String Theory Seminar
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Mon, 21/04/2008 12:00 |
Mohab Abou Zeid (Institute for Mathematical Sciences) |
String Theory Seminar  |
L3 |
| I will present a modification of twistor string theory which gives the spectrum of super Yang-Mills theory coupled to Einstein supergravity instead of the higher derivative conformal supergravity arising in the original twistor strings of Witten and of Berkovits. After reviewing the world-sheet formulation of the Berkovits model, I will describe the symmetries of the so-called beta-gamma systems and their gauging. I will then explain how the analysis can be applied to the construction of a family of new gauged Berkovits twistor strings which are free from world-sheet anomalies. The new theories include one with the spectrum of N=8 supergravity, two theories with the spectrum of N=4 supergravity coupled to N=4 Yang-Mills, a family of N>0 models with the spectra of self-dual supergravity coupled to self-dual super-Yang-Mills, and a non-supersymmetric string with the spectrum of self-dual gravity coupled to self-dual Yang-Mills and a scalar. Time permitting, I will discuss what is known about the interactions in the new theories. | |||
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Mon, 28/04/2008 12:00 |
Spiro Karigiannis (Oxford) |
String Theory Seminar |
L3 |
Abstract: Compact  manifolds with isolated conical singularities arise naturally in M-theory. I will discuss such manifolds, and explain a method to “desingularize” them by glueing in pieces of asymptotically conical manifolds. There are topological obstructions to such desingularizations that depend on the rate of convergence to the cone at the singularities, and on the geometry of the links of the cones. If time permits, I will also briefly discuss a new related project with Dominic Joyce which could provide the first examples of such manifolds, as well as a possible new construction of smooth compact manifolds. |
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Mon, 05/05/2008 12:00 |
Paul Mansfield (Durham) |
String Theory Seminar |
L3 |
| Abstract: I will talk about how the reformulation of perturbative Yang-Mills theory in terms of MHV rules accounts for one-loop amplitudes for gluons of the same helicity, and some of the effects of introducing a regulator. | |||
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Mon, 12/05/2008 12:00 |
Fernando Quevedo (Cambridge) |
String Theory Seminar |
L3 |
| Abstract: Different phenomenological scenarios emerging from the exponentially large volume string constructions will be discussed with and without low energy supersymmetry. | |||
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Mon, 19/05/2008 12:00 |
Dan Freedman (Cambridge and MIT) |
String Theory Seminar |
L3 |
| Abstract: We study n-point tree amplitudes of N=4 super Yang-Mills theory and N=8 supergravity for general configurations of external particles of the two theories. We construct generating functions for n-point MHV and NMHV amplitudes with general external states. Amplitudes derived from them obey SUSY Ward identities, and the generating functions characterize and count amplitudes in the MHV and NMHV sectors. The MHV generating function provides an efficient way to perform the intermediate state helicity sums required to obtain loop amplitudes from trees. The NMHV generating functions rely on the MHV-vertex expansion obtained from recursion relations associated with a 3-line shift of external momenta involving a reference spinor |X]. The recursion relations remain valid for a subset of N=8 supergravity amplitudes although they do not vanish asymptotically for all |X]. The MHV-vertex expansion of the n-graviton NMHV amplitude for n=5,6,...,11 is independent of |X] and exhibits the asymptotic behavior z^{n-12}. This presages difficulties for n > 12. Generating functions show how the symmetries of supergravity can be implemented in the quadratic map between supergravity and gauge theory embodied in the KLT and other similar relations between amplitudes in the two theories. | |||
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Mon, 26/05/2008 10:00 |
Daniel Grayson (UIUC) |
String Theory Seminar |
L3 |
| Abstract: I will describe how computations are done using "Groebner bases" in quotient rings of polynomial rings, and I will describe explicitly the form of a particular Groebner basis for the ideal defining the ring parametrizing all factorizations of a monic polynomial of degree a+b+...+e into monic factors of degree a,b,...,e. That can be and is used in practice to compute intersection numbers involving of algebraic cycles arising as Chern classes on flag bundles of vector bundles. Simplest example: how many lines in 3-space meet four fixed lines? | |||
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Mon, 26/05/2008 12:00 |
Andy Neitzke (IAS, Princeton) |
String Theory Seminar |
L3 |
| Abstract: Quantum field theories and string theories constructed from geometric data, say Calabi-Yau threefolds, are expected to provide integer "invariants". In many cases these "invariants" display a rather intricate wall-crossing behavior. I will discuss two examples – one which was studied in the early 1990's by Cecotti-Vafa, and another currently under investigation by Kontsevich-Soibelman – emphasizing the close physical and mathematical parallels between them. | |||
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Wed, 04/06/2008 12:00 |
Giulia Zanderighi (Oxford) |
String Theory Seminar |
L3 |
| Abstract: We discuss recent techniques to compute one-loop amplitudes in QCD and show that all N-gluon one-loop helicity amplitudes can be computed numerically for arbitrary N with an algorithm which has a polynomial growth in N. | |||
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Mon, 09/06/2008 12:00 |
Dan Waldram (Imperial College) |
String Theory Seminar |
L3 |
| Abstract: We discuss an extension of Hitchin's generalised geometry, based on the exceptional groups, that provides a unified geometrical description of supersymmetric flux backgrounds in eleven-dimensional supergravity. We focus on N=1 seven-dimensional compactifications. The background is characterised by an element phi, the analogue of the generalised complex structure, that lies in a particular orbit of the 912 representation of E7. As an application we show that the four-dimensional effective superpotential takes a universal form, that is, a homogeneous E7-invariant functional of phi. | |||
