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We discuss the special holonomy metrics of Gibbons, Lu, Pope and Stelle, which were constructed as nilmanifold bundles over a line by uplifting supersymmetric domain wall solutions of supergravity to 11 dimensions. We show that these are dual to intersecting brane solutions, and considering these leads us to a more general class of special holonomy metrics. Further dualities relate these to non-geometric backgrounds involving intersections of branes and exotic branes. We discuss the possibility of resolving these spaces to give smooth special holonomy manifolds., 39 pages

We continue our study of effective field theory via homotopy transfer of $L_\infty$-algebras, and apply it to tree-level non-Wilsonian effective actions of the kind discussed by Sen in which the modes integrated out are comparable in mass to the modes that are kept. We focus on the construction of effective actions for string states at fixed levels and in particular on the construction of weakly constrained double field theory. With these examples in mind, we discuss closed string theory on toroidal backgrounds and resolve some subtle issues involving vertex operators, including the proper form of cocycle factors and of the reflector state. This resolves outstanding issues concerning the construction of covariant closed string field theory on toroidal backgrounds. The weakly constrained double field theory is formally obtained from closed string field theory on a toroidal background by integrating out all but the doubly massless' states and homotopy transfer then gives a prescription for determining the theory's vertices and symmetries. We also discuss consistent truncation in the context of homotopy transfer., 53 pages. v2: References added

We use the dictionary between general field theories and strongly homotopy algebras to provide an algebraic formulation of the procedure of integrating out of degrees of freedom in terms of homotopy transfer. This includes more general effective theories in which some massive modes are kept while other modes of a comparable mass scale are integrated out, as first explored by Sen in the context of closed string field theory. We treat $L_\infty$-algebras both in terms of a nilpotent coderivation and, on the dual space, in terms of a nilpotent derivation (corresponding to the BRST charge of the field theory) and provide explicit formulas for homotopy transfer. These are then shown to govern the integrating out of degrees of freedom at tree level, while the generalization to loop level will be explored in a sequel to this paper., 70 pages. v2: Minor changes, references added

A class of special holonomy spaces arise as nilmanifolds fibred over a line interval and are dual to intersecting brane solutions of string theory. Further dualities relate these to T-folds, exotic branes, essentially doubled spaces and spaces with R-flux. We develop the doubled geometry of these spaces, with the various duals arising as different slices of the doubled space., Comment: 38 pages

We define $(p,q)$ hermitian geometry as the target space geometry of the two dimensional $(p,q)$ supersymmetric sigma model. This includes generalised K\"{a}hler geometry for $(2,2)$, generalised hyperk\"{a}hler geometry for $(4,2)$, strong K\"{a}hler with torsion geometry for $(2,1)$ and strong hyperk\"{a}hler with torsion geometry for $(4,1)$. We provide a generalised complex geometry formulation of hermitian geometry, generalising Gualtieri's formulation of the $(2,2)$ case. Our formulation involves a chiral version of generalised complex structure and we provide explicit formulae for the map to generalised geometry., Comment: 20 pages

Strong Kahler with Torsion is the target space geometry of $(2,1)$ and $(2,0)$ supersymmetric nonlinear sigma models. We discuss how it can be represented in terms of Generalised Complex Geometry in analogy to the Gualtieri map from the geometry of $(2,2)$ supersymmetric nonlinear sigma models to Generalised Kahler Geometry.

We present a new class of dualities relating non-geometric Calabi-Yau compactifications of type II string theory to T-fold compactifications of the heterotic string, both preserving four-dimensional $\mathcal{N}=2$ supersymmetry. The non-geometric Calabi-Yau space is a $K3$ fibration over $T^2$ with non-geometric monodromies in the duality group $O(\Gamma_{4,20})$; this is dual to a heterotic reduction on a $T^4$ fibration over $T^2$ with the $O(\Gamma_{4,20})$ monodromies now viewed as heterotic T-dualities. At a point in moduli space which is a minimum of the scalar potential, the type II compactification becomes an asymmetric Gepner model and the monodromies become automorphisms involving mirror symmetries, while the heterotic dual is an asymmetric toroidal orbifold. We generalise previous constructions to ones in which the automorphisms are not of prime order. The type II construction is perturbatively consistent, but the naive heterotic dual is not modular invariant. Modular invariance on the heterotic side is achieved by including twists in the circles dual to the winding numbers round the $T^2$, and this in turn introduces non-perturbative phases depending on NS5-brane charge in the type II construction., Comment: 50 pages, no figure

We find the T-duality transformation rules for 2-dimensional (2,1) supersymmetric sigma-models in (2,1) superspace. Our results clarify certain aspects of the (2,1) sigma model geometry relevant to the discussion of T-duality. The complexified duality transformations we find are equivalent to the usual Buscher duality transformations (including an important refinement) together with diffeomorphisms. We use the gauging of sigma-models in (2,1) superspace, which we review and develop, finding a manifestly real and geometric expression for the gauged action. We discuss the obstructions to gauging (2,1) sigma-models, and find that the obstructions to (2,1) T-duality are considerably weaker., Comment: 45 pages, Minor Corrections

We study superstring theory in three dimensional Anti-de Sitter spacetime with NS-NS flux, focusing on the case where the radius of curvature is equal to the string length. This corresponds to the critical level k=1 in the Wess-Zumino-Witten description. Previously, it was argued that a transition takes place at this special radius, from a phase dominated by black holes at larger radius to one dominated by long strings at smaller radius. We argue that the infinite tower of modes that become massless at k=1 is a signal of this transition. We propose a simple two-dimensional conformal field theory as the holographic dual to superstring theory at k=1. As evidence for our conjecture, we demonstrate that at large N our putative dual exactly reproduces the full spectrum of the long strings of the weakly coupled string theory, including states unprotected by supersymmetry., Comment: 29 pages, one figure. An equivalent construction of the dual orbifold CFT has been added, together with a discussion of the short string spectrum and additional observations on interactions

Off-shell $(4,0)$ supermultiplets in 2-dimensions are formulated. These are used to construct sigma models whose target spaces are vector bundles over manifolds that are hyperk\"ahler with torsion. The off-shell supersymmetry implies that the complex structures are simultaneously integrable and allows us to write actions using extended superspace and projective superspace, giving an explicit construction of the target space geometries., Comment: 13 pages

We consider compactifications of type IIA superstring theory on mirror-folds obtained as K3 fibrations over two-tori with non-geometric monodromies involving mirror symmetries. At special points in the moduli space these are asymmetric Gepner models. The compactifications are constructed from non-geometric automorphisms that arise from the diagonal action of an automorphism of the K3 surface and of an automorphism of the mirror surface. We identify the corresponding gaugings of N=4 supergravity in four dimensions, and show that the minima of the potential describe the same four-dimensional low-energy physics as the worldsheet formulation in terms of asymmetric Gepner models. In this way, we obtain a class of Minkowski vacua of type II string theory which preserve N=2 supersymmetry. The massless sector consists of N=2 supergravity coupled to 3 vector multiplets, giving the STU model. In some cases there are additional massless hypermultiplets., Comment: 48 pages, no figures; v2: few typos corrected, one rephrased statement in subsection 5.2

We investigate the behaviour of the bosonic string on AdS3 with H-flux at stringy scales, looking in particular for a ‘tensionless’ limit in which there are massless higher spin gauge fields. We do this by revisiting the physical spectrum of the sl(2, R)k WZW model and considering the limit in which k becomes small. At k = 3 we find that there is an infinite stringy tower of massless higher spin fields which are part of a continuum of light states. This can be viewed as a novel tensionless limit, which appears to be distinct from that inferred from the symmetric orbifold description of superstring AdS3 vacua., Journal of High Energy Physics, 2017, ISSN:1126-6708, ISSN:1029-8479

The recently derived expressions for finite gauge transformations in double field theory with duality group O(d,d) are generalised to give expressions for finite gauge transformations for extended field theories with duality group SL(5,R), SO(5,5) and E6. The generalised metrics are discussed., 24 pages

When the United States—the traditional supplier of arms to Cuba—decided not to supply Fulgencio Batista's government after March 1958 and the dictator approached Britain, this presented a dilemma for Harold Macmillan's administration so soon after the 1956 Suez crisis. Whitehall was keen to sell arms but wary of doing business with a country so firmly located in the United States' sphere of influence. It will be seen that the Foreign Office conferred with Washington at every stage in its decision process, both in deciding to arm Batista after gaining US acquiescence, and later in acceding to pressure and not supplying military hardware to Fidel Castro's nascent regime.

Recently proposed forms for gauge transformations with finite parameters in double field theory are discussed and problematic issues are identified. A new form for finite gauge transformations is derived that reveals the underlying gerbe structure and the close relationship with generalised geometry. The nature of generalised tensors is elucidated, and in particular it is seen that the presence of a constant metric with split signature does not restrict the doubled geometry, provided it is a generalised tensor rather than a conventional tensor., Comment: 28 pages

We consider the maximal super-Yang-Mills theory in 5 Euclidean dimensions with SO(5) R-symmetry and 16 supersymmetries. We argue that the strong coupling limit of this theory (with a possible UV completion) has an emergent time dimension and gives a description of the 5+1 dimensional Lorentz invariant (2,0) theory of the M5-brane, compactified on a timelike circle with radius R=g^2/4\pi^2 . Our discussion involves issues of quantization of Euclidean theories without time., Comment: 1+40 pages

The SO(32) heterotic string can be obtained from the type IIB string by gauging a discrete symmetry that acts as $(-1)^{F_L}$ on the perturbative string states and reverses the parity of the D-string. Consistency requires the presence of 32 NS 9-branes -- the S-duals of D9-branes -- which give SO(32) Chan-Paton factors to open D-strings. At finite string coupling, there are SO(32) charges tethered to the heterotic string world-sheet by open D-strings. At zero-coupling, the D-string tension becomes infinite and the SO(32) charges are pulled onto the world-sheet, and give the usual SO(32) world-sheet currents of the heterotic string., 12 Pages, Tex, Phyzzx Macro

A non-abelian generalisation of a theory of gravity coupled to a 2-form gauge field and a dilaton is found, in which the metric and 3-form field strength are Lie algebra-valued. In the abelian limit, the curvature with torsion is self-dual in four dimensions, or has SU(n) holonomy in $2n$ dimensions. The coupling to self-dual Yang-Mills fields in 4 dimensions, or their higher dimensional generalisation, is discussed. The abelian theory is the effective action for (2,1) strings, and the non-abelian generalisation is relevant to the study of coincident branes in the (2,1) string approach to M-theory. The theory is local when expressed in terms of a vector pre-potential., 14 pages, phyzzx macro. Minor corrections

It is argued that D=10 type II strings and M-theory in D=11 have D-5 branes and 9-branes that are not standard p-branes coupled to anti-symmetric tensors. The global charges in a D-dimensional theory of gravity consist of a momentum $P_M$ and a dual D-5 form charge $K_{M_1...M_{D-5}}$, which is related to the NUT charge. On dimensional reduction, P gives the electric charge and K the magnetic charge of the graviphoton. The charge K is constructed and shown to occur in the superalgebra and BPS bounds in $D\ge 5$, and leads to a NUT-charge modification of the BPS bound in D=4. $K$ is carried by Kaluza-Klein monopoles, which can be regarded as D-5 branes. Supersymmetry and U-duality imply that the type IIB theory has (p,q) 9-branes. Orientifolding with 32 (0,1) 9-branes gives the type I string, while modding out by a related discrete symmetry with 32 (1,0) 9-branes gives the SO(32) heterotic string. Symmetry enhancement, the effective world-volume theories and the possibility of a twelve dimensional origin are discussed., Comment: 54 pages, TeX, Phyzzx Macro. Added reference

For supergavrity solutions which are the product of an anti-de Sitter space with an Einstein space X, we study the relation between the amount of supersymmetry preserved and the geometry of X. Depending on the dimension and the amount of supersymmetry, the following geometries for X are possible, in addition to the maximally supersymmetric spherical geometry: Einstein-Sasaki in dimension 2k+1, 3-Sasaki in dimension 4k+3, 7-dimensional manifolds of weak G_2 holonomy and 6-dimensional nearly Kaehler manifolds. Many new examples of such manifolds are presented which are not homogeneous and have escaped earlier classification efforts. String or M theory in these vacua are conjectured to be dual to superconformal field theories. The brane solutions interpolating between these anti-de Sitter near-horizon geometries and the product of Minkowski space with a cone over X lead to an interpretation of the dual superconformal field theory as the world-volume theory for branes at a conical singularity (cone branes). We propose a description of those field theories whose associated cones are obtained by (hyper-)Kaehler quotients.

The effective action for type II string theory compactified on a six torus is $N=8$ supergravity, which is known to have an $E_{7}$ duality symmetry. We show that this is broken by quantum effects to a discrete subgroup, $E_7(\Z)$, which contains both the T-duality group $SO(6,6;\Z)$ and the S-duality group $SL(2;\Z)$. We present evidence for the conjecture that $E_7(\Z)$ is an exact \lq U-duality' symmetry of type II string theory. This conjecture requires certain extreme black hole states to be identified with massive modes of the fundamental string. The gauge bosons from the Ramond-Ramond sector couple not to string excitations but to solitons. We discuss similar issues in the context of toroidal string compactifications to other dimensions, compactifications of the type II string on $K_3\times T^2$ and compactifications of eleven-dimensional supermembrane theory., Comment: 45 pages. Some minor corrections made and some references added

Two-dimensional (2,2) supersymmetric nonlinear sigma models can be described in (2,2), (2,1) or (1,1) superspaces. Each description emphasizes different aspects of generalized K\"ahler geometry. We investigate the reduction from (2,2) to (2,1) superspace. This has some interesting nontrivial features arising from the elimination of nondynamical fields. We compare quantization in the different superspace formulations., Comment: 22 pages, published version

The string theory defined by gauging W∞/2, the subalgebra of W∞ generated by the currents of even spin, is discussed. The critical value of the central charge is calculated using zeta-function techniques and shown to be c=1. A critical string theory is constructed by coupling a free boson to W∞/2-gravity and the physical states are analyzed.

The gauging of isometries in general sigma-models which include fermionic terms which represent the interaction of strings with background Yang-Mills fields is considered. Gauging is possible only if certain obstructions are absent. The quantum gauge anomaly is discussed, and the (1,0) supersymmetric generalisation of the gauged action given., 10 pages, phyzzx, QMW-93-25 (Blank lines created by mailer removed, so this version should be TeXable)

Using (1,0) superfield methods, we determine the general scalar potential consistent with off-shell (p,0) supersymmetry and (1,1) supersymmetry in two-dimensional non-linear sigma models with torsion. We also present an extended superfield formulation of the (p,0) models and show how the (1,1) models can be obtained from the (1,1)-superspace formulation of the gauged, but massless, (1,1) sigma model., Comment: 11 pages

The generalized metric is a T-duality covariant symmetric matrix constructed from the metric and two-form gauge field and arises in generalized geometry. We view it here as a metric on the doubled spacetime and use it to give a simple formulation with manifest T-duality of the double field theory that describes the massless sector of closed strings. The gauge transformations are written in terms of a generalized Lie derivative whose commutator algebra is defined by a double field theory extension of the Courant bracket., 36 pages, v2: minor corrections

In the neighborhood of a regular point, generalized Kahler geometry admits a description in terms of a single real function, the generalized Kahler potential. We study the local conditions for a generalized Kahler manifold to be a generalized Calabi-Yau manifold and we derive a non-linear PDE that the generalized Kahler potential has to satisfy for this to be true. This non-linear PDE can be understood as a generalization of the complex Monge-Ampere equation and its solutions give supergravity solutions with metric, dilaton and H-field., Comment: 26 pages

We investigate the target space geometry of supersymmetric sigma models in two dimensions with Euclidean signature, and the conditions for N=2 supersymmetry. For a real action, the geometry for the N=2 model is not the generalized Kahler geometry that arises for Lorentzian signature, but is an interesting modification of this which is not a complex geometry., 13 pages

String backgrounds with a local torus fibration such as T-folds are naturally formulated in a doubled formalism in which the torus fibres are doubled to include dual coordinates conjugate to winding number. Here we formulate and explore a generalisation of this construction in which all coordinates are doubled, so that the doubled space is a twisted torus, i.e. a compact space constructed from identifying a group manifold under a discrete subgroup. This incorporates reductions with duality twists, T-folds and a class of flux compactifications, together with the non-geometric backgrounds expected to arise from these through T-duality. It also incorporates backgrounds that are not even locally geometric, and suggests a generalisation of T-duality to a more general context. We discuss the effective field theory arising from such an internal sector, give a world-sheet sigma model formulation of string theory on such backgrounds and illustrate our discussion with detailed examples., 81 pages

The zero modes of closed strings on a torus — the torus coordinates plus dual coordinates conjugate to winding number — parameterize a doubled torus. In closed string field theory, the string field depends on all zero-modes and so can be expanded to give an infinite set of fields on the doubled torus. We use string field theory to construct a theory of massless fields on the doubled torus. Key to the consistency is a constraint on fields and gauge parameters that arises from the L[subscript 0]−[bar over L][subscript 0] = 0 condition in closed string theory. The symmetry of this double field theory includes usual and `dual diffeomorphisms', together with a T-duality acting on fields that have explicit dependence on the torus coordinates and the dual coordinates. We find that, along with gravity, a Kalb-Ramond field and a dilaton must be added to support both usual and dual diffeomorphisms. We construct a fully consistent and gauge invariant action on the doubled torus to cubic order in the fields. We discuss the challenges involved in the construction of the full nonlinear theory. We emphasize that the doubled geometry is physical and the dual dimensions should not be viewed as an auxiliary structure or a gauge artifact., United States. Dept. of Energy (Grant De-FC02-94eR40818)

For any theory of WN or W∞ gravity coupled to matter, there are W-gravity anomalies depending only on the W-gravity gauge fields, corresponding to the central charge terms in the current algebra. For left-right symmetric theories, these can be cancelled at the expense of introducing anomalies in symmetries which are higher-spin generalisations of Weyl symmetry. Integrating out the matter fields gives a term in the effective action proportional to ΣsR(s)□−1R(s) where R(s) is the curvature scalar for the spin-s gauge field and this is the induced effective action for non-critical quantum W-gravity. In a conformal gauge, this reduces to a generalisation of the Liouville action involging N − 1 scalar fields. In a chiral gauge, there is a Kac-Moody symmetry corresponding to a group G, where G = SL(∞) forW∞ and G is a group contraction of SL(N) forWN (e.g. ISL(2) for W3). If the W-gravity symmetries are non-linearly realised, there are in addition anomalies depending on the matter fields. These are discussed in a separate paper.

The coupling of scalars fields to chiral W3 gravity is reviewed. In general the quantum current algebra generated by the spin-two and three currents does not close when the "natural" regularization (corresponding to the normal ordering with respect to the modes of ∂ϕi) is used, and the non-closure reflects matter-dependent anomalies in the path integral quantization. We consider the most general modification of the current, involving higher derivative "background charge" terms, and find the conditions for them to form a closed algebra in the "natural" regularization. These conditions can be satisfied only for the two-boson model. In that case, it is possible to cancel all the matter-dependent anomalies by adding finite local counter terms to the action and modifying the transformation rules of the fields.

The complete non-linear structure of two-dimensional w∞-gravity and its symmetries is derived. For a scalar field o (x) on a Riemann surface M, the lagrangian is given in terms of a function on the cotangent bundle T∗M which satisfies a non-linear differential constraint similar to one arising in the study of self-dual geometries in four dimensions. Explicit solutions of these constraints lead to particular parameterizations of the w∞-action. The symmetry group is a subgroup of the symplectic diffeomorphisms of T∗M.

Classical two-dimensional conformal field theories such as Toda models that are invariant under classical Virasoro or W-algebra symmetries with non-zero central charges are discussed. A naive gauging of such symmetries by coupling to gravity or W-gravity is shown not to work if there are central charges, and two successful ways of gauging are presented. The first is Weyl invariant and requires the introduction of spin-zero fields which can be thought of as gauge fields for gauge transformations generated by the central charges. The second involves, for the Virasoro algebra, a “non-geometric” coupling to all components of the metric and violates Weyl invariance. For the formulation with gauge fields corresponding to the central charges, BRST quantization requires ghosts for the transformations generated by the central charge. The resulting BRST charge is nilpotent for any value of the central charge, i.e. Q2 = 0 for any conformal field theory. However, there are no non-trivial BRST cohomology classes unless the total central charge, including ghost contributions, is zero. Non-critical strings are quantized using this approach. There are no anomalies for any value of the space-time dimension D and if D = 26, the usual string spectrum is obtained. However, if D ≠ 26, all Fock-space physical states are BRST-trivial, so that there are no particles in the spectrum. In this sense, non-critical strings can be thought of as topological field theories. Models in which the central charges arise dynamically are studied. In these models, the role of each central charge is played by the expectation value of a spin-zero field, the field equations of which force it to be constant. The field is the Noether current for a spin-zero symmetry. Examples include Toda theories coupled to certain topological field theories, with the “central charge scalars” arising as currents in the topological sector.