Preprint typeset in JHEP style. - PAPER VERSIONDFTT/7/2010 LAPTH 023/10 NSF-KITP-10-107 F-theoretic vs microscopic description of a conformal N= 2SYM theory Marco Bill`o1,2, Laurent Gallot3, Alberto Lerda4and Igor Pesando1 1Dipartimento di Fisica Teorica, Universit`a di Torino and I.N.F.N. - sezione di Torino Via P. Giuria 1, I-10125 Torino, Italy 2Kavli Institute for Theoretical Physics, University of California, Santa Barbara, CA 93106-4030, USA 3LAPTH, Universit´e de Savoie, CNRS 9, Chemin de Bellevue 74941 Annecy le Vieux Cedex, France 4Dipartimento di Scienze e Tecnologie Avanzate, Universit`a del Piemonte Orientale and I.N.F.N. - Gruppo Collegato di Alessandria - sezione di Torino Viale T. Michel 11, I-15121 Alessandria, Italy billo,lerda,ipesando@to.infn.it; laurent.gallot@lapp.in2p3.fr Abstract:The F-theory background of four D7 branes in a type I′orientifold was con- jectured to be described by the Seiberg-Witten curve for the superconformal SU(2) gauge theory with four flavors.This relation was explained by considering in this background a probe D3 brane, which supports this theory with SU(2) realized as Sp(1). Here we ex- plicitly compute the non-perturbative corrections to the D7/D3 system in type I′due to D-instantons. This computation provides both the quartic effective action on the D7 branes and the quadratic effective action on the D3 brane; the latter agrees with the F-theoretic prediction. The action obtained in this way is related to the one derived from the usual instanton calculus `a la Nekrasov (or from its AGT realization in terms of Liouville confor- mal blocks) by means of a non-perturbative redefinition of the coupling constant. We also point out an intriguing relation between the four-dimensional theory on the probe D3 brane with SO(8) flavor symmetry and the eight-dimensional dynamics on the D7 branes.On the latter, SO(8) represents a gauge group and the flavor masses correspond to the vacuum expectation values of an adjoint scalar fieldm:what we find is that theexacteffective coupling in four dimensions is obtained from itsperturbativepart by taking into account in its mass dependence the full quantum dynamics of the fieldmin eight dimensions. Keywords:Superstrings, D-branes, Gauge Theories, Instantons, F-theory. arXiv:1008.5240v2 [hep-th] 15 Nov 2010
Contents 1.Introduction and motivations2 2.F-theory and the D7/D3 system in type I′4 2.1The local case5 2.2F-theoretic description and D3 brane probes6 2.2.1The SW curve for the D3 gauge theory8 3.D-instanton corrections11 3.1Instanton moduli spectrum11 3.2BRST structure of moduli space13 3.3Moduli integration via localization and instanton partition function16 3.4Non-perturbative prepotentials18 4.Eight-dimensional effective prepotential and chiral ring20 5.Four-dimensional effective prepotential23 6.An intriguing relation25 7.Comparison with the SU(2) instanton calculus26 7.1Instanton calculus27 7.2The Nekrasov prepotential from the AGT realization28 8.Summary and conclusions30 A.Notations and theta-function conventions32 B.Flavor invariants33 B.1Triality extension of SL(2,Z)34 C.Details on the D-instanton computation34 D. Details on the AGT correspondence36 E.Decoupling limits toNf= 3,2,1,0.38 1
1. Introduction and motivations Phenomenologically viable string models based on consistent D-brane configurations have attracted a lot of attention in the last years [1]–[3]. In such constructions it is necessary to take into account possible non-perturbative corrections due D-instantons and (wrapped) euclidean branes; for a review see, for example, [4].Some of these instantonic branes reproduce gauge instantons [5]–[8], other provide inherently stringy (or “exotic”) instanton effects; these latter can be responsible of important terms in the effective action which would be perturbatively forbidden [9]–[11]. Recently there has been much progress in the explicit computation of such contributions, both ordinary and exotic, at least in supersymmetric cases (again, see [4] and references therein). Another framework where phenomenological models with highly desirable features can be set up, and where in particular Grand Unified Theories occur naturally and consistently [12]–[14], is represented by F-theory compactifications [15] (for reviews see, for instance, [16, 17]). F-theory gives anon-perturbativegeometric description of type IIB backgrounds containing D7 branes and orientifold planes; it somehow resums the non-perturbative cor- rections arising from certain instantonic branes. Understanding in detail how this resumma- tion takes place would improve our knowledge of the relation between the two descriptions. This could be useful for a better comprehension of further non-perturbative effects in F- theory through a lift of their type IIB counterparts, a subject that is recently1receiving quite some attention [19]–[24]. In this paper we work out a simple, yet non-trivial, example where we are able to compute the D-instanton effects in the IIB description and show that they reconstruct the F-theory curve.This example was considered by A. Sen in [25], and is given by the compactification of F-theory on the orbifold limit of an elliptically fibered K3 surface, for which the complex structure modulusτof the fiber is constant.This background was shown to correspond to the so-called type I′theory, which is T-dual to type I theory compactified on a 2-torusT2, and thus possesses one O7 plane at each of the four fixed points ofT2with four D7 branes on top of it. Focusing on the vicinity of one orientifold fixed-plane, and allowing the four D7 branes to move out of it, Sen conjectured that the corresponding F-theory background should be described by the Seiberg-Witten (SW) curve [26] for theN= 2 superconformal Yang-Mills theory with gauge group SU(2) andNf= 4 flavors [27].This relation was later explained by T. Banks, M. Douglas and N. Seiberg [28] by considering a D3 brane in this background, which indeed supports such a four- dimensional gauge theory on its world-volume, with SU(2) realized as Sp(1). The SO(8) flavor symmetry of theNf= 4 theory is nothing else but the gauge group on the D7 branes. Here we explicitly compute the non-perturbative corrections to the D7/D3 system in type I′due to D-instantons.This requires to identify the spectrum of moduli,i.e.of excitations of the strings with at least one end-point on the D-instantons, and the moduli action that arises from disks with at least part of their boundary on a D-instanton, which was already discussed in [29].To obtain the non-perturbative effects it is necessary to integrate over the moduli; we explicitly perform this integration by applying the by-now 1For an earlier discussion in anN= 2 context see [18]. 2
standard techniques based on the BRST structure of the moduli spectrum and action and its deformation by means of suitable RR backgrounds [30]–[34]. This induces a complete localization of the integral, similarly to what happens in supersymmetric instanton calculus in field theory [35]–[40]. These techniques have been recently applied to similar brane set- ups, such as the D7 system in type I′[31, 33], and the D7/D3 system onT4/Z2[34]. D-instanton effects induce corrections to both the quartic effective action on the D7 branes and the quadratic effective action on the D3 brane. We use the prescription proposed in [34] to disentangle the two contributions. The non-perturbative action on the D7’s turns out to be exactly the same of the D7 system in type I′theory considered in [31]. The non- perturbative effective coupling on the D3 brane agrees with the one extracted from the SW curve, that is with the F-theoretic prediction. One interesting question is the precise relation between the eight-dimensional quartic effective action on the D7 branes and the F-theory curve.This problem was already addressed in the past using the duality of certain F-theory compactifications, including the one corresponding to type I′theory, to heterotic models. Despite some interesting results [41, 42], this relation is not yet totally clear. Since in our case the F-theory curve is nothing else but the SW curve encoding the effective theory on the D3 probe, another way to state the above question is: what is the relation between the non-perturbative effective actions on the D7 branes and on the D3 brane? More generally, how are the quantum dynamics on the D7’s and that on the D3 related to each other? We do not have a full answer to this question, but we uncover an intriguing relation that goes as follows.On the D7’s, the SO(8) flavor symmetry represents a gauge group and the flavor massesmicorrespond to the vacuum expectation values of an adjoint scalar fieldm(X):we find that theexacteffective four-dimensional coupling is obtained from itsperturbativepart by taking into account in its mass dependence the eight-dimensional quantum dynamics of the fieldm, and in particular the so-called “chiral ring” formed by the correlators〈Trm2l〉.In this way the F-theory geometry is related explicitly to these eight-dimensional quantities. It would be very interesting to investigate whether such sort of perturbative propagation of the full quantum dynamics on a brane stack (in our case, the D7’s) to another one (in our case the D3 probe) takes place also in other systems2. The theory that, in our example, lives on the D3 brane is a four-dimensional conformal N= 2 theory. This class of theories have recently attracted much attention in relation to the so-called AGT conjecture put forward by L. F. Alday, D. Gaiotto and Y. Tachikawa [44]. This conjecture relates the effective actions obtained from usual instanton calculus `a la Nekrasov [35] to suitable correlators of the Liouville theory in two dimensions [45] (see also [46]–[48]). In particular, the non-perturbative action for the SU(2) theory withNf= 4 can be extracted from the 4-point functions on the sphere. It is interesting to compare these results to what we get in the D7/D3 system in type I′, where the conformal theory is realized as an Sp(1) gauge theory. It turns out that the effective action derived from the Nekrasov’s prescription for the SU(2),Nf= 4 case or, more efficiently, from its AGT realization in 2Recently, D3 probes in F-theory have been considered in [20, 43]. In particular in [43] it is pointed out that the interplay between the eight-dimensional theory on the D7 and the four-dimensional theory on the probe, and the interpretation of parameters in the latter as adjoint fields on the former, plays a crucial rˆole. 3
terms of Liouville conformal blocks does not coincide, at first sight, with our results, nor with the SW curve proposed in [27]. There is a discrepancy already at the massless level: in this case from the SW curve (and from our microscopic computation) we see that the tree- level coupling receives no corrections. When computed following Nekrasov’s prescription for SU(2), or by the AGT method, instead, the coupling gets non-perturbatively modified3. This suggests that a redefinition of the coupling constant is needed in order to compare the two approaches; after such a redefinition is performed, remarkably the two methods are reconciled and the two results agree completely also in the massive case. The paper is subdivided into several sections as we now describe.In Section 2 we introduce the model and its F-theory description through the SW curve; from this curve we extract the instanton expansion of the effective coupling.In Section 3 we describe the microscopic computation of D-instanton contributions in our model.The resulting effective action on the D7 branes is discussed in Section 4, while in Section 5 we write the D-instanton induced effective action on the D3 brane, which is in full agreement with the F- theoretic description. In Section 6 we put forward our conjecture about the exact effective coupling on the D3 being determined by its perturbative part plus the eight-dimensional dynamics of the mass parameters. Finally, in Section 7 we discuss how our results compare to Nekrasov instanton calculus (or its AGT realization) for the SU(2) theory withNf= 4, and in Section 8 we present our conclusions. In the appendices some technical details and the extension of some results to asymptotically free cases withNf<4 are given. 2. F-theory and the D7/D3 system in type I′ In F-theory compactifications over an elliptically fibered manifold [15], the complex struc- ture modulusτof the fiber corresponds to the varying axio-dilaton profile of a suitable type IIB compactification on the base manifold. In [25] A. Sen studied F-theory on an ellipti- cally fibered K3 surface, which is conjectured to be dual to heterotic string compactified on a two-dimensional torus. He considered the particular case in which the K3 is at the orbifold limit in moduli space where it is described by the following curve in Weierstrass form y2=x3−1 4G2(z)x−1 4G3(z).(2.1) Herezis the coordinate on the base of the fibration and G2(z)∝Q2(z),G3(z)∝Q3(z),Q(z) = 4∏ I=1 (z−fI),(2.2) withfIconstants. The absolute modular invariant of this curve J=G3 2 G3 2−27G2 3 (2.3) 3A discrepancy in this sense was already noticed at the two-instanton level in [49, 50], where the direct integration over the moduli was performed without resorting to localization techniques. 4
isz-independent, and so is its complex structure modulusτwhich can be determined from Jby inverting the relation J= (ϑ8 2(τ) +ϑ8 3(τ) +ϑ8 4(τ) 24η8(τ) )3 (2.4) where theϑa’s are the Jacobi theta-functions andηis the Dedekind function. By studying the metric on the base space, it can be seen that the latter has the geometry of a torus orbifold of the typeT2/Z2, withZ2acting as parity reflection alongT2, in which thefI’s appearing in (2.2) correspond to the points ofT2that are fixed under theZ2action. This specific F-theory background can be identified with the so-called type I′theory, namely type IIB compactified on a torusT2and modded out by Ω =ω(−1)FLI2,(2.5) whereωis the world-sheet parity reversal,FLis the left-moving space-time fermion number andI2the inversion onT2. This is the T-dual version of type I theory compactified onT2, and possesses four O7 orientifold planes located at the points ofT2that are fixed under I2(see Fig. 1a).Each orientifold plane carries (−4) units of 7-brane charge, which need to be neutralized by putting 16 D7 branes transverse toT2. If we place them in groups of 4 over each orientifold plane, the tadpole cancellation becomes local and the axio-dilaton is constant overT2.From now on we take a local perspective and focus on one of the orientifold fixed planes (say, the one atz=f1) and its associated stack of 4 D7 branes. 2.1 The local case The action of the orientifold projection Ω is such that each group of 4 D7 branes supports an eight-dimensional theory with gauge group SO(8).Indeed, the massless degrees of freedom of the 7/7 open strings build up an eight-dimensional chiral superfield in the adjoint representation of SO(8), whose first few terms are M(X,Θ) =m(X) +√2 Θσ(X) + 1 2 ΘγM NΘfM N(X) +. . . ,(2.6) wherefM Nis the field-strength,σis the gaugino,mis a complex scalar, and (X,Θ) are the eight-dimensional super-coordinates. If the D7 branes are moved away from the orientifold plane,i.e.if we give a diagonal vacuum expectation value to the scalar fieldm, the charges no longer cancel locally, and correspondingly the solution of the equation of motion forτdisplays logarithmic singu- larities at the orientifold and D7 brane locations. Let us parametrize the region near the orientifold fixed point with a coordinate4wand let the D7 branes and their images be located atw=±mi/√2 withi= 1, . . . ,4 (see Fig. 1b).This corresponds to choose the following vacuum expectation values5 〈m〉= diag(m1/√2, . . . , m4/√2,−m1/√2, . . .−m4/√2).(2.7) 4With respect to the global coordinatezused above,w∝(z−f1). 5Notice thatmis a complex field in the adjoint of SO(8),i.e.it is a complex antisymmetric matrix. If mwere real, its eigenvaluesmiwould be imaginary. 5
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