Measurement of the CP violation in neutrino flavour oscillations with Hyper-Kamiokande

Long baseline neutrino oscillation experiments have a high sensitivity to the CP violation phase of the neutrino mixing matrix through the $\nu_\mu\rightarrow\nu_e$ and $\bar\nu_\mu\rightarrow\bar\nu_e$ appearance channels. For instance, the measurements of the T2K experiment in Japan, using Super-Kamiokande as a far detector, favour a close to maximal CP violation and excluded the CP conserving values at a 90% CL. However, these results are largely limited by statistics. With a fiducial volume approximately 8 times larger than the existing Super-Kamiokande, the future water Cherenkov detector Hyper-Kamiokande (HK) being built in Japan is one example of a next generation of long-baseline neutrino experiment. Thanks to a bigger far detector (8 times larger) and an upgraded beam facility (3 times more intense), using the same baseline as the current T2K experiment , HK will improve the measurements of neutrino oscillation parameters. With this increased statistics, the CP violation phase measurement will be limited by the poor understanding of systematic effects among which the neutrino interaction modelling is a major source of uncertainties. To improve the measurement of the CP violation phase, the HK collaboration is studying in depth the systematics errors that impact the measurements. At the same time it is upgrading the near detectors suite to be able to improve the knowledge of the neutrino flux and cross-section before oscillations. In particular, HK plans to measure the $\nu_e$ and $\bar\nu_e$ cross-sections (that are expected to be a major source of uncertainty for the measurement of the CP violation phase), with a new intermediate water Cherenkov detector located at 850 m from the neutrino source and an upgraded near detector (inherited from the T2K experiment ) at 280 m. Depending on the true value of the CP violation phase, the measurement could also be impacted by degeneracy with the neutrino Mass Ordering. This obstacle can be overcome using the combined analysis of accelerator and atmospheric neutrinos in HK.