Neutrino physics in Japan
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For more than a decade the IRL teams have been actively involved in
the very successful Japan-based neutrino program. Reinforcing the existing
collaborations inside the T2K and Super-Kamiokande (SK) experiments
and developing new joint fields of research for the recently approved
Hyper-Kamiokande (HK) project will be the goal of this program.

The on-going T2K experiment has already achieved important physics results,
in particular via the discovery of neutrino oscillation in appearance mode
and via the first hints of CP-violation in the lepton sector. On the hardware
side, the IRL teams are now focused on the near detector (ND280) upgrade
for the second phase of the T2K project aiming at detailed measurements
of neutrino interactions and further studies of CP-violation at the 3-sigma
sensitivity level. The IN2P3 and UoT physicists are playing a key coordination
role in the ND280-upgrade project. On the analysis side, the IRL teams
are leading the development of the very first joint analysis between
atmospheric and accelerator-produced neutrinos. This result will not only
pave the way for every future HK analyses, but will also provide
the world-leading sensitivity to both CP violation and to the neutrino mass
hierarchy.

Super-Kamiokande, the largest water Cherenkov detector ever built so far,
is about to start a new phase of its history where the water of the tank
will be doped with a water-soluble chemical compound of Gadolinium,
allowing an efficient tagging of the neutron produced by the inverse beta
decay. This upgrade should open a new era in the search for the Diffuse
Supernova Neutrino Background (DSNB) produced by the supernovae which
occurred since the beginning of the Universe. The IRL team participates
actively in this fascinating search with a high potential of discovery,
and having an impact on cosmology.

The next-generation neutrino experiment which has recently been approved
in Japan is the HK project. It requires a huge common effort of a large
international collaboration. One of the main physics goals of the experiment
is the discovery of CP violation in the lepton sector, but it will also be
the most sensitive detector to proton decay as well as a unique observatory
for neutrinos from astrophysical sources, such as supernovae neutrinos.
The HK detector will be the largest (8 times larger than the existing SK)
underground water Cherenkov detector with a 68 m diameter and 72 m height
cylindrical tank, equipped with up to 50,000 photo-sensors. The IRL teams
have strongly contributed to the HK sensitivity studies in both the low
and high energy domains, including different photo-sensor configurations.
They have especially demonstrated that introducing the same photo-sensors
that will be used for the future Intermediate Detector (multi-PMTs) into
the HK detector will boost the detector capabilities in both energy sectors.
The IN2P3 and UoT teams are now working very closely to demonstrate
the potential and feasibility of these multi-PMT photo-sensors, using
complementary test setups located at the UoT and APC-Paris (MEMPHYNO).

Timing synchronization of each PMT signal is crucial for a precise event
reconstruction in HK. Moreover, the association between the local time base
and the Coordinated Universal Time (UTC) is fundamental to synchronize
the data acquisition with the beam sent from the J-PARC accelerator and
to correlate observed astrophysical events with other detectors world-wide
via the SNEWS network. Physicists from IN2P3 and University of Tokyo
have already established a strong collaboration and are co-responsible
for both of these tasks. The IRL will support extending these to other key
activities, including joint analysis efforts.