  Excited state contamination is one of the most challenging sources of
systematics to tackle in the determination of nucleon matrix elements and form
factors. The signal-to-noise problem prevents one from considering large
source-sink time separations for the three-point functions to ensure ground
state dominance. Instead, relevant analyses consider multi-state fits. Excited
state contributions are particularly significant in the axial channel. In this
work, we confront the problem directly. Since the major source of contamination
is understood to be related to pion production, we consider three-point
correlation functions with a nucleon operator at the source and a nucleon-pion
interpolating operator at the sink, which allows studies of $N \to N\pi$ matrix
elements. We discuss the construction of these three-point correlation
functions and we solve the generalized eigenvalue problem (GEVP) using
different sets of nucleon and nucleon-pion interpolators. The analysis is
performed on the CLS ensemble A653 with $m_\pi \approx 420$ MeV. Results were
generated with valence quark masses corresponding to $m_\pi \approx 1750$ MeV
and $m_\pi \approx 420$ MeV.
