T2K Experiment Fully Started
 Figure 1 : Setup of the T2K experiment. |
 Figure 2 : The near detector (ND280 and INGRID). |
 Figure 3 : The first neutrino reaction event observed at INGRID. |
 Figure 4 : A neutrino reaction event observed at ND280. |
 Figure 5 : The first neutrino reaction event of the T2K experiment observed at Super-Kamiolande. |
The next-generation neutrino oscillation experiment in Japan, T2K, has now officially commenced its full-scale physics run. Previously, when introducing the T2K experiment, we reported the successful generation of a neutrino beam at J-PARC. Here, we would like to discuss the various developments that have occurred since then.
The T2K experiment involves shooting muon neutrinos (νμ) from J-PARC, located in Tokai Village, Naka District, Ibaraki Prefecture, towards Super-Kamiokande in Kamioka Town, Hida City, Gifu Prefecture. The experiment studies the phenomenon where νμ oscillates into other flavors over the 295 km distance, thereby investigating the underlying physics. Figure 1 provides a schematic showing the positions of the detectors in the experiment. As shown in the figure, the generated neutrinos are observed at two points: within the J-PARC site and at Super-Kamiokande. By thoroughly investigating the state of neutrinos before they oscillate near the accelerator and comparing this with the observations at Super-Kamiokande, the extent of oscillation can be determined. The detector within the J-PARC site is called the “near detector”, and this near detector is further divided into INGRID, located on the central axis of the neutrino beam, and ND280, positioned in line with Super-Kamiokande (Figure 2). The reason for slightly shifting the central axis of the neutrino beam from the direction of Super-Kamiokande is to narrow the energy range of the neutrinos directed towards Super-Kamiokande, but this will be discussed in detail elsewhere.
INGRID was constructed mainly by Kyoto University, with participation from Osaka City University, and the major components were installed by August 2009. INGRID has a sandwich structure with alternating layers of plastic scintillator tracking planes and iron targets. By observing the tracks of muons generated through charged-current quasi-elastic scattering (CCQE reaction: νμ + n → μ− + p) in the iron targets, INGRID monitors the neutrino beam profile, central axis position, and beam intensity. During the beam commissioning in November 2009, INGRID observed its first neutrino reaction event (Figure 3).
ND280 is positioned in line with Super-Kamiokande, with the primary purpose of measuring the energy distribution and intensity of the neutrino beam directed towards Super-Kamiokande, as well as the contamination rate of background such as νe. Therefore, it is a complex of detectors including a tracking detector and an electromagnetic calorimeter, and like INGRID, it measures CCQE reactions. ND280 was almost completed by December 2009, and currently, neutrino reactions like those shown in Figure 4 are being observed.
Finally, regarding Super-Kamiokande, located 295 km from J-PARC, this detector is a 50-kiloton water Cherenkov detector that detects neutrinos by observing the Cherenkov rings produced by high-energy charged particles generated through neutrino reactions. In February 2010, Super-Kamiokande succeeded in detecting neutrinos believed to originate from the J-PARC beam. Figure 5 shows an event where a neutrino likely caused a neutral current reaction with water, generating a π0 meson. Moving forward, we will increase the statistical data to derive physical results related to neutrino oscillations.