How does AdS/CFT help us understand non-perturbative aspects of QCD?

Question

I've heard AdS/CFT has found applications in many areas of physics where nonperturbative aspects leave us crippled in making any simple calculations.

Among these applications, I also have heard that this method also applies to QCD.

My question as someone who has not studied the duality formally yet, is how this is possible, given that all instances of AdS/CFT are supersymmetric, while QCD is not supersymmetric or at least up to now the symmetry breaking from ${\cal N}=1$ to ${\cal N}=0$ is not yet understood well within the framework of Witten-Seiberg theory, and confinement is only proven in the case of ${\cal N}=2$ to ${\cal N}=1$ SUSY breaking.

Can anyone elucidate the general ideas behind AdS/QCD and why it has numerical predictions in simple terms?

My Knowledge is good enough to understand Witten-Seiberg theory formally but have not studied super gravity or string theory, and just know the general ideas.

Answer 1

AdS/QCD was never developed in any kind of exact form, although perhaps the idea is sound. Roughly speaking the main problem is that QCD at small scales approaches a free theory, but a weakly coupled gauge theory corresponds to a highly quantum AdS theory for which the semiclassical approximation breaks down. To do actual computations people take a UV cutoff which is not much higher than $\Lambda_{QCD}$ and this means that the calculated particle spectrum has all sorts of artifacts which are not present in real QCD.

As far as supersymmetry is concerned, the main idea underlying AdS/QCD and Witten's model of holographic Yang-Mills (arXiv:hep-th/9803131, section 4) is that there is some compactified direction on the AdS side with thermal boundary conditions (fermions are antiperiodic) and this breaks supersymmetry explicitly.