  In this work, we investigate the worldvolume fermion on the flavor brane in
the D0-D4/D8 model which is holographically equivalent to the four-dimensional
QCD with a theta term. The action for the worldvolume fermion is obtained by
the T-duality rules in string theory and we accordingly derive its effectively
five-dimensional, canonical four-dimensional forms by using the systematical
dimensional reduction and decomposition of spinor. Afterwards, we employ the
AdS/CFT dictionary in order to evaluate the two-point correlation function as
the spectral function for the worldvolume fermion and interpret the fermion as
baryon by analyzing its quantum number with the baryon vertex in holography. In
this sense, the interacted action involving the worldvolume fermion and gauge
field on the flavor brane are finally derived in holography which describes the
various interaction of meson and baryon with the theta angle in large-N limit.
Therefore, this work provides a holographic picture to describe baryon and its
interactions based on string theory, in particular, in the presence of a theta
angle.
