Past String Theory Seminar

20 October 2014
12:00
Andreas Braun
Abstract

I will discuss several recent developments regarding the construction of fluxes for F-theory on Calabi-Yau fourfolds. Of particular importance to the effective physics is the structure of the middle (co)homology groups, on which new results are presented. Fluxes dynamically drive the fourfold to Noether-Lefschetz loci in moduli space. While the structure of such loci is generally unknown for Calabi-Yau fourfolds, this problem can be answered in terms of arithmetic for K3 x K3 and a classification is possible.

  • String Theory Seminar
16 June 2014
12:00
Abstract
<em>I will discuss the validity of the claims that eternal inflation populates the landscape and show the reasons for the mathematical inconsistency between the two. Within the formalism of quantum cosmology, solutions for the wavefunction of the universe to the landscape are Anderson localized. The latter can give rise to observational tests of the landscape, some of which are already supported from Planck's findings of anomalies at large scales.</em>
  • String Theory Seminar
9 June 2014
12:00
Igor Buchberger
Abstract
<em>I will discuss four-dimensional minimally supersymmetric toroidal orientifold models with D-branes and worldvolume flux. With flux but no orbifold projection, these models are well understood. They have been used extensively in various phenomenological contexts, and are T-dual to D-branes at angles. With orbifold, supersymmetry is reduced also in the closed string sector, and T-duality relations are not always straightforward. I will discuss work in progress on aspects of the one-loop string effective actions of these more general models.</em>
  • String Theory Seminar
26 May 2014
12:00
Lara Anderson
Abstract
<em>We systematically analyze a broad class of dual heterotic and F-theory models that give four-dimensional supergravity theories, and compare the geometric constraints on the two sides of the duality. In this talk I will show that F-theory gives new insight into the conditions under which heterotic vector bundles can be constructed. We show that in many cases the F-theory geometry imposes a constraint on the extent to which the gauge group can be enhanced, corresponding to limits on the way in which the heterotic bundle can decompose. We explicitly construct all dual F-theory/heterotic pairs in the class under consideration where the common twofold base surface is toric, and give both toric and non-toric examples of the general results. Finally, we provide evidence for important new aspects of G-flux in four-dimensional compactifications.</em>
  • String Theory Seminar
19 May 2014
12:00
Abstract
<em> We will introduce a deformed version of the 3d hyperkahler sigma model which arises from the compactification of d=4,N=2 gauge theories on a Gibbons-Hawking space. After discussing extensions of the relevant hyperkahler identities from the standard story, we will derive the condition for which the deformed sigma model preserves 4 out of the 8 supercharges. Using supersymmetry considerations, we will also demonstrate that the contribution of the NUT center to the sigma model path integral is a holomorphic section of a certain holomorphic line bundle over the hyperkahler target. As a concrete example, we will discuss the case where the original 4d theory is a U(1) super Yang-Mills and show that the NUT center contribution in this case is the Jacobi theta function.</em>
  • String Theory Seminar
12 May 2014
12:00
Christoph Seig
Abstract
<em>The gamma_i-deformed N=4 SYM was proposed as the conformal field theory in a non-supersymmetric deformation of the AdS/CFT correspondence. As we have shown, conformal invariance is, however, broken in this theory by running double-trace couplings. Although these couplings are apparently suppressed in 't Hooft's planar limit, they give rise to finite size corrections in the planar spectrum. In particular, they should be considered in the integrability-based formulation of the planar spectral problem. In the talk, I will explain our results in detail and also discuss possible implications for the conjectured integrability-based approach.</em>
  • String Theory Seminar
5 May 2014
12:00
Mathew Bullimore
Abstract
<em>String theory predicts the existence of a class of interacting superconformal field theories in six dimensions which arise on the world-volume of coincident M5 branes. There are important non-local operators in these theories corresponding to intersecting M2 and M5 branes. I will explain how to compute the superconformal index in the presence of such operators using five-dimensional supersymmetric gauge theory. The answers are in 1-1 correspondence with characters of representations of a class of `chiral algebras’. I will discuss potential applications of this result for bootstrapping correlation functions.</em>
  • String Theory Seminar
28 April 2014
12:00
Xenia de la Ossa
Abstract
<em>We describe the tangent space to the moduli space of heterotic string theory compactifications which preserve N=1 supersymmetry in four dimensions, that is, the infinitesimal parameter space of the Strominger system. We establish that if we promote a connection on TX to a field, the moduli space corresponds to deformations of a holomorphic structure \bar{D} on a bundle Q. The bundle Q is constructed as an extension by the cotangent bundle T^*X of the bundle E= End(V) \oplus End(TX) \oplus TX with an extension class {\cal H} which precisely enforces the anomaly cancelation condition. The deformations corresponding to the bundle E are simultaneous deformations of the holomorphic structures on the poly-stable bundles V and TX together with those of the complex structure of X. We discuss the fact that the ``moduli'' corresponding to End(TX) cannot be physical, but are however needed in our mathematical structure to be able to enforce the anomaly cancelation condition. This is work done in collaboration with Eirik Svanes.</em>
  • String Theory Seminar
10 March 2014
12:00
James Drummond
Abstract
<em> We describe the analytic properties of scattering amplitudes in N=4 super Yang-Mills theory, with the focus on high order corrections to the six-particle MHV amplitude. By making an ansatz for the analytic structure and imposing physical constraints, including matching the BFKL expansion in multi-Regge kinematics and the operator product expansion for the dual Wilson loop in the near-collinear regime, we are able to explicitly construct the amplitude to four loops in perturbation theory.</em>
  • String Theory Seminar
3 March 2014
12:00
Toby Wiseman
Abstract
<em>I will review the link between 1+p dimensional maximally supersymmetric Yang-Mills and the black hole thermodynamics of Dp-branes via the gauge/string correspondence. The finite temperature behaviour of Dp-brane supergravity black holes looks very alien from the perspective of the dual strongly coupled Yang-Mills. However, I will argue that in a natural set of Yang-Mills variables, the classical moduli (which unfortunately are still strongly coupled), certain features of these thermodynamics become quite transparent. A physical picture then emerges of the black holes as a strongly interacting 'soup' of these moduli.</em>
  • String Theory Seminar

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