QMW-PH-97-22 On Mirror Symmetry for Manifolds of Exceptional Holonomy. B.S.Acharya1, Department of Physics, Queen Mary and Westfield College, Mile End Road, London. E1 4NS. U.K. Abstract n m We consider Type II string theories on T /Z2 Joyce orbifolds. This class contains orbifolds which can be desingularised to give man- ifolds of G2 (n=7) and Spin(7) holonomy (n=8). In the G2 holon- omy case we present two types of T -duality transformation which are clearly generalisations of the T -duality/mirror transformation in Calabi-Yau spaces. The first maps Type IIA theory on one such space from this class to Type IIB theory on another such space. The sec- ond maps Type IIA (IIB) to Type IIA (IIB). In the case of Spin(7) holonomy we present a T -duality transformation which maps Type IIA (IIB) theory on one such space to Type IIA (IIB) on another such space. As orbifold conformal field theories these T -dual target spaces are related via the inclusion/exclusion of discrete torsion and the T - duality is proven to genus g in string perturbation theory. We then apply a Strominger, Yau, Zaslow type argument which suggests that manifolds of G2 holonomy which have a “mirror” of the first (second) 3 4 arXiv:hep-th/9707186v1 22 Jul 1997 type admit supersymmetric T (T ) fibrations and that manifolds of Spin(7) holonomy for which a mirror exists admit fibration by su- persymmetric 4-tori. Further evidence for this suggestion is given by examining the moduli space structure of wrapped D-branes. [email protected] Work Supported by PPARC. 1 1 Introduction. Mirror symmetry has proven to be a powerful and fascinating area of string theory (for a review see [1]). Mirror symmetry is at present only defined for string theory on Calabi-Yau spaces. Recent work [2, 3, 4] has suggested the possible existence of an analagous symmetry for string theory on mani- folds of exceptional holonomy. This evidence is perhaps not surprising since manifolds of exceptional holonomy share many properties with Calabi-Yau manifolds. They admit Ricci flat metrics and covariantly constant forms leading to the important fact that they give supersymmetric compactifica- tions of string theory and M theory. Another remarkable general feature is that Euclidean D-branes wrapped around supersymmetric cycles in these manifolds have world-volume theories which are cohomological [5, 6, 7, 8, 9]. This latter feature is almost certainly related to what we will have to say in this paper, but will not be discussed here. String theories on orbifold limits of such manifolds can provide a simple framework in which to study a variety of physical and mathematical phenom- ena which often persist as one moves away from the orbifold limit. This is due to the fact that strings remarkably “know how to” blow up the orbifold. We will exploit these simplifications here. The purpose of this paper is to provide evidence for the existence of a symmetry in Type IIA/IIB string theory compactified on manifolds of exceptional holonomy. As we will see, this symmetry is very much analogous to the mirror symmetry of Type II strings on Calabi-Yau n-folds. In the case of Calabi-Yau threefolds, Strominger, Yau and Zaslow (SYZ) considered some non-perturbative implications of mirror symmetry [10]. By considering the mirror BPS soliton spectra in the two theories they ar- gued that mirror symmetry is T -duality. The analogue of their argument for Calabi-Yau n-folds was given in [13]. The outcome of the SYZ argu- ment is that Calabi-Yau n-folds which have a mirror admit supersymmetric Tn fibrations. Mirror symmetry is then T -duality on these supersymmetric fibers. Owing to the simplifications in studying orbifolds, one well understood 6 example of mirror symmetry is the pair of T /Z2×Z2 orbifolds first con- structed in [11]. This pair of orbifolds differ by the inclusion/exclusion of 2 discrete torsion2. In this case, for each choice of orbifold isometry group, there is only one consistent choice for the discrete torsion leading to the fact that there are only two such orbifolds, one without discrete torsion and the other with. As pointed out in [11] this pair of orbifolds have mirror Hodge numbers. Let us denote these two target spaces as X and Y. This pair of orbifolds were further studied in [14] where it was shown that Type IIA the- ory on X is equivalent to Type IIB theory on Y. This equivalence was proven to all orders in string perturbation theory. Moreover the mirror symmetry transformation between these two theories was shown to be T -duality. This example thus provides strong evidence that the SYZ argument is correct3. In [16] we tested the SYZ argument for Calabi-Yau fourfolds by studying 8 3 T /Z2 orbifolds. The mirror transformation is also R → 1/R T -duality, but this time on supersymmetric T4 fibers. In this case also, it is the turning on of discrete torsion via the T -duality transformation which “produces” the mirror target space. As in [14] equivalence between these mirror theories can be checked to genus g in string perturbation theory. In [20, 21, 22] Joyce showed that manifolds with G2 and Spin(7) holonomy n m can also be constructed by desingularising T /Z2 orbifolds, where n = 7 and 8 for G2 and Spin(7) holonomy respectively. We studied Type II theories on such orbifolds (for n = 7) in [4] where we proved that Type IIA theory on a given orbifold from this class is equivalent to Type IIB theory on another orbifold from the same class. This verified conjectures made in [2, 3]. Again, these examples follow the same pattern: the transformation between the two theories is T -duality on supersymmetric tori, they differ via the inclusion/exclusion of discrete torsion and the equivalence can be checked to all orders in string perturbation theory. In this sense, this transformation between Type IIA and IIB theories on spaces of G2 holonomy can be regarded as a direct generalisation of mirror symmetry for Calabi-Yau n-folds. In this paper we will further develop the notion of a “mirror” symmetry for manifolds of exceptional holonomy. The plan of the paper is as follows. Motivated by the results which follow we first give a simple definition of mirror manifolds in Type II string theory. This definition is a very general one and, for the cases which we study here, is likely to be replaced with a much more specific conjecture in the future. The results of this paper are 2Discrete torsion was defined in [12]. 3Further aspects of this mirror pair were studied in [15] in light of [10]. 3 independent of this definition, but we make this definition for two reasons: (i): it allows a simplified presentation of the results; (ii): since, in the exam- ples studied here, a symmetry analogous to mirror symmetry for Calabi-Yau spaces exists, it deserves to have a name. For obvious reasons we choose to call it exceptional mirror symmetry, or just mirror symmetry when the context is obvious. Throughout this paper a generic pair of mirror target spaces will be denoted by X and Y repsectively. 7 n In section three we study Type II strings on T /Z2 orbifolds. This class of orbifolds includes those which can be blown up to give manifolds of G2 holonomy [20, 21]. We describe two types of T -duality/mirror transforma- tions which map between pairs of target spaces in this class. The first are of the type studied in [4] and map Type IIA on one target space X from this category to Type IIB on the mirror Y, where the mirror is defined in the previous section. The second type of transformation maps Type IIA on X to Type IIA on Y and similarly maps IIB to IIB. 8 n In section four we study Type II theories on T /Z2 orbifolds, some of which admit a resolution to manifolds of Spin(7) holonomy. We present a T -duality/mirror transformation which maps Type IIA (IIB) theory on one such space (X) to Type IIA (IIB) theory on the mirror Y. In section five we consider the Strominger, Yau, Zaslow argument [10] applied to all these three cases. Even though the results are not as strong as in the Calabi-Yau cases, the results suggest that manifolds of G2 holonomy with a mirror of the first type admit supersymmetric T3 fibrations and that those with a mirror of the second type admit supersymmetric T4 fibrations. Similarly, it is conjectured that manifolds of Spin(7) holonomy for which a mirror exists admit supersymmetric T4 fibrations. In section six we give further evidence for the conjectures of section five by considering the geometry of the classical orbifolds themselves. We first point out that all the orbifolds we have discussed indeed have the predicted fibration structure. We then show that the moduli space of the relevant D- brane wrapped around the corresponding p-torus is also an orbifold in the class we are considering. 4 2 A Definition of Mirror Symmetry. As we have discussed in the introduction, we will describe in this paper a symmetry analogous to mirror symmetry for Calabi-Yau n-folds in Type II string theories on manifolds of exceptional holonomy. In order to make the presentation of these results clearer as well as to give this symmetry a name it is useful to give a definition of such a symmetry here. Mirror symmetry in the conventional sense applies to two Type II theories compactified on differ- ent manifolds of SU(n) holonomy.