  Techniques to compute hadron properties from lattice QCD rely upon the limit
of long time separation. For baryons, the signal-to-noise problem often
restricts one to time separations that are not ideally long, and for which
couplings to excited states can obstruct the isolation of ground-state baryon
properties. We consider excited-state contamination in nucleon two- and
three-point functions. Using chiral perturbation theory, we determine couplings
to pion-nucleon and pion-delta excited states. In two-point functions, these
contributions are small, in accordance with general properties of the spectral
weights on a torus. For the axial-current correlation function in the nucleon,
the sign of excited-state contributions suggests overestimation of the nucleon
axial charge. Thus contamination from pion-nucleon excited states will not
likely explain the trend in lattice QCD data.
