Performance of the High-Angle Time Projection Chambers in the T2K Near Detector Upgrade

The T2K experiment in Japan is a long baseline neutrino oscillation experiment that aims to probe CP violation in the leptonic sector. To reduce dominant interaction-model systematics and improve acceptance for high-angle, low-momentum particles, the ND280 near detector was recently upgraded with several new subdetectors, including two High-Angle Time Projection Chambers (HA-TPCs) instrumented with Encapsulated Resistive Anode Micromegas (ERAM) readout and a lightweight composite field cage. The HA-TPCs were installed in Autumn 2023 and Spring 2024 and commissioned with cosmic rays and beam. Since June 2024 they have operated as part of the fully upgraded ND280. This proceeding reports initial HA-TPC performance in cosmic and beam data using the common ND280 reconstruction, with comparisons to Monte Carlo simulations. Key results include spatial, momentum, and d𝐸/d𝑥 resolutions consistent with design expectations, and drift-direction studies that reveal the anticipated field non-uniformities near detector boundaries. The upgraded configuration delivers 4𝜋 angular coverage and lower tracking thresholds (to ∼ 300 MeV/c), meeting ND280 design targets and strengthening constraints on neutrino-nucleus interaction systematics for T2K. Looking ahead, the upgraded ND280, including the HA-TPCs discussed here, is expected to serve in the Hyper-Kamiokande near detector program, delivering high-precision neutrino-nucleus constraints for next-generation long baseline neutrino oscillation experiments.