The K2K experiment established the method of the acceler- ator-based long baseline neutrino oscillation experiment and successfully confirmed the neutrino oscillation phenomena discovered by natural cosmic neutrinos from earth's atmosphere and the sun. Until now, several experiments have measured all 3 neutrino mixing angles and 2 mass differences using accelerator, atmospheric, solar and reactor neutrinos.
The one of original T2K's goals has been already achieved by its initial data, that is to establish electron neutrino appearance phenomena in muon neutrino beam. This result is interpreted by nonzero θ13 - last unknown neutrino mixing angle - which was also clearly demonstrated by reactor neutrino experiments in 2012. The revised T2K's goal is to explore CP violation in the lepton sector that became theoretically and technically feasible by the established electron neutrino appearance measurement in T2K.
Another important purpose of this experiment is precise measurement of θ23 and Δm2 32 parameters. By high statistical neutrino observation, the precisions of these parameters are expected to be almost one order of magnitude better than before. So far, sin2 2θ23 is consistent with maximum (=1) from the SK, K2K and the MINOS experiments. If sin2 2θ23 is exactly unity, it may suggest an underlying new symmetry.
The intense neutrino beam is produced by using a new high intensity proton synchrotron accelerator complex (J-PARC) constructed at JAERI site in Tokai village. As a far detector to study neutrino oscillation phenomena, the T2K experiment utilizes the Super-Kamiokande (SK) detector, which is located at 295 km away from the beam production target. In designing the neutrino beam line for T2K, the idea of off-axis beam (Long Baseline Neutrino Oscillation Experiment BNL E889 proposal, (1995)) is conducted. With this method, we can produce sub-GeV energy neutrino beam with narrow energy spread efficiently from a 30 GeV proton beam. In the T2K experiment, the initial peak position of the neutrino beam energy is adjusted to ～650MeV by setting the off-axis angle to 2.5°to maximize the neutrino oscillation effects at the SK detector. The generated neutrino beam is primarily muon neutrino with a small contamination of electron neutrino, which is estimated to be 0.4% at the flux peak. The T2K neutrino beam is expected to become almost two orders of magnitude more intense compared to the K2K neutrino beam. In Super-Kamiokande, the front-end electronics were replaced in 2008 and we have achieved very stable data taking. The beam timing transfer system and Super- Kamiokande event selection by using the beam timing have been established.
The construction of the J-PARC accelerator complex for the T2K experiment was completed and physics run were started in January 2010. On February 24th 2010, we succeeded in observing the first J-PARC neutrino interaction event at Super- Kamiokande. Of the 88 neutrino events accumulated until just before the big earthquake on March 11th 2011, 6 electron neutrino candidates has been found (Figure1). The indication of this electron neutrino appearance were published in June 2011. We resumed neutrino beam data taking in January 2012 and established the electron neutrino appearance phenomena by observed 28 candidate events in the updated analysis by using data taken by 2013. This discovery means that an experimental test of CP violation is actually feasible and that the test is already started. By accumulating anti-neutrino data as well as more neutrino data, T2K aims to find out all unknown quantities of the neutrino world.