of QCD, string theory took a different direction, becoming a possible candidate for a unified theory of all the forces. Nevertheless, some string interpretation of hadron spectra was not abandoned; for example, a meson is sometimes described as a quark and an anti-quark connected by a tube of strong interaction flux \[59-60\]. This picture establishes a link between QCD and string theory, which becomes even more evident in the limit of large number of colors N \[61\]. ’t Hooft proposed that in this limit the gauge theory may have a description in terms of a tree level string theory; in particular, the leading Feynman diagrams in the 1/N expansion are planar and look like the worldsheet of a string theory. For example, a meson can be represented by two quark lines propagating in time connected by a dense “sheet” of gluons, reminding the worldsheet swept out by a string through time. In 1997 these studies found a possible new framework in the so-called AdS/CFT correspondence \[12\], a conjecture introduced by Maldacena relating a supergravity theory in ten dimensions to a supersymmetric gauge theory in four dimensions. This correspondence has been extended to a gauge theory as $SU(3)_c$ ,thus proving some 