Instantons in Supersymmetric Yang–Mills and D-Instantons in IIB Superstring Theory
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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by CERN Document Server ROM2F-98-25; DAMTP-98-69; hep-th/9807033 Instantons in supersymmetric Yang–Mills and D-instantons in IIB superstring theory Massimo Bianchia, Michael B. Greenb, Stefano Kovacsa and Giancarlo Rossia a Dipartimento di Fisica, Universit`a di Roma “Tor Vergata” I.N.F.N. - Sezione di Roma “Tor Vergata”, Via della Ricerca Scientifica, 1 00173 Roma, ITALY b Department of Applied Mathematics and Theoretical Physics, Silver Street, Cambridge CB3 9EW, UK ABSTRACT The one-instanton contributions to various correlation functions of su- perconformal currents in four-dimensional N = 4 supersymmetric SU(2) Yang–Mills theory are evaluated to the lowest order in perturbation theory. Expressions of the same form are obtained from the leading effects of a sin- gle D-instanton extracted from the IIB superstring effective action around 5 the AdS5 × S background. This is in line with the suggested AdS/Yang– Mills correspondence. The relation between Yang–Mills instantons and D- instantons is further confirmed by the explicit form of the classical D-instanton 5 solution in the AdS5 × S background and its associated supermultiplet of zero modes. Speculations are made concerning instanton effects in the large- Nc limit of the SU(Nc) Yang–Mills theory. 1 Introduction Four-dimensional N = 4 supersymmetric Yang–Mills theory is a very special quantum field theory. It is the original example of a theory possessing exact electromagnetic duality [1, 2, 3] which is connected to the fact that it con- tains an infinite set of stable dyonic BPS states [4] and also has a vanishing renormalization group β function. Whereas the abelian theory is free, the nature of the non-abelian theory depends on whether scalar fields have vac- uum expectation values. In the Coulomb phase, reached by giving vacuum expectation values (vev’s) to the scalars in the Cartan subalgebra, the two- derivative Wilsonian effective action is believed not to be renormalized either in perturbation theory or by non-perturbative effects. The superconformal invariance of the theory is, however, broken in this phase. The phase in which all scalar fields have vanishing vev’s is expected to describe a highly nontriv- ial superconformal field theory. Although it is very difficult to understand the nature of this phase from direct perturbative calculations, according to the recent flurry of work [5, 6, 7, 8] certain properties of the theory should be understandable in terms of type IIB superstring theory compactified on 5 AdS5 × S , where the Yang–Mills theory is located on the four-dimensional boundary of the five-dimensional anti de-Sitter space. According to the proposal made by Maldacena in [6] properties of SU(Nc) N = 4 Yang–Mills theory in the large-Nc limit may be determined by semi- classical approximations to the superstring theory. In this limit the boundary Yang–Mills theory is interpreted as the world-volume theory for a large-Nc 5 collection of coincident D3-branes. The AdS5 × S geometry is the near- horizon description of the classical D3-brane solution [9] which is a source of non-vanishing self-dual Ramond–Ramond (R ⊗ R) five-form field strength, F5 = ∗F5 [10]. In fact, the D3-brane solution plays the rˆole of an interpolating soliton between two maximally supersymmetric configurations (with 16 + 16 supercharges) — flat ten-dimensional Minkowski space at infinity and AdS5× S5 at the horizon. The extra 16 Killing spinors at the horizon are in one-to- one correspondence with the special supersymmetry transformations of the boundary theory [11]. The SO(6) isometry of the S5 factor corresponds to the gauging of the SU(4) R-symmetry group while the SO(4, 2) isometry of AdS5 coincides with the conformal group of the boundary theory at the horizon. In this picture the boundary values of the fields of the bulk superstring 1 5 theory compactified on AdS5 × S are sources that couple to gauge-invariant operators of the four-dimensional boundary N = 4 supersymmetric Yang– Mills theory. The lowest Kaluza–Klein modes of the graviton supermultiplet couple to the superconformal multiplet of Yang–Mills currents. These fields and currents will be reviewed in more detail in section 2. More generally, all the Kaluza-Klein excitations of the bulk supergravity theory can be put in one-to-one correspondence with gauge-singlet composite Yang–Mills opera- tors [7, 8, 12, 13]. According to this idea the effective action of type IIB supergravity, eval- uated on a solution of the equations of motion with prescribed boundary conditions, is equated with the generating functional of connected gauge- invariant correlation functions in the Yang–Mills theory. The parameters of 5 the N = 4 Yang–Mills theory and the IIB superstring on AdS5 × S are related by g2 L2 g = YM , 2πC˜(0) = θ , = g2 N , (1) s 4π YM α0 YM c q φ˜ ˜ ˜(0) where gs = e is the string coupling (φ is the constant dilaton), C is the constant R ⊗ R axionic background, gYM is the Yang–Mills coupling, θYM is 5 the vacuum angle and L is the radius of both the AdS5 and S factors of the bulk background. The complex Yang–Mills coupling is therefore identified with the constant boundary value of the complex scalar field of the IIB superstring, θ 4πi i YM + =C˜(0) + . (2) 2π g2 g YM s Most of the tests of this conjecture have so far amounted to the computation of two-point and three-point correlations of currents [14] based on the semi- classical approximation to the bulk supergravity (gs << 1) which is valid at length scales much larger than the string scale or, equivalently, in the limit 0 2 2 α /L << 1. This is the limit Nc →∞and gYM → 0withgYMNc >> 1. This can also be viewed as the large-Nc limit introduced by ’t Hooft [15] with 2 2 the couplingg ˆ = gYMNc fixed at a large value. The explicit connection between the bulk theory and the boundary theory can be expressed symbolically as [8, 7, 16] exp(−SIIB [Φm(J)]) = DA exp(−SYM[A]+O∆[A]J), (3) Z 2 where SIIB is the effective action of the IIB superstring or its low energy supergravity limit which is evaluated in terms of the ‘massless’ supergravity fields and their Kaluza–Klein descendents, that we have generically indi- cated with Φ(z; ω), where ω are the coordinates on S5 and zM ≡ (xµ,ρ) (M =0,1,2,3,5andµ=0,1,2,3) are the AdS5 coordinates (ρ ≡ z5 is the coordinate transverse to the boundary). The notation in (3) indicates that the action depends on the boundary values, J(x), of the bulk fields. The fluctuating boundary N = 4 supersymmetric Yang–Mills fields are denoted by A and O(A) in (3) is the set of gauge-invariant composite operators to which J couples. The recipe for computing correlations involves the ‘bulk- to-boundary’ Green functions which are defined as specific normalized limits of bulk-to-bulk Green functions [7, 8, 17] when one point is taken to the AdS boundary. The precise forms of these propagators depend on the spin and mass of the field. For example, the normalized bulk-to-boundary Green function for a dimension ∆ scalar field is given by 0 0 µ 0µ G∆(x, ρ, ω; x , 0,ω)=c∆K∆(x ,ρ;x ,0), (4) 2 which is independent of ω and where c∆ = Γ(∆)/(π Γ(∆ − 2)) and ∆ µ 0µ ρ K∆(x ,ρ;x ,0) = . (5) (ρ2 +(x−x0)2)∆ The expression (4) is appropriate for an ‘S-wave’ process in which there are 5 no excitations in the directions of the five-sphere, S .IntermsofK∆the bulk field 4 0 0 0 Φm(z; J)=c∆ dxK∆(x, ρ; x , 0)J∆(x )(6) Z satisfies the boundary condition as ρ → 0, 4−∆ Φm(x, ρ; J) ≈ ρ J∆(x)(7) since K∆ reduces to a δ-function on the boundary. The conformal dimension of the operator is related to the AdS mass of the corresponding bulk field 2 2 by (mL) =∆(∆−4), so that ∆± =2± 4+(mL) and only the positive branch, ∆ = ∆+, is relevant for the lowest-‘mass’q supergravity multiplet. In the case of a massless scalar field (∆+ = 4) the propagator reduces to δ(4)(xµ − x0µ) in the limit ρ → 0. 3 For our considerations it will prove crucial in the following that the expres- sion (5) in the case ∆+ = 4 has exactly the same form as the contribution of − 2 − a Yang–Mills instanton to Tr(Fµν ) (where Fµν is the non-abelian self-dual field strength) when the fifth coordinate ρ is identified with the instanton scale. At the same time, we will see that in this case (5) has precisely the same form as the five-dimensional profile of a D-instanton centered on the M 0µ point z in AdS5 and evaluated at the boundary point (x , 0).Thisisa key observation in identifying D-instanton effects of the bulk theory with Yang–Mills instanton effects in the boundary theory. It is related to the fact that the moduli space of a Yang–Mills instanton has an AdS5 factor. Two-point and three-point correlation functions of superconformal cur- rents are not renormalized from their free-field values due to the N =4 superconformal invariance so they do not get interesting interaction correc- tions [17, 18, 19, 20]. However, higher-point correlation functions do receive nontrivial interaction corrections. Here we will be concerned with calcula- tions of processes in which the one-instanton contributions can be evaluated exactly to lowest order in perturbation theory.