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.
TRIUMF AND T2K
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.
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