T2K: Tokai to Kamioka

T2K (Tokai to Kamioka) is the first experiment to artificially generate neutrinos and explore how they change flavour as they travel, a critical area of research in the search for beyond-Standard Model physics.

The Japan-based T2K is one of TRIUMF’s major international collaborations, with TRIUMF scientists, engineers and technicians providing expertise and material contributions from conception, through design, construction, operation, data collection and data analysis.

Neutrinos are elementary particles that come in three flavours (electron, muon, and tau) and interact only very slightly with other matter. T2K builds on the 2015 Nobel Prize-winning discovery that neutrinos change flavor, or oscillate, on their journey from the Sun to the Earth, for example an electron neutrino becoming a muon neutrino. The discovery and prize were shared by the team running Japan’s Super-Kamiokande (Super-K) detector, the “K” in T2K.

In T2K, an intense beam of muon neutrinos is created at the Japan Proton Accelerator Research Complex (JPARC) on Japan’s east coast and sent 295 km through the earth to Super-K in western Japan. Before it leaves J-PARC, the neutrino beam is characterized using the Near Detector 280 (ND280), located 280 meters from the beam’s source. The ND280 and Super-K enable a before and after oscillation measurement of the muon neutrino beam.

Since initial beam in 2010, T2K has provided the first experimental evidence that muon neutrinos oscillate into other flavours and continues to characterize this rate with the highest precision. The experiment has also discovered that muon and anti-muon neutrinos appear to oscillate at different rates, a finding that points to a violation of CP-symmetry and may help explain the dominance of matter over antimatter in the universe.

The T2K international collaboration involves approximately 500 scientists and engineers from 63 institutions in 11 countries. The Canada T2K group involves TRIUMF and the University of Alberta, University of British Columbia, University of Regina, University of Toronto, University of Victoria, University of Winnipeg and York University.

TRIUMF and Canadian university collaborators are currently participating in the proposal and detailed planning for a successor to the T2K experiment the Hyper-Kamiokande, a neutrino detector 20-times the volume of Super-K. Hyper-Kamiokande will provide greater sensitivity and remove key uncertainties from current measurements, as well as searching for supernova neutrinos and proton decay.

T2K consists of three main components: the muon neutrino beam; the Near Detector 280; and the Super-K detector.

Making the muon neutrino beam

To create T2K’s intense muon neutrino beam, a high-intensity proton beam from the J-PARC synchrotron is fired at a 90-centimeter-long target of graphite, a form of pure carbon more commonly used as pencil lead.

Collisions between the beam protons and carbon nucleons produces pions, particles with a half-life of just 26 billionths-of-a-second consisting of two quarks. The pions travel through an approximately 110-meter-long transition beamline, the “decay pipe” where they decay to muons and muon neutrinos. The muons and any remaining protons and pions are stopped in a second layer of graphite, while the muon neutrinos pass through this layer on their way to the ND280 and Super-K detectors. In 2014, T2K began also producing a muon-antineutrino beam in order to compare antineutrino oscillations with neutrino oscillations.

The Near Detector (ND280)

ND280 determines the muon neutrino beam’s initial energy and flux in a way that’s analogous to reconstructing a two-vehicle collision by examining the location of the debris, except in this case scientists observe the particle debris from a neutrino’s collisions with a neutron.

To accomplish this, ND280 is an 8-by-8 m, cube-shaped, detector consisting of a powerful magnet surrounding two core parts: a TRIUMF-built Tracking detector and a Pi-Zero detector.

The Tracker itself consists of two different kinds of detectors: three gas-filled Time Project Chambers (TPC) surrounding two solid Fine-Grained Detectors (FGD). Some of the muon neutrinos passing through ND280 collide head-on with a neutron in the FGD creating a proton and a negatively charged muon, a short-lived heavier version of an electron. By reconstructing the proton and muon trajectories from neutrino collisions, T2K scientists are able to identify a muon neutrinos energy.

The FGD is made from 10,000 one-by-one centimeter-square and two-meters-long polystyrene scintillator bars. A scintillator is a material that produces light when it interacts with a charged particle. Protons produced by neutrino collisions hit the scintillator and produce flashes of light detected by surrounding photon counters, enabling the T2K scientists to pinpoint the locations of neutrino collision.

The TPC’s play a similar tracking function but using gas. Each TPC is filled with a gaseous mix of argon, isobutane and carbon fluoride. One side of the gas chamber positively charged the other negatively charged. Muons produced from neutrino-neutron collisions travel through the gas, ionizing it and creating free electrons which are attracted to the positive side of the chamber triggering an electrical signal. The signal enables T2K scientists to identify the muon’s trajectory and energy.

The Pi-Zero component monitors the neutral pion backgrounds.

Most of the neutrinos pass through the ND280 without interacting, and these travel at near the speed of light to Super Kamiokande (Super K).

Super Kamiokande Detector

Located one kilometer underground in the Kamioka mine, Hida city, Gifu, Japan, Super K is the world’s largest neutrino detector. The detector is a huge barrel-shaped stainless-steel tank, 39.3m diameter and 41.4m tall, filled with 50,000 tons of ultra-pure water. The inner walls of the tank are lined with more than 13,000 dinner plate-sized photomultipliers which detect the faint light from the interaction of a neutrino and nucleons or electrons in the water.

When a neutrino hits a nucleon or an electron in Super-K’s water, it generates an energized charged particle, usually a muon or electron. This charged particle has a higher velocity in water than the speed of light in water resulting in the emission of Cherenkov light, a special kind of electromagnetic radiation. The Cherenkov light is emitted outwards in the charged particle’s direction of travel in a cone shape. The photomultiplier tubes lining Super-K’s interior detect this Cherenkov light, enabling T2K scientists to use the Cherenkov light’s energy, direction and shape to reconstruct the collision event and identify the type of neutrino involved. For examples, muons produce a sharp ring of Cherenkov light while electrons produce a more diffuse ring.

The 1000 meters of rock overburden covering Super-K reduces the cosmic ray muon background to 1/100,000 of that at the surface. Nonetheless, most of the millions of events recorded at Super-K are caused by neutrinos from the Sun or the Earth’s atmosphere. Only a few hundred neutrino interactions per year are due to beam neutrinos traveling from J-PARC.

From the initial discussions that launched the T2K collaboration in 2001, TRIUMF has been extensively involved in the design, construction, and operation of the experiment.

The Near Detector (ND280)

TRIUMF contributed the two core ND280 detector components: the Time Projection Chambers (TPCs) and the Fine-Grained Detectors, and has operated and maintained these components from the start. (TRIUMF is currently transferring these roles to other members of the collaboration.) These detectors have provided the primary data for the T2K neutrino oscillation analyses from ND280.

The Fine-Grained Detector scintillators were extruded at a facility in Richmond, B.C. and TRIUMF scientists and engineers adapted a titanium-oxide coating technology developed at the U.S. Department of Energy’s FermiLab to light-shield the scintillators, which were assembled in a TRIUMF clean room. TRIUMF also provided several pieces of common ND280 infrastructure, including the global slow controls system which monitors the status of all detector electronics and power supplies.

All TRIUMF-built components were constructed and tested at TRIUMF and shipped to the T2K experiment. ND280 was built by collaborators from 10 countries, including six Canadian participants: TRIUMF, the University of Regina, University of Toronto, York University, University of Victoria and the University of British Columbia.

Design and Data Analysis

One of TRIUMF’s key design contributions to T2K is the use of a first of its kind off-axis neutrino beamline to provide a narrower neutrino energy spectrum than an on-axis beam. The neutrino beamline is located at 2.5 degrees off centre at both J-PARC and Super-K. This provides muon neutrinos with energy peaked at the neutrino oscillation maximum in order to increase the number of possible observed oscillations.

TRIUMF and Canada T2K scientists also developed a new Super-K event reconstruction algorithm, the computational model that turns the detector’s raw data into physics results. The algorithm significantly reduces systemic uncertainties, including increasing background rejections.

TRIUMF also hosted one of the two T2K Tier-1 data storage facilities, acting as a critical back-up to data storage at T2K, and providing additional computing resources for data analysis. At its peak, TRIUMF hosted one of the largest analysis groups in the T2K collaboration and TRIUMF staff act as conveners in various aspects of the T2K collaboration.

Target Station Hot Cell Remote Handling Facility

The T2K target station’s unique TRIUMF-designed remote handling system is based on the TRIUMF ISAC Target Hall remote handling system’s design. TRIUMF’s remote handling group also built a hot cell for remotely exchanging the target and a beam monitor system for the final focus beam section upstream of the T2K target.

Servicing the target’s focusing magnetic horn and target itself is done by moving the module to the hot cell. The remote handling system was essential following the Great East Japan earthquake near J-PARC in 2011, at which time TRIUMF remote handling experts traveled to J-PARC to remotely assess possible damage to the horn focusing magnet and target set-up.

Optical Transition Radiation Monitor

In collaboration with the University of Toronto and York University, TRIUMF scientists and engineers built a unique Optical Transition Radiation (OTR) monitor to observe the profile of the proton beam at the target in an extremely high-radiation environment in which most materials melt.

Monitoring the exact angle at which the proton beam hits the target is critical to T2K’s success since it, in turn, determines the off-axis neutrino beam’s position. The proton beam must be accurately positioned to within five-one-hundredths of a degree.

The OTR monitor involves a thin titanium foil which the proton beam passes through producing transition radiation, a form of visible light. The OTR uses a series of four parabolic mirrors to carry the image from the titanium foil through the target-shielding module to a radiation-hard camera. The Canadian T2K group operates and maintains the OTR monitor.

T2K Photosensor Facility at TRIUMF

TRIUMF-T2K scientists have created a unique photomultiplier testing facility at TRIUMF to improve the accuracy of data taken by Super-K and to prepare for the Hyper-K facility.

The facility is evaluating the angular dependence of the photomultiplier tubes that line the Super-K tank and detect neutrino collisions via Cherenkov light. Does the photomultiplier’s detection sensitivity vary with the angle and direction from which a photon arrives? At present, this factor is a significant uncertainty in the T2K and SuperK experiments.

The photosensor facility contains two computer-controlled gantry arms that aim laser light at different angles and positions at the photomultiplier tubes and monitors the reflected light. The whole system is enclosed by an electromagnet to provide a controlled, uniform magnetic field and is also temperature controlled to +/- 1 degree Celsius, to characterize the temperature dependence of the laser and optical fibres.

The TRIUMF group has discovered that the detector signal varies significantly with the photon angle and is working to characterize its response.