Hyper-Kamiokande, the next generation of water Cherenkov neutrino observatory, will open fascinating new window on the universe, including the search for Grand Unified Theory through proton decay, neutrino astronomy, and determination of the CP violation in the lepton sector. In the first part of this seminar, we will present some questions remaining in these various physics domains, as well as the answers we may be able to provide with Hyper-Kamiokande.

The standard neutrino oscillation parametrization is defined by six independent parameters: three mixing angles (θ23, θ12 and θ13), two squared mass differences and a CP violation phase (δcp). By 2010, only θ13 and δcp were still unknown. Despite the non-zero measured values of θ12 and θ23, a non zero value of θ13 is also necessary to open the window to explore CP violation on the lepton sector. In 2011, T2K (accelerator based) and Double Chooz (reactor based) experiments showed an evidence of non-zero θ13.

T2K (Tokai to Kamioka) is a long-baseline neutrino oscillation experiment designed to measure (anti-)neutrino flavour oscillations using an off-axis (anti-)neutrino beam. After the measurement of a non-zero value for the last unknown mixing angle, theta13, and the most precise measurement of  theta23, T2K has started a campaign to measure CP violation in the leptonic sector taking data with both neutrino and an anti-neutrino beam.

I discuss perspectives for modern multi-messenger astroparticle physics
including cosmic rays and neutrinos from the theory point of view.
Examples are the cosmogenic neutrino flux expected from cosmic ray
observations, the possible common origin of the neutrinos and cosmic rays at the
highest energies from the same sources, and the implications
of the neutrinos observed from blazar TXS 0506+056.