D3 D7 r$_{5}$ r$_{6}$ Figure 3.1: If we place the D7 brane(s) at any non-zero distance from the D3 branes, we break the conformal symmetry The exact embedding of N$_{f}$ D7-branes has been chosen so that the directions of r$_{5}$ and r$_{6}$ are orthogonal to the D7-branes’ world-volume (table 3.1). There is now an N = 2 chiral supermultiplet connecting the D3 and D7 brane stack, and this interacts with the N = 4 supermultiplet on the surface of the D3 branes. How to know which symmetries are preserved and which are broken in some setup of Dp and Dp’ branes is not an obvious one, and involves some detailed calculations [81]. In this particular case, the N = 4 supermultiplet is broken completely by the presence of the D7 brane, leaving just the N = 2 supermultiplet. When the D7 brane is separated from the D3 stack, the scalar associated with the r$_{5}$ and r$_{6}$ directions gains a VEV and gives a mass to the quarks: However when when we have separated the D3-D7 system, there is still a U(1) symmetry because of our freedom to re-parameterise the r$_{5}$ − r$_{6}$ coordinates. This U(1) symmetry corresponds to the axial symmetry of a single-flavour QCD model. As explained in section 1.2.8, in QCD this sym- metry is spontaneously broken by the chiral condensate 〈 ¯ qq 〉 . We don’t see 47 ¯ Qλ$_{56}$Q −→ m ¯ QQ as λ$_{56}$ acquires a VEV (3.2)