String Theory Seminar
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Mon, 27/04/2009 12:00 |
Andrew Hodges (Oxford) |
String Theory Seminar  |
L3 |
| Abstract: The recent paper by Arkani-Hamed, Cachazo, Cheung and Kaplan on 'The S-matrix in Twistor Space' (hep-th/0903.2110v2) has envigorated the project of expressing scattering amplitudes for (supersymmetric) gauge theory and gravity entirely in terms of twistor geometry. I shall review these new developments of twistor diagram theory, with some illustrations of its computational value. I shall also emphasise the many outstanding problems in the formalism. One of these, which Arkani-Hamed has highlighted, is the asymmetry in the representation of the amplitudes and the 'spurious poles' that arise. So far, the twistor diagram formalism has simply reproduced the less than satisfactory features of the (supersymmetrised) BCFW recursion. I will outline some new twistor-geometric results which address and partially resolve this problem. | |||
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Mon, 04/05/2009 12:00 |
Ilarion Melnikov (Potsdam) |
String Theory Seminar |
L3 |
| Abstract: I will discuss techniques for the computation of correlators in (0,2) Landau-Ginzburg models. After introducing these theories from the point of view of heterotic compactifications, I will describe the associated half-twisted models and their basic algebraic structure. This structure enables direct computation of correlators and suggests a generalization of the Grothendieck residue. | |||
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Mon, 11/05/2009 12:00 |
David Skinner (Oxford) |
String Theory Seminar |
L3 |
| Abstract: Modern techniques for computing multi-particle and multi-loop scattering amplitudes rely on a sophisticated use of on-shell recursion relations and generalised unitarity methods. I will show that these methods are ideally suited to interpretation in twistor space, where superconformal properties become manifest. In fact, the recursion relations of Britto, Cachazo, Feng & Witten provide a clear framework for the twistor diagram program initiated in the 1970s. Tree-level scattering amplitudes in N=4 SYM are now known to possess a Yangian symmetry, formed by combining the original PSU(2,2|4) superconformal invariance with a second "dual" copy. I will also discuss very recent work constructing scattering amplitudes in a twistor space in which this dual superconformal symmetry acts geometrically. | |||
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Mon, 18/05/2009 12:00 |
Fay Dowker (Imperial College) |
String Theory Seminar |
L3 |
| Abstract: Despite the high regard in which physicists hold General Relativity, the spacetime nature of reality has not yet fully been taken to heart in addressing the question of the interpretation of quantum mechanics. Partial progress was made by Dirac and Feynman by casting the dynamical content of quantum theory in terms of a Sum Over (spacetime) Histories (SOH). Recently it has been suggested by Sorkin that this SOH is part of an interpretive framework in which the rules of inference that are used to reason about physical reality are themselves subject to dynamical law. Just as General Relativity showed that geometry is not fixed and absolute, so Quantum Mechanics may be telling us that logical rules of inference are not fixed but part of physics. | |||
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Mon, 25/05/2009 12:00 |
John Dixon |
String Theory Seminar |
L3 |
| Abstract: Cybersusy is a new approach to supersymmetry breaking, based on the BRS cohomology of composite operators in the supersymmetric standard model (analyzed using spectral sequences). The cohomology generates a new kind of supersymmetry algebra and a new effective action. When the gauge symmetry is broken (from the vacuum expectation value of a scalar field), supersymmetry breaking is also induced. Applied to the leptons, the result is consistent with experiment, and the vacuum energy remains zero, and no annoying mass sum rules are present. | |||
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Mon, 01/06/2009 12:00 |
David Tong (Cambridge) |
String Theory Seminar |
L3 |
| Abstract: I will give an introduction to the phenomenon of non-Abelian Berry phase. This process, which describes the holonomy of degenerate quantum states as parameters are varied, is governed by a non-Abelian gauge connection. I will explain why this set of ideas is particularly natural in supersymmetric quantum mechanics and will show that the connection is governed by well known equations of mathematical physics such as the Hitchin equation, the Bogomolnyi equation and generalizations. | |||
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Mon, 15/06/2009 12:00 |
Sergei Dubovsky (Stanford) |
String Theory Seminar |
L3 |
| String theory suggests the simultaneous presence of many ultralight axions possibly populating each decade of mass down to the Hubble scale 10^-33eV. Conversely the presence of such a plenitude of axions (an "axiverse") would be evidence for string theory, since it arises due to the topological complexity of the extra-dimensional manifold and is ad hoc in a theory with just the four familiar dimensions. We investigate how upcoming astrophysical experiments will explore the existence of such axions over a vast mass range from 10^-33eV to 10^-10eV. Axions with masses between 10^-33eV to 10^-28eV cause a rotation of the CMB polarization that is constant throughout the sky. The predicted rotation angle is of order \alpha~1/137. Axions in the mass range 10^-28eV to 10^-18eV give rise to multiple steps in the matter power spectrum, that will be probed by upcoming galaxy surveys and 21 cm line tomography. Axions in the mass range 10^-22eV to 10^-10eV affect the dynamics and gravitational wave emission of rapidly rotating astrophysical black holes through the Penrose superradiance process. When the axion Compton wavelength is of order of the black hole size, the axions develop "superradiant" atomic bound states around the black hole "nucleus". Their occupation number grows exponentially by extracting rotational energy from the ergosphere, culminating in a rotating Bose-Einstein axion condensate emitting gravitational waves. This mechanism creates mass gaps in the spectrum of rapidly rotating black holes that diagnose the presence of axions. The rapidly rotating black hole in the X-ray binary LMC X-1 implies an upper limit on the decay constant of the QCD axion f_a<2*10^17GeV, much below the Planck mass. This reach can be improved down to the grand unification scale f_a<2*10^16GeV, by observing smaller stellar mass black holes. | |||
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Fri, 19/06/2009 16:15 |
Nima Arkani-Hamed (Institute for Advanced Study) |
String Theory Seminar |
NAPL |
