  The Witten-Sakai-Sugimoto construction of holographic QCD in terms of D4
color branes and D8 flavor branes in type IIA string theory is used to
investigate the role of topological charge in the chiral dynamics of quarks in
QCD. The QCD theta term arises from a compactified 5-dimensional Chern-Simons
term on the D4 branes. This term couples the QCD topological charge to the
Ramond-Ramond $U(1)$ gauge field of IIA string theory. The nonzero topological
susceptibility of pure-glue QCD can be attributed to the presence of D6 branes,
which constitute magnetic sources of the RR gauge field. The topological charge
of QCD is required, by an anomaly inflow argument, to coincide in space-time
with the intersection of the D6 branes and the D4 color branes. This clarifies
the relation between D6 branes and the coherent, codimension-one topological
charge membranes observed in QCD Monte Carlo calculations. Using
open-string/closed-string duality, we interpret a quark loop (represented by a
D4-D8 open string loop) in terms of closed-string exchange between color and
flavor branes. The role of the RR gauge field in quark-antiquark annihilation
processes is discussed. RR exchange in the s-channel generates a 4-quark
contact term which produces an $\eta'$ mass insertion and provides an
explanation for the observed spin-parity structure of the OZI rule. The $(\log
{\rm Det\;U})^2$ form of the $U(1)$ anomaly emerges naturally. RR exchange in
the t-channel of the $q\overline{q}$ scattering amplitude produces a Nambu-Jona
Lasinio interaction which may provide a mechanism for spontaneous breaking of
$SU(N_f)\times SU(N_f)$.

High Energy Physics - Theory;High Energy Physics - Lattice;High Energy Physics - Phenomenology