String Theory Seminar (past)
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Mon, 13/05 12:00 |
Pau Figueras (DAMTP) |
String Theory Seminar  |
L3 |
| In this talk I will explain a new method to numerically construct stationary black holes with non-Killing horizons. As an example, I will use AdS/CFT to describe a time-independent CFT plasma flowing through a static spacetime which asymptotes to Minkowski in the flow's past and future, with a varying spatial geometry in-between. When the boundary geometry varies slowly, the holographic stress tensor is well-described by viscous hydrodynamics. For fast variations it is not, and the solutions are stationary analogs of dynamical quenches, with the plasma being suddenly driven out of equilibrium. We find evidence that these flows become unstable for sufficiently strong quenches and speculate that the instability may be turbulent. The gravitational dual of these flows are the first examples of stationary black holes with non-Killing horizons. | |||
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Mon, 06/05 12:00 |
Mario Garcia Fernandez (EPFL) |
String Theory Seminar |
L3 |
| I will present a new derivation of the equations of motion of Heterotic supergravity using generalized geometry, inspired by the geometric description of 11-dimensional and type II supergravity by Coimbra, Strickland-Constable and Waldram. From a mathematical point of view, this arises from the study of torsion-free generalized connections on a non-exact Courant algebroid. We will find that the freedom provided by the dilaton field in the physical theory can be interpreted as the freedom of choice of Levi-Civita connection in generalized geometry. | |||
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Mon, 29/04 12:00 |
Rhys Davies (Oxford) |
String Theory Seminar |
L3 |
| I will discuss a class of isolated singularities, given by finite cyclic quotients of a threefold node (conifold), which arise naturally in compact Calabi-Yau threefolds. These singularities admit projective crepant resolutions, and thereby give rise to topological transitions between compact Calabi-Yau spaces. Among the interesting properties of such 'hyperconifold transitions' is that they can change the fundamental group, and are related by mirror symmetry to familiar conifold transitions. Having established these mathematical properties, I will briefly discuss some applications, as well as the physics of type IIB string theory compactified on a space with a hyperconifold singularity. | |||
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Mon, 25/02 12:00 |
Lotte Hollands (Oxford) |
String Theory Seminar |
L3 |
| Fenchel-Nielsen coordinates play a central role in constructing partition functions for theories of class S with gauge group SU(2). Having an analogue of these coordinates for higher rank gauge groups is a first step in finding partition functions for strongly coupled gauge theories of the Minahan-Nemeschansky type. We find such a generalization through the formalism of spectral networks and the non-abelianization map, that was originally introduced by Gaiotto, Moore and Neitzke to find a better understanding of BPS states in the theories of class S. This is joint work with Andy Neitzke. | |||
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Mon, 18/02 12:00 |
Mike Duff (Imperial College) |
String Theory Seminar |
L3 |
| I will give a division algebra R,C,H,O description of D = 3 Yang-Mills with N = 1,2,4,8 and hence, by tensoring left and right multiplets, a magic square RR, CR, CC, HR, HC, HH, OR, OC, OH, OO description of D = 3 supergravity with N = 2, 3, 4, 5, 6, 8, 9, 10, 12, 16. | |||
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Mon, 11/02 12:00 |
David Marsh (Oxford) |
String Theory Seminar |
L3 |
| I will describe recent efforts to understand the mediation of supersymmetry breaking in stabilized compactifications of type IIB string theory. By geometrically separating the visible sector from the supersymmetry breaking effects one may hope to achieve sequestered supersymmetry breaking and much ameliorated constraints from bounds on flavor changing neutral currents. However, in this talk I will discuss how non-perturbative superpotential cross-couplings between the visible sector and the Kähler moduli may spoil sequestering and introduce a sensitivity to the global details of the compactification. As a simple example, I will describe the structure of these `de-sequestering’ operators for a class of visible sectors realized by D-branes probing an orbifold singularity, and I will discuss their importance in the KKLT and LVS moduli stabilization scenarios. | |||
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Mon, 04/02 12:00 |
Lotte Hollands (Oxford) |
String Theory Seminar |
L3 |
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Mon, 28/01 12:00 |
Mariana Graña (CEA/Saclay) |
String Theory Seminar |
L3 |
| We will discuss supersymmetric reductions of type II and M-theory down to four dimensions, in the language of exceptional generalized geometry (EGG). EGG is the extension of generalized complex geometry which also geometrizes the RR fields, and is therefore the relevant language to use in M-theory. After a brief introduction to EGG, we will present the algebraic structures that encode all information about the lower-dimensional action, concentrating on the case of N=2 supersymmetry. We will show, in particular, that these structures have a nice description using an 8-dimensional tangent space, where they look like pure spinors as in generalized complex geometry. | |||
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Mon, 21/01 12:00 |
Miranda Cheng (Jussieu) |
String Theory Seminar |
L3 |
| Mock modular forms are generalizations of modular forms first introduced by Ramanujan. Their properties had been mysterious for 80 years until various breakthroughs in the past 10 years. In the last century, the Monstrous Moonshine Conjecture initiated the study of the fascinating relation between modular forms and sporadic groups. In this talk I will report a conjecture on a new type of "umbral moonshine" relating a set of mock modular forms, including many of Ramanujan's original examples, and the representation theory of a set of finite groups. One instance of such a surprising umbral moonshine phenomenon relates the largest Mathieu group to the elliptic genus of K3 surfaces, as was first observed by Euguchi-Ooguri-Tachikawa in 2010. Moreover, there are hints suggesting that all occurrences of umbral moonshine have a close relation to K3-compactifications of string theory. However, despite of these tantalising hints the origin and the explanation of this umbral moonshine is still unclear at the moment. This talk is based on the arXiv pre-print: 1201.4140, 1204.2779 with John Duncan and Jeff Harvey. | |||
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Mon, 14/01 12:00 |
Magdalena Larfors (Oxford) |
String Theory Seminar |
L3 |
| Strings are extended objects, and this means that they can be compactified not only on Riemannian manifolds, but also on more exotic spaces with generalized transition functions. An example of this is the T-fold, where T-duality is used to glue the Neveu-Schwarz fields of the background. Collectively, these exotic set-ups are known as non-geometric string compactifications, and in this talk I will discuss two of their aspects. First, I will present a field redefinition in the Neveu-Schwarz sector that allows a ten-dimensional, effective description of certain non-geometric backgrounds. This redefinition is inspired by Generalized Geometry and Double Field Theory. Second, I will show that closed strings become non-commuting when non-geometric fluxes are turned on. This will be done through the analysis of a three-torus with H-flux and its T-dual configurations. | |||
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Mon, 26/11/2012 12:00 |
James Gray (LMU Munich) |
String Theory Seminar |
L3 |
| I will describe methods for searching for bundles which are only holomorphic for isolated complex structures of a base Calabi-Yau threefold. These can be used, in the hidden sector of heterotic compactifications, to stabilize the associated moduli fields. Various bundle constructions will be covered, and the possibility and consequences of resolving the potentially singular threefolds which result will be discussed. If time permits, I will also briefly mention a large set of Calabi-Yau fourfolds which is currently being classified. | |||
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Fri, 23/11/2012 12:00 |
Jonathan Toledo (Perimeter Institute) |
String Theory Seminar |
Gibson 1st Floor SR |
| We consider the strong coupling limit of 4-point functions of heavy operators in N=4 SYM dual to strings with no spin in AdS. We restrict our discussion for operators inserted on a line. The string computation factorizes into a state-dependent sphere part and a universal AdS contribution which depends only on the dimensions of the operators and the cross ratios. We use the integrability of the AdS string equations to compute the AdS part for operators of arbitrary conformal dimensions. The solution takes the form of TBA-like integral equations with the minimal AdS string-action computed by a corresponding free-energy-like functional. These TBA-like equations stem from a peculiar system of functional equations which we call a \chi-system. In principle one could use the same method to solve for the AdS contribution in the N-point function. An interesting feature of the solution is that it encodes multiple string configurations corresponding to different classical saddle-points. The discrete data that parameterizes these solutions enters through the analog of the chemical-potentials in the TBA-like equations. Finally, for operators dual to strings spinning in the same equator in S^5 (i.e. BPS operators of the same type) the sphere part is simple to compute. In this case (which is generically neither extremal nor protected) we can construct the complete, strong-coupling 4-point function. | |||
-Systems for Correlation Functions|
Mon, 19/11/2012 12:00 |
Sara Pasquetti (University of Surrey) |
String Theory Seminar |
L3 |
| We show that sphere partition functions and indices of 3 dimensional, N = 2, gauge theories can be decomposed into a sum of products of a universal set of holomorphic blocks. The blocks count BPS states of a theory on R2 × S1 and are in one-to-one correspondence with the theory’s massive vacua. The blocks turn out to have a wealth of surprising properties such as a Stokes phenomenon and have interesting dual interpretations in analytically continued Chern-Simons theory and open topological strings. | |||
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Mon, 12/11/2012 12:00 |
Arthur Lipstein (Oxford) |
String Theory Seminar |
L3 |
| I will describe scattering amplitudes of 3d Yang-Mills and Chern-Simons theories and what they may imply about string theory and M-theory. | |||
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Mon, 05/11/2012 12:00 |
Sakura Schafer-Nameki (Kings College London) |
String Theory Seminar |
L3 |
| F-theory compactifications on singular elliptic Calabi-Yau fourfolds provide an ideal framework to study supersymmetric Grand Unified Theories. Recent years have seen much progress in local F-theory model building. Understanding the global constraints for realizing local models are key in estabilishing a consistent F-theoretic realization. We will address these questions by analyzing the structure of the singular elliptic CY fourfolds, which form the geometric foundation for these compactification, as well as the construction of globally consistent G_4 flux. | |||
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Mon, 29/10/2012 12:00 |
Magdalena Larfors (Oxford) |
String Theory Seminar |
L3 |
| In the absence of background fluxes and sources, the compactification of string theories on Calabi-Yau threefolds leads to supersymmetric solutions.Turning on fluxes, e.g. to lift the moduli of the compactification, generically forces the geometry to break the Calabi-Yau conditions, and to satisfy, instead, the weaker condition of reduced structure. In this talk I will discuss manifolds with SU(3) structure, and their relevance for heterotic string compacitications. I will focus on domain wall solutions and how explicit example geometries can be constructed. | |||
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Mon, 22/10/2012 12:00 |
Jock McOrist (University of Surrey) |
String Theory Seminar |
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| Even once the F-theory dust has settled, the heterotic string remains a viable route to N=1 d=4 phenomenology and is a fertile ground for developing the mathematics of holomorphic vector bundles. Within this context, there has been recent progress in using worldsheet techniques to understand the F-terms of certain heterotic compactifications. Less is understood about their D-term cousins. In this talk I will describe some steps towards rectifying this, writing down a moduli space metric for vector bundle deformations and describing some of its properties. Such metrics are relevant physically ( to normalise Yukawa couplings) as well as in the mathematics of vector bundles (they extend the metric of Kobayashi). | |||
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Mon, 15/10/2012 12:00 |
Andrei Constantin (Oxford) |
String Theory Seminar |
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| Even a cursory inspection of the Hodge plot associated with Calabi-Yau threefolds that are hypersurfaces in toric varieties reveals striking structures. These patterns correspond to webs of elliptic-K3 fibrations whose mirror images are also elliptic-K3 fibrations. Such manifolds arise from reflexive polytopes that can be cut into two parts along slices corresponding to the K3 fibers. Any two half-polytopes over a given slice can be combined into a reflexive polytope. This fact, together with a remarkable relation on the additivity of Hodge numbers, explains much of the structure of the observed patterns. | |||
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Mon, 08/10/2012 12:00 |
Philip Candelas (Oxford) |
String Theory Seminar |
L3 |
I will discuss some of the subtleties involved in counting lines on Calabi-Yau threefolds and then discuss the lines on the Dwork pencil of quintic threefolds. It has been known for some time that the manifolds of the pencil contain continuous families of lines and it is known from the work of Angca Mustata that there are 375 discrete lines and also two families parametrized by isomorphic curves that are 125:1 covers of genus six curves  . The surprise is that an explicit parametrization of these families is not as complicated as might have been anticipated. We find, in this way, what should have anticipated from the outset, that the curves  are the curves of the Wiman pencil. |
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Mon, 11/06/2012 12:00 |
David Shih (Rutgers University) |
String Theory Seminar |
L3 |
