Technical challenges for the new T2K ND280 High Angle TPCs

Abstract The Near Detector of the T2K experiment at J-PARC has recently been upgraded in order to reduce the present systematic uncertainties affecting the oscillation parameters measurements and to exploit the increased neutrino beam power of the J-PARC complex. One of the major improvements to the T2K ND280 detector consisted in the integration of two large size (∼ 3m 3 each) new horizontal High-Angle Time-Projection Chambers (HATPC). The new HATPCs are based on a gaseous active volume contained in a Field Cage made of lightweight composite material, combining optimal mechanical and electrical properties with minimal radiation length and dead volume. The readout is performed by innovative Resistive Micromegas modules featuring a resistive layer for charge spreading on top of the readout plane to enhance spatial resolution performance. The detector technologies have been studied in test beams and cosmic ray campaigns. After the installation at J-PARC in Fall 2023, commissioning periods of data taking with cosmics and then with a neutrino beam have been performed. The details about the detector concepts, the design, and construction methods are presented with the technological challenges, and the solution adopted to cope with the challenging requirements of the upgrade. Furthermore, the results of the characterization and commissioning performance of the HA-TPCs at CERN and J-Parc will be illustrated, including the first results using beam neutrino interactions.