Neutrino oscillation measurements with long baseline experiments have entered the precision era with the discovery of electron neutrino appearance followed by the first hints of Charge-Parity violation (CPV) observed by T2K.
Long baseline experiments are composed by an accelerator producing the neutrino beam and a far detector at few hundreds of km of distance that measure neutrino spectra after the oscillations. The third critical components of any long baseline experiment is a Near Detector complex that is used to measure neutrino fluxes and cross-sections before the oscillations, reducing systematic uncertainties in the oscillation analyses.
In the case of T2K, the near detector is a magnetised off-axis Near Detector (ND280) that we employed to reduce systematic uncertainties from ~15-20% to ~5%.
ND280 has been recently upgraded with a new set of detectors consisting in a plastic scintillator high granularity detector (Super-FGD), two High-Angle Time Projection Chambers (HATPC) and six time-of-flight (TOF) planes. The upgraded near detector started taking data in December 2023 and will improve ND280 measurements by reconstructing leptons emitted at any angle with respect to the beam direction and sensibly reducing the threshold to reconstruct protons and neutrons emitted in neutrino and antineutrino interactions.
The upgraded ND280 will also be used as the near detector for the next generation long baseline experiments that is being built in Japan, Hyper-Kamiokande, that will start taking data in 2027 and, for large values of dCP, will observe CPV within 3 years from the beginning of the data taking.