Super-ORCA: Measuring the leptonic CP-phase with Atmospheric Neutrinos and Beam Neutrinos

Studying the atmospheric neutrino oscillation probabilities below 2 GeV with a multi-megaton Cherenkov detector allows for a measurement of the leptonic CP-phase $\delta_{CP}$. The most relevant CP-sensitive energy range is below the neutrino detection threshold of KM3NeT/ORCA, which is an underwater Cherenkov detector optimised to determine the neutrino mass ordering by measuring the oscillation pattern of 3-30 GeV atmospheric neutrinos. With Super-ORCA, a $\sim 10$ times more-densely instrumented version of ORCA, the detection threshold can be lowered and the event reconstruction capabilities improved. In this paper, the key detector performance indicators for a possible Super-ORCA detector and the sensitivity to $\delta_{CP}$ with atmospheric neutrinos are presented. Including systematics, a 1$\sigma$-resolution on $\delta_{CP}$ of about $38^\circ$ ($23^\circ$) is achieved for $\delta_{CP}=0$ ($\delta_{CP}=\pi/2$) after 10 years. In addition, the potential of using a neutrino beam from the Protvino accelerator facility to the Super-ORCA detector is discussed. With this, a 1$\sigma$-resolution on $\delta_{CP}$ of about $10^\circ$ ($16^\circ$) is achieved for $\delta_{CP}=0$ ($\delta_{CP}=\pi/2$) after 10 years.