Abstract
We review recent progress in studying mesons within gauge/gravity duality, in the context of adding flavour degrees of freedom to generalizations of the AdS/CFT correspondence. Our main focus is on the `top-down approach' of considering models constructed within string theory. We explain the string-theoretical constructions in detail, aiming at non-specialists. These give rise to a new way of describing strongly coupled confining large N gauge gauge theories similar to large N QCD. In particular, we consider gravity dual descriptions of spontaneous chiral symmetry breaking, and compare with lattice results. A further topic covered is the behaviour of flavour bound states in finite temperature field theories dual to a gravity background involving a black hole. We also describe the `bottom up' phenomenological approach to mesons within AdS/QCD. -- Some previously unpublished results are also included.
Highlights
String theory1 originated as a theory of hadrons in the 1960s, when it was noticed that the hadron spectra contain Regge trajectories that can be reproduced by the properties of a rotating relativistic string
E ddx φ0(x)O(x) φ(0,x)=φ0 (x) i.e. the generating functional of particular gauge-invariant operators in the conformal field theory coincides with the generating functional for tree diagrams in supergravity, with the boundary values of the supergravity fields coinciding with the sources. This suggests that the AdS/CFT correspondence may be tested by comparing correlation functions of N = 4 quantum field theory with classical correlation functions on AdS5
We will be brief in our review of this work having spent considerable time on the simplest N = 2 theory and wishing to proceed to models with dynamical chiral symmetry breaking in the spirit of Quantum Chromodynamics (QCD)
Summary
String theory originated as a theory of hadrons in the 1960s, when it was noticed that the hadron spectra contain Regge trajectories that can be reproduced by the properties of a rotating relativistic string. One has AdS5 × S5 generated by the D3 branes with the D7 brane probe wrapping —for massless quarks— an AdS5 × S3 subspace This corresponds to a four-dimensional N = 2 supersymmetric large-N gauge theory with the field content of N = 4 plus a small number of fundamental hypermultiplets. At high temperature or density, mesons become unstable and melt into the quark-gluon plasma This phenomenon is obtained in the gauge/gravity dual description, but is associated with a particular first-order phase transition [18, 33,34,35,36] which is not expected to be present in QCD. For readers unfamiliar with the subject, we recommend reading the following sections first (in the order given): 2, 3.1, 3.2, 3.3.1, 6, 7 and 8
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