  Top-down holographic QCD models often work in the "probe" (or "quenched")
limit, which assumes that the number of colors is much greater than the number
of flavors. Relaxing this limit is essential to a fuller understanding of
holography and more accurate phenomenological predictions. In this work, we
focus on a mixing of glueball and meson mass eigenstates that arises from the
DBI action as a finite Nf/Nc effect. For concreteness, we work in the
Witten-Sakai-Sugimoto model, and show that this mixing must be treated in
conjunction with the backreaction of the flavor branes onto the background
geometry. Including the backreaction with the simplification that it is
"smeared out" over the compact transverse direction, we derive a corrected
effective action for the vector glueball and scalar states. Along the way, we
observe a Stueckelberg-like mechanism that restores translation invariance in
the transverse direction. We also derive a general technique, that lends itself
easily to numerics, for finding mass eigenstates of Lagrangians with
vector-scalar mixing. We then calculate the first order corrections to the mass
spectra of both the vector and scalar particles, and show that the term that
explicitly mixes vector and scalar states is the most significant correction to
the masses of low-lying scalar mesons.
