Towards the cross-section measurement of the charged current muon antineutrino single pion production in the T2K near detector.

The goal of the presented analysis is the cross-section measurement of the muon antineutrino single $\pi^{-}$ production ($\bar\nu_{\mu} + N \rightarrow \mu^{+} + \pi^{-} + X$) in the T2K near detector. This interaction mode is a background in Charged Current (CC) quasi-elastic sample in T2K oscillation analysis and its modelling needs to be constrained. The measurement will be double differential in lepton kinematics: $p_{\mu}$ and cos$\theta_{\mu}$. The extraction of the cross-section will be done with a binned likelihood fit. Validations of the method, done with *mock data studies*, will be presented. The main background in the CC1$\pi^{-}$ signal sample consists of events originating from neutrino CC interactions containing 1 $\mu^{-}$ and 1 $\pi^{+}$ in the final state. The selection performance, effect of systematic errors and control samples will be discussed.