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Recently, we proposed a holographic description of 4 dim QCD by
using a D4/D8-brane system in type IIA string theory. (hep-th/
0412141 with T. Sakai) It has been shown that the model nicely
catches a lot of features of QCD and hadron physics. For example,
the chiral symmetry breaking in QCD is easily understood from the
geometry of the background. Mesons (such as pions, rho mesons, a1
mesons) are beautifully unified in a five dimensional gauge field.
We can explicitly calculate the effective action of these mesons
and it is shown that the structure of the interaction is consistent
with various phenomenological models. Furthermore, the masses and
couplings are roughly in good agreement with the experimental
values, although the approximation used in our analysis is still
crude. Baryons are constructed as instantons in a five dimensional
gauge theory and the baryon spectrum is obtained by considering a
quantum mechanics on the instanton moduli space. (Refs: hep-th/
0412141, 0507073, 0701280)
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