A measurement of $\theta_{13}$ with the CNGS beam off axis and a deep underwater Cherenkov detector in the Gulf of Taranto

We propose to use the CERN Neutrino beam to Gran Sasso (CNGS) in off-axis mode for a direct measurement of $\nu_\mu \rightarrow \nu_e$ oscillations. Monochromatic neutrinos of 0.8 GeV energy are detected through quasi-elastic charged current interactions in a movable underwater Cherenkov detector of 2 megatons fiducial mass installed in a deep sea trench in the Gulf of Taranto, off the south coast of Italy. The detector can be positioned at different baselines over a range of several hundred kilometres, allowing for a direct demonstration of neutrino flavour transitions with no need for absolute flux normalisation. The experiment provides a precise measurement of the oscillation parameters $\Delta m^2_{23}$ and $\sin^2 \theta_{23}$ and will determine $\sin^2 \theta_{13}$ with a sensitivity of about 0.002.