The T2K Experiment

Abstract

Over the last dozen years neutrino physics has crossed a threshold in its basic understanding of the fundamental nature of neutrinos. The first compelling demonstration that neutrinos have mass came when the Super-Kamiokande experiment showed a zenith-angle dependent suppression of muon neutrinos arising from cosmic ray interactions with the atmosphere. That had already been suspected, because initial indications of an anomaly in the flux of neutrinos from the sun from the pioneering experiments of Ray Davis and his collaborators and from the Kamiokande experiment, supported by measurements of the low-energy solar neutrino fluxes by the SAGE and GALLEX experiments, were hard to explain without invoking new neutrino physics. The SNO experiment then demonstrated that the shortfall of solar neutrino was caused by neutrino coming from the sun changing flavor, further pointing to the existence of neutrino oscillations. This was confirmed by the observation (or rather non-observation!) of the disappearance of reactor neutrinos by the KamLAND experiment and of accelerator neutrinos by the K2K long-baseline neutrino oscillation experiment (subsequently confirmed by the MINOS experiment). The combination of all these experimental results clearly demonstrates the existence of neutrino oscillations (the first confirmed physics beyond the Standard Model of particle physics).