T2K Collaboration in Canada
The Standard Model (SM) of particle physics is such a successful theory that a great deal of experiemental effort is focused on finding results the theory cannot explain. The search for "physics beyond the Standard Model" has become a crusade-like quest, and it seems that physicists have seen their first glimpse of the promised land: the neutrino, long thought to be massless, actually has a tiny mass.
Neutrinos are fundamental subatomic particles, which interact with matter only via the weak-interaction. For example, they are produced when neutrons or protons decay inside a radioactive nucleus. In the SM there are three types (or flavours) of massless neutrinos, which always maintain their identity (flavour conservation). However, if neutrinos have mass, as they travel they can oscillate through quantum mechanical interference from one flavour type to another, and back again. Proving the existence of neutrino oscillations would show that neutrinos have mass, and that lepton flavour is in fact not conserved.
Such convincing evidence recently has been obtained in measurements of neutrinos produced in the sun (solar neutrinos), nuclear reactors, and in the upper atmosphere (atmospheric neutrinos) by experimental groups in Japan and at the Sudbury Neutrino observatory (SNO) in Ontario. These experiments gave the first evidence of physics beyond the SM, and the very light neutrino masses (compared to charged leptons) implied by the data indicates new physics effects at an extremely high energy scale, such as grand unification or extra dimensions of space-time.
Measuring the solar and reactor neutrino (atmospheric) oscillations has provided the coupling between first and second (second and third) flavours of neutrinos, θ12(θ23) respectively. The next step is an experiment to determine the remaining parameters describing neutrino oscillation (the so-called MNSP matrix), including the coupling between first and third flavour (θ13) and its complex CP-violating phase (∆CP), which will help shed light on the unknown very high energy scale physics.
Neutrinos are fundamental subatomic particles, which interact with matter only via the weak-interaction. For example, they are produced when neutrons or protons decay inside a radioactive nucleus. In the SM there are three types (or flavours) of massless neutrinos, which always maintain their identity (flavour conservation). However, if neutrinos have mass, as they travel they can oscillate through quantum mechanical interference from one flavour type to another, and back again. Proving the existence of neutrino oscillations would show that neutrinos have mass, and that lepton flavour is in fact not conserved.
Such convincing evidence recently has been obtained in measurements of neutrinos produced in the sun (solar neutrinos), nuclear reactors, and in the upper atmosphere (atmospheric neutrinos) by experimental groups in Japan and at the Sudbury Neutrino observatory (SNO) in Ontario. These experiments gave the first evidence of physics beyond the SM, and the very light neutrino masses (compared to charged leptons) implied by the data indicates new physics effects at an extremely high energy scale, such as grand unification or extra dimensions of space-time.
Measuring the solar and reactor neutrino (atmospheric) oscillations has provided the coupling between first and second (second and third) flavours of neutrinos, θ12(θ23) respectively. The next step is an experiment to determine the remaining parameters describing neutrino oscillation (the so-called MNSP matrix), including the coupling between first and third flavour (θ13) and its complex CP-violating phase (∆CP), which will help shed light on the unknown very high energy scale physics.
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Fig.1. |
High intensity neutrino beam will be sent from the newly constructed neutrino beamline at J-PARC facility to the Super-Kamiokande detector which is located away. |
The most promising avenue for measuring the MNSP parameters is to study neutrino oscillations using so-called "superbeams" -- high intensity neutrino beams derived from the decays of pions. The Japanese T2K experiment will send a neutrino beam from a new 1 megawatt accelerator at JPARC (Japan Proton Accelerator Research Complex) in Tokai to the world largest water Čerenkov detector hundreds of kilometres away at Super-Kamiokande (Fig.1). The project has been approved by the Japanese government and the neutrino beamline construction started in April 2004. The commissioning of the J-PARC accelerator is scheduled for 2008 and the first T2K neutrino beam is expected in the spring of 2009.
The T2K collaboration in Canada consists of scientists and students at TRIUMF, the Universities of Alberta, UBC, Carleton, Montreal, Toronto, Victoria, and York. Among the earliest collaborators, the Canadian group introduced essential design concepts, such idea of the νµ→νe oscillation analysis and narrow band off-axis beam, and contributed to the accelerator beam dynamics study, dual abort/extraction kicker design, and the combined function magnet design. Japanese T2K scientists visited TRIUMF to tap our expertise in beamline and target station design. Neutrinos are produced by a high intensity proton beam, which hits a target, creating pions, which decay subsequently producing neutrinos (π→µν). The target station, where much of the mega-watt beam energy is released, will require full remote maintenance similar to the (much smaller) ISAC target station at TRIUMF. Detailed engineering design of the target station is in progress and TRIUMF is expected to contribute to the remote handling and shielding designs. Other accelerator components like the beam damper system in the main ring, the extraction kicker magnet, and radiation hard magnets are also being considered as potential TRIUMF contributions.
The neutrino source will be monitored at three locations (the primary proton beam monitor, the muon monitor at the beam dump, and an on-axis neutrino monitor), which are essential to understand and control the neutrino beam. The Canadian group is responsible for providing a proton beam profile monitor in front of the target based on an optical transition radiation (OTR) detector, and for the muon monitor based on an array of diamond sensors. OTR detectors have been used for electron machines and recently adopted for proton machines at CERN and FNAL, and are likely the only monitors that would work in the target's extremely high radiation environment. The diamond sensor has been tested successfully at a current neutrino beam line at KEK.
Neutrino oscillations are observed by measuring neutrino flux in the far detector, Super-Kamiokande, compared with that in an off-axis near detector placed in the same direction as the far detector. The off-axis near detector will consist of fine-grained calorimeters (FGD) and time projection chambers (TPC), and an electromagnetic calorimeter, all placed inside a large dipole magnet formerly at CERN (Fig.2). The Canadian group is in charge of the three 2.4m × 2.4m × 0.9m TPCs, and of a FGD based on water-soluble scintillator. The FGD also acts as a neutrino target, with a granularity of about 1cm × 1cm required to be able to detect recoil protons from the quasi-elastic scattering process, νµn→ µ- p. Because the far detector is water, near-detector scintillator with high water content is desired. By adding a surfactant, we have achieved 70% water content with water soluble scintillator contained in 1cm × 1cm corrugated polypropylene panels. The emitted light is read out by wavelength shifting fibers, perhaps to be read out with multipixel Geiger-mode avalanche photodiodes. R&D for the scintillator and photosensor are underway. Prototypes of both the TPC and the water-soluble scintillator will be constructed and tested at TRIUMF this year.•
The neutrino source will be monitored at three locations (the primary proton beam monitor, the muon monitor at the beam dump, and an on-axis neutrino monitor), which are essential to understand and control the neutrino beam. The Canadian group is responsible for providing a proton beam profile monitor in front of the target based on an optical transition radiation (OTR) detector, and for the muon monitor based on an array of diamond sensors. OTR detectors have been used for electron machines and recently adopted for proton machines at CERN and FNAL, and are likely the only monitors that would work in the target's extremely high radiation environment. The diamond sensor has been tested successfully at a current neutrino beam line at KEK.
Neutrino oscillations are observed by measuring neutrino flux in the far detector, Super-Kamiokande, compared with that in an off-axis near detector placed in the same direction as the far detector. The off-axis near detector will consist of fine-grained calorimeters (FGD) and time projection chambers (TPC), and an electromagnetic calorimeter, all placed inside a large dipole magnet formerly at CERN (Fig.2). The Canadian group is in charge of the three 2.4m × 2.4m × 0.9m TPCs, and of a FGD based on water-soluble scintillator. The FGD also acts as a neutrino target, with a granularity of about 1cm × 1cm required to be able to detect recoil protons from the quasi-elastic scattering process, νµn→ µ- p. Because the far detector is water, near-detector scintillator with high water content is desired. By adding a surfactant, we have achieved 70% water content with water soluble scintillator contained in 1cm × 1cm corrugated polypropylene panels. The emitted light is read out by wavelength shifting fibers, perhaps to be read out with multipixel Geiger-mode avalanche photodiodes. R&D for the scintillator and photosensor are underway. Prototypes of both the TPC and the water-soluble scintillator will be constructed and tested at TRIUMF this year.•
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| Fig. 2 | Conceptual design of the off-axis near detector |
Akira Konaka
