The long-baseline neutrino experiment Tokai-to-Kamiokande (T2K) is in Japan and measures neutrino oscillation parameters. The muon neutrino charged current interactions in the near detector (ND280) are used to predict the far detector's event rate, particularly constraining the neutrino flux and neutrino-nucleus interaction cross sections, the dominant systematic uncertainties in the oscillation analysis.

We present a study of charged current interactions on carbon with a muon and a single positively charged pion in the final state (CC1$\pi^+$) at the T2K off-axis near detector with a 4$\pi$ solid angle acceptance. This channel constitutes the main background for the muon neutrino disappearance measurement when we do not observe the charged pion in the Super-Kamiokande water Cherenkov detector. A precise understanding of it is relevant for all current and planned neutrino oscillation experiments. Single positive pion production is primarily sensitive to resonant processes, non-resonant contributions, and coherent pion production. Additionally, final-state interactions in the nuclear target have to be considered.

A fascinating characterization of CC1$\pi^+$ interactions through the measurement of Adler Angles is presented. These observables carry information about the polarization of the $\Delta$ resonance and the interference with the non-resonant single pion production.