## Chapter 9 

## Conclusions 

QCD is far from being fully understood. The non-perturbative aspects of the theory are particularly intriguing: confinement, asymptotic freedom, chiral symmetry breaking, the hadron spectrum, hadronization, and the QCD phase diagram all offer exciting research opportunities. The AdS/CFT correspondence has proved to be a valuable tool in our understanding of strongly-coupled gauge theories, and the related AdS/QCD models have reproduced many aspects of QCD very successfully. 

In chapter 1 we described some of the more beguiling aspects of QCD, and reviewed the very basic fundamentals of string theory. In chapter 2 we introduced the AdS/CFT correspondence, in particular explaining how every operator in the field theory can be uniquely matched with a field on the gravity side. We used chapter 3 to show how to include flavour into the correspondence, via a D7 brane probe. This gave a relationship between the quark mass and quark condensate, and we showed that in this geometry, chiral symmetry breaking could only be induced by massive quarks: the vacuum does not spontaneously break the chiral symmetry. We went on to calculate the (scalar) meson spectrum in this set-up by looking at the scalar fields on the surface of the D7 brane. We also showed that the vector meson 

