstrated that these anomalies are caused by neutrino oscillations, whereby one type of neutrino turns into another.

UK scientists from 9 institutions, who are among the 508 physicists from 12 countries involved, have made a significant contribution to the experiment, producing vital hardware for both the accelerator and detectors. The UK is also playing a leading role in the analysis software for the experiment and will be fully involved in using the data to explore the properties of neutrinos.

Professor John Womersley, Director, Science Programmes at STFC said; “STFC is proud to be funding an experiment that could make such a significant contribution to our understanding of these elusive particles and indeed to what we know about the formation of the universe”.

The first initial science results from this experiment are expected within a few months, but it will be several years before any definitive answers are found.

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Notes to editors

The T2K collaboration consists of 508 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 beamline using the recently constructed 30 GeV synchrotron at the J-PARC laboratory in Tokai, Japan, a set of near detectors constructed 280m from the neutrino production target, and the Super-Kamiokande detector in western Japan.

View the complete list of institutions (PDF-external link) (link opens in a new window).

Images and captions

• Image 1: A schematic of a neutrino's journey from the neutrino beamline at J-PARC, through the near detectors (yellow dot) which are used to determine the properties of the neutrino beam, and then 295 km underneath Japan to Super-Kamiokande.
  Credit: T2K Collaboration
  
  
• Image 2: A cutaway drawing of the Super-Kamiokande Detector. The detector is a 40m diameter by 40m high cylinder filled with ultrapure water and surrounded by more than 10,000 50cm phototubes (PMTs), each sensitive enough to see a single photon.
  Credit: Kamioka Observatory, ICRR (Institute for Cosmic Ray Research), The University of Tokyo
  
  
• Image 3: The first T2K event seen in Super-Kamiokande. Each dot is a PMT which has detected light. The two circles of hits indicate that a neutrino has probably produced a particle called a p0, perfectly in time with the arrival of a pulse of neutrinos from J-PARC. Another faint circle surrounds the viewpoint of this image, showing a third particle was created by the neutrino.
  Credit: T2K Collaboration

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Further Information

T2K UK collaboration

• STFC Daresbury Laboratory
• STFC Rutherford Appleton Laboratory
• Imperial College London
• Liverpool University
• Warwick University
• Lancaster University
• Sheffield University
• Queen Mary University London
• Oxford University

UK involvement in T2K

The UK has invested £14.3M in the T2K project.

Producing a neutrino beam produces significant engineering challenges, and STFC engineers from the UK were heavily involved in surmounting those challenges. The first challenge is producing a high-power proton accelerator, which is what the KEK accelerator group have built at J-PARC with 30-GeV Main Ring proton synchrotron. The beam from that accelerator is then handed over to the T2K group to produce their neutrino beam. First the proton beam must be bent through a tight angle, focussed, and then collided with a target to produce pions, which are focussed by a set of horn magnets until a He-filled decay volume about 80 meters long and several meters square, where they decay to produce the neutrino beam. The remains of the beam are then absorbed by a graphite beam dump.

An engineering group at the STFC Rutherford Appleton Laboratory (RAL), lead by Dr. Chris Densham, designed and built the high-power window which separates the vacuum of the proton beam line from the He gas of the target station and a baffle to protect the target assembly from a mis-steered beam. They helped design the target itself (a carbon rod about a metre long in a complex titanium enclosure which must remain intact while being hit by a 750 kW proton beam, but must be small and light to avoid attenuating the pions), and they helped design the beam dump. All these UK-supplied components are in place and worked well during the weekend beam run. This project has given the UK group key experience in building components for high-power proton facilities.

The T2K experiment also needs three neutrino detectors – two near detectors which measure the neutrino properties before they leave the laboratory, and the Super Kamiokande detector in western Japan to measure the neutrinos after they oscillate. The two near detectors have slightly different roles – one, called the INGRID, measures the neutrino beam direction and profile, the other (just called the off-axis detector) measures its spectrum, composition, and interaction properties. The INGRID detector is designed to see the most neutrinos and determine the direction and profile of the neutrino beam. This detector was built by groups from Japan and France, but T2K UK collaborators have made significant contributions, providing much of the electronics and its operating software systems (called DAQ software by particle physicists). The electronics were designed, built, and tested at RAL and at Imperial College in London, while the DAQ software has been written by physicists from RAL and Oxford.

The major UK contribution to the experiment is the design and construction of the electromagnetic calorimeter for the off-axis detector, which was designed and is being built by many of the UK groups (and is partially installed). First data from this detector is expected next month. The UK is also playing a leading role in the analysis software for the experiment.

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