The initial idea of a string/gauge duality is due to ’t Hooft , who realized that the perturbative expansion of SU() gauge field theory in the large limit can be reinterpreted as a genus expansion of discretized two-dimensional surfaces built from the field theory Feynman diagrams. Here counts the genus of the Feynman diagram, while the ’t Hooft coupling (with denoting the gauge theory coupling constant) enumerates quantum loops. The genus expansion of the free energy of a SU() gauge theory in the ’t Hooft limit ( with fixed), for example, takes the pictorial form
The AdS/CFT correspondence is the first concrete realization of this idea for four-dimensional gauge theories. In its purest form - which shall also be the setting we will be interested in - it identifies the “fundamental” type IIB superstring in a ten-dimensional anti-de-Sitter cross sphere () space-time background with the maximally supersymmetric Yang-Mills theory with gauge group SU() ( SYM) in four dimensions. The Super Yang-Mills model is a quantum conformal field theory, as its -function vanishes exactly. The string model is controlled by two parameters: the string coupling constant and the “effective” string tension , where is the common radius of the and geometries. The gauge theory, on the other hand, is parameterized by the rank of the gauge group and the coupling constant , or equivalently, the ’t Hooft coupling . According to the AdS/CFT proposal, these two sets of parameters are to be identified as
The equations (2) relate the coupling constants, but there is also a dictionary between the excitations of the two theories. The correspondence identifies the energy eigenstates of the string, which we denote schematically as - with being a multi-index - with (suitable) composite gauge theory operators of the form , where are the elementary fields of SYM (and their covariant derivatives) in the adjoint representation of SU(), i.e. hermitian matrices. The energy eigenvalue of a string state, with respect to time in global coordinates, is conjectured to be equal to the scaling dimension of the dual gauge theory operator, which in turn is determined from the two point function of the conformal field theory1|4) of the two theories, which furnishes the representations under which and transform. This then yields a hint on how one could set up an explicit string state/gauge operator dictionary.
Clearly, there is little hope of determining either the all genus (all orders in ) string spectrum, or the complete dependence of the gauge theory scaling dimensions . But the identification of the planar gauge theory with the free () string seems feasible and fascinating: Free string theory should give the exact all-loop gauge theory scaling dimensions in the large limit! Unfortunately though, our knowledge of the string spectrum in curved backgrounds, even in such a highly symmetric one as , remains scarce. Therefore, until very recently, investigations on the string side of the correspondence were limited to the domain of the low energy effective field theory description of strings in terms of type IIB supergravity. This, however, is necessarily limited to weakly curved geometries in string units, i.e. to the domain of by virtue of (2). On the gauge theory side, one has control only in the perturbative regime where , which is perfectly incompatible with the accessible supergravity regime . Hence, one is facing a strong/weak coupling duality, in which strongly coupled gauge fields are described by classical supergravity, and weakly coupled gauge fields correspond to strings propagating in a highly curved background geometry. This insight is certainly fascinating, but at the same time strongly hinders any dynamical tests (or even a proof) of the AdS/CFT conjecture in regimes that are not protected by the large amount of symmetry in the problem.
This situation has profoundly changed since 2002 by performing studies of the correspondence in novel limits where quantum numbers (such as spins or angular momenta in the geometric language) become large in a correlated fashion as . This was initiated in the work of Berenstein, Maldacena and Nastase , who considered the quantum fluctuation expansion of the string around a degenerated point-like configuration, corresponding to a particle rotating with a large angular momentum on a great circle of the space. In the limit of with held fixed (the “BMN limit”), the geometry seen by the fast moving particle is a gravitational plane-wave, which allows for an exact quantization of the free string in the light-cone-gauge [88, 90]. The resulting string spectrum leads to a formidable prediction for the all-loop scaling dimensions of the dual gauge theory operators in the corresponding limit, i.e. the famous formula for the simplest two string oscillator mode excitation. The key point here is the emergence of the effective gauge theory loop counting parameter in the BMN limit. By now, these scaling dimensions have been firmly reproduced up to the three-loop order in gauge theory [21, 12, 53]. This has also led to important structural information for higher (or all-loop) attempts in gauge theory, which maximally employ the uncovered integrable structures to be discussed below. Moreover, the plane wave string theory/ SYM duality could be extended to the interacting string () respectively non-planar gauge theory regime providing us with the most concrete realization of a string/gauge duality to date (for reviews see [94, 97, 104, 103]).
In this review we shall discuss developments beyond the BMN plane-wave correspondence, which employ more general sectors of large quantum numbers in the AdS/CFT duality. The key point from the string perspective is that such a limit can make the semiclassical (in the plane-wave case) or even classical (in the “spinning string” case) computation of the string energies also quantum exact [61, 62], i.e. higher -model loops are suppressed by inverse powers of the total angular momentum on the five sphere2. These considerations on the string side then (arguably) yield all-loop predictions for the dual gauge theory. Additionally, the perturbative gauge theoretic studies at the first few orders in led to the discovery that the spectrum of scaling dimensions of the planar gauge theory is identical to that of an integrable long-range spin chain [92, 21, 24]. Consistently, the string is a classically integrable model , which has been heavily exploited in the construction of spinning string solutions. This review aims at a more elementary introduction to this very active area of research, which in principle holds the promise of finding the exact quantum spectrum of the string or equivalently the all-loop scaling dimensions of planar Super Yang-Mills. It is intended as a first guide to the field for students and interested “newcomers” and points to the relevant literature for deeper studies. We will discuss the simplest solutions of the string corresponding to folded and circular string configurations propagating in a subspace, with the lying within the . On the gauge theory side we will motivate the emergence of the spin chain picture at the leading one-loop order and discuss the emerging Heisenberg model and its diagonalization using the coordinate Bethe ansatz technique. This then enables us to perform a comparison between the classical string predictions in the limit of large angular momenta and the dual thermodynamic limit of the spin chain spectrum. Finally, we turn to higher-loop calculations in the gauge-theory and discuss conjectures for the all-loop form of the Bethe equations, giving rise to a long-range interacting spin chain. Comparison with the obtained string results uncovers a discrepancy from loop order three onwards and the interpretation of this result is also discussed.
A number of more detailed reviews on spinning strings, integrability and spin chains in the AdS/CFT correspondence already exist: Tseytlin’s review  mostly focuses on the string side of the correspondence, whereas Beisert’s Physics Report  concentrates primarily on the gauge side. See also Tseytlin’s second review , on the so-called coherent-state effective action approach, which we will not discuss in this review. Recommended is also the shorter review by Zarembo  on the SU(2) respectively subsector, discussing the integrable structure appearing on the classical string - not covered in this review. For a detailed account of the near plane-wave superstring, its quantum spectroscopy and integrability structures see Swanson’s thesis .
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