【文章內容簡介】 mesons and Y shape baryons under the influence of symmetric and asymmetric disturbance. We also performed a parturbative analysis The conclusion from both the simulations and the perturbative analysis is that indeed the Y shape string configuration is unstable to asymmetric deformations. Thus an excited baryon is an unbalanced single string with a quark on one side and a diquark and the baryonic vertex on the other side. Baryons in confining SUGRA backgrounds Holographic baryons have to include a baryonic vertex embedded in a gravity background ``dual” to the YM theory with flavor branes that admit chiral symmetry breaking A suitable candidate is the Sakai Sugimoto model which is based on the incorporation of D8 anti D8 branes in Witten?s model Witten’s model a prototype of confining model A way to get a confining background is to cut the radial direction and introduce a scale. One approach is indeed to cut by hand an Ads space. This is not a solution of the SUGRA equations of motion. People use it to examine phenomenological properties (AdsQCD) The approach of Witten was to pactify one coordinate of D3 (D4) brane background with a “cigarlike” solution. UL R One imposes antiperiodic boundary conditions on fermions. This kills supersymmetry. In the dual gauge theory the gauginos and the scalars acquire a mass ~1/R and hence in the small R limit they decouple and we are left only with the gauge fields. For a Dp brane, in the small R limit we loose one space dimension and we end up with a pure gauge theory in p1 space dimensions. The gravity theory associated with D3 branes namely the AdS5xS5 case pactified on a circle is dual to a pure YM theory in 3d ( with KK contamiation) The same mechanism for near extremal D4 branes yields a dual theory of pure YM in 4d. D4 D4 R ?The gauge theory and sugra parameters are related via 5d coupling 4d coupling glueball mass String tension ?The gravity picture is valid only provided that l5 R ?At energies E 1/R the theory is effectively 4d. ?However it is not really QCD since Mgb ~ MKK ?In the opposite limit of l5 ?? R we approach QCD To add fundamental quarks one adds flavor branes. Lets go for a moment from the SUGRA background back to the brane configuration. If we add to the original stack of Nc D3 ( or D4 ) branes another set of Nf Dp branes there will be strings connecting the D3 (D4) and Dp branes. These strings map in the dual field theory to bifundamental “quarks” that transform as the(Nc, Nf) representation of the gauge symmetry U(Nc)xU(Nf) For Nc Nf the U(Nf) can be treated as a global symmetry and hence we get fundamental quarks. Coming back to the SUGRA background, in the case of Nc Nf we can safely neglect the backreaction of the additional branes on the background. Thus we have introduced in fact flavor probe branes into a background gravity model dual of a YM (SYM) theory. This is the gravity analog of using a quenched approximation in lattice gauge theories. We would like to introduce probe flavor branes to Witten’s model. What type of Dp branes should we add D4, D6 or D8 branes? How do we incorporate a full chiral flavor global symmetry of the form U(Nf)xU(Nf), with left and right handed chiral quarks? Adding flavor probe branes The mass is the string endpoint masss discussed before U(Nf)xU(Nf) global flavor symmetry in the UV calls for two separate stacks of branes. To have a breakdown of this chiral symmetry to the diagonal U(Nf)D we need the two stacks of branes to merge one into the other. This requires a U shape profile of the probe branes. The opposite orientations of the probe branes at their two ends implies that in fact these are stacks of Nf D8 branes and a stack of Nf anti D8 bran