C2GT: intercepting CERN neutrinos to Gran Sasso in the Gulf of Taranto to measure $\theta_13$

Today's greatest challenge in accelerator-based neutrino physics is to measure the mixing angle \thonethree\ which is known to be much smaller than the solar mixing angle \thonetwo\ and the atmospheric mixing angle \thtwothree . A non-zero value of the angle \thonethree\ is a prerequisite for observing CP violation in neutrino mixing. In this paper, we discuss a deep-sea neutrino experiment with 1.5~Mt fiducial target mass in the Gulf of Taranto with the prime objective of measuring \thonethree. The detector is exposed to the CERN neutrino beam to Gran Sasso in off-axis geometry. Monochromatic muon-neutrinos of $\approx 800$~MeV energy are the dominant beam component. Neutrinos are detected through quasi-elastic, charged-current reactions in sea water; electrons and muons are detected in a large-surface, ring-imaging Cherenkov detector. The profile of the seabed in the Gulf of Taranto allows for a moveable experiment at variable distances from CERN, starting at 1100 km. From the oscillatory pattern of the disappearance of muon-neutrinos, the experiment will measure \sthtwothree\ and especially \dmtwothree\ with high precision. The appearance of electron-neutrinos will be observed with a sensitivity to $P(\numu \rightarrow \nue$) as small as 0.0035 (90\% CL) and $\sin^2\thonethree$ as small as 0.0019 (90\% CL; for a CP phase angle $\delta = 0^\circ$ and for normal neutrino mass hierarchy).