The ND280 detector, an essential part of the T2K (Tokai to Kamioka) neutrino experiment in Japan, has needed a significant upgrade to enhance its ability to precisely measure neutrino interactions. T2K aims to improve our understanding of CP violation in the lepton sector, a phenomenon that could explain matter-antimatter asymmetry in the universe. Thanks to a more intense neutrino beam, which can produce both anti-neutrinos and neutrinos with an energy of 0.6 GeV, the researchers expect to triple the neutrinos interacting with the target. To cope with the increasing number of neutrino interactions and lowering statistical errors from currently 30 % down to 1 %, an upgrade of ND280 became essential.

With the start of its second data-taking phase in June 2024, the upgraded near detector ND280 was used for the first time, which included multiple subdetectors assembled and tested at CERN's East Area. This marks the end of an extraordinary journey that started in 2017 as an R&D project between T2K and CERN’s Neutrino Platform.

The ND280 detector comprises the Super Fine Grained detector (SFGD), which is surrounded by time-projection chambers reconstructing the particle’s passage through the detector, as well as six time-of-flight chambers. SFGD is made of 2 million scintillator cubes that allow the researchers to reconstruct the trajectories of particles coming from a neutrino interacting with the target material in a 3D view. To achieve this, each cube is equipped with three fibres to read out the signal, which turns the SFGD into an active target. One of the challenges for the team was the SFGD’s weight of about 2 tonnes as they needed to place each cube inside the holding structure.

Time-projection chambers track particles emitted at large angles and are based on a novel read-out technique using encapsulated resistive micromegas detectors. These detectors improve the measurement accuracy by spreading the electric charge of a particle over a large area on a thin resistive layer, similar to those already being used in the ATLAS experiment.

Together with six new scintillator planes for measuring the time of flight of outgoing particles surround the SFGD, the new time-projection chambers improve the rejection of background particles from outside the detector.

After all parts arrived in Japan, the detector was tested on the surface before lowering it into its final position about 50 metres below ground, which happened in the second half of 2023. “ND280 passed its first crucial test with flying colours in December 2023 as we just could hope that it performs equally well in-situ as on the surface,” says Stefano Levorato,  researcher at INFN. “It was so incredible to see the first event displays with the neutrino tracks.”