LONG BASELINE NEUTRINO EXPERIMENTS

Abstract

The evidence of neutrino oscillation in the atmospheric neutrinos discovered by Super-Kamiokande (SK) [1] is being confirmed by the first accelerator-based long baseline (LBL) oscillation experiment, K2K [2]. Also, implications of neutrino oscillation in the solar neutrino [3] is now confirmed by a reactor-based LBL experiment, KamLAND [4]. Existence of finite neutrino masses and large flavor mixing becomes almost unambiguous. They are the first observations which are contrary to the standard model. Next step in LBL experiments along this direction is to establish (or refute) the framework of 3-flavor mixing. One of the most important things for that purpose is to discover the only remaining oscillation mode νμ → νe or finite mixing angle θ13. The mixing angle is known to be much smaller than the other two mixing angle [?]. Discovery and precise measurement of θ13 would provide a key to explore the physics at high energy scale. It is also important to measure oscillation parameters precisely by checking the spectrum shape after oscillation. Deviation from the predicted oscillation pattern would imply non-standard model physics, such as extra dimension. Furthermore, firm confirmation of νμ → ντ oscillation by (1)direct observation of ντ or (2) observation of neutral current interactions are also important. This would provide a constraint on sterile neutrino. Once the finite θ13 is found, search for the CP violation in the lepton sector becomes realistic. Since the large mixing angle region is found to be the solution of the solar neutrino problem, the expected size of the CP asymmetry is within the reach of the next generation LBL experiments provided that θ13 is not extremely small. Discovery of the CP violation in the lepton sector would be a big step to understand matter anti-matter asymmetry in the universe. One or two order of magnitude higher intensity neutrino beam than current on-going experiment is necessary to achieve the purposes. Several high statistics LBL experiments with conventional horn-focused νμ beam produced by a MWclass proton accelerator are being proposed. Such attempts are recently called “superbeam” experiments, when it is contrasted from LBL experiments with neutrino factory. In my presentation, the proposed superbeam experiments and their physics potential are summarized.