Forthcoming events in this series


Mon, 07 Jun 2010

12:00 - 13:00
L3

The torsional conifold: fivebranes and the Klebanov-Strassler theory

Dario Martelli
(Kings College London)
Abstract
We study a gravity solution corresponding to fivebranes wrapped on the S^2 of the resolved conifold. By changing a parameter the solution continuously interpolates between the deformed conifold with flux and the resolved conifold with branes. Therefore, it displays a geometric transition, purely in the supergravity context. The solution is a simple example of torsional geometry and may be thought of as a non-Kahler analog of the conifold. By U-duality transformations we can add D3 brane charge and recover the solution in the form originally derived by Butti et al. This describes the baryonic branch of the Klebanov-Strassler theory. Far along the baryonic branch the field theory gives rise to a fuzzy two-sphere. This corresponds to the D5 branes wrapping the two-sphere of the resolved conifold in the gravity solution.
Mon, 24 May 2010

12:00 - 13:00
L3

String Theory and Many-Body Physics

Djordje Minic
(VirginiaTech)
Abstract
Recent theoretical advances in string theory relate in an unexpected way the physics of gravity in certain D dimensional space-times with the dynamics of quantum field theories living on the associated (D-1) dimensional space-time boundary. This unsuspected relationship offers a remarkable new tool for dealing with some outstanding problems in condensed matter physics. In the first part of the talk I aim to explain both the intuitive and technical underpinnings of these new developments. In the second half of the talk I will present some recent results on aging in systems far from equilibrium and also some new avenues for research in condensed matter physics which involve the interplay of gauge fields, membranes and many-body systems. In particular this last work opens up an exciting possibility for fundamentally new states of condensed matter.
Mon, 17 May 2010

12:00 - 13:00
L3

Aspects of heterotic Calabi-Yau compactifications

James Gray
(Oxford)
Abstract
I will discuss various aspects of Calabi-Yau compactifications appropriate for use in models of string phenomenology. Topics covered will include transitions between and deformations of bundles as well as consequences of stability walls for phenomenology.
Mon, 10 May 2010

12:00 - 13:00
L3

Crystal Melting and Wall Crossing for Donaldson-Thomas Invariants

Masahito Yamazaki
(Tokyo)
Abstract
I will describe the wall crossing phenomena for (generalized) Donaldson-Thomas invariants (also known as BPS invariants) from a physicist's perspective; the topics include crystal melting and its thermodynamic limit, M-theory derivation of wall crossing, and open wall crossing.
Mon, 26 Apr 2010

12:00 - 13:00
L3

On Fields over Fields

Yang-Hui He
(Oxford)
Abstract
We investigate certain arithmetic properties of field theories. In particular, we study the vacuum structure of supersymmetric gauge theories as algebraic varieties over number fields of finite characteristic. Parallel to the Plethystic Programme of counting the spectrum of operators from the complex geometry, we investigate the Hasse-Weil zeta functions and the associated Dirichlet expansions. We find curious dualities wherein the geometrical properties and asymptotic behaviour of one gauge theory is governed by the number theoretic nature of another.
Mon, 22 Feb 2010

12:00 - 13:00
L3

Generalized scaling and integrability from AdS5 x S5

Riccardo Ricci
(Imperial College)
Abstract
According to AdS/CFT a remarkable correspondence exists between strings in AdS5 x S5 and operators in N=4 SYM. A particularly important case is that of fast-spinning folded closed strings and the so called twist-operators in the gauge theory. This is a remarkable tool for uncovering and checking the detailed structure of the AdS/CFT correspondence and its integrability properties. In this talk I will show how to match the expression of the anomalous dimension of twist operators as computed from the quantum superstring with the result obtained from the Bethe ansatz of SYM. This agreement resolves a long-standing disagreement between gauge and string sides of the AdS/CFT duality and provides a highly nontrivial strong coupling test of SYM integrability.
Mon, 15 Feb 2010

12:00 - 13:00
L3

N=2 Superconformal Theories and M5 branes

Sergio Benvenuti
(Imperial College)
Abstract
In 2009 there was progress in understanding and classifying the set of four dimensional field theories with N=2 SUSY. These models arise as M5 branes wrapped over a Riemann surface. We review this construction and describe a five dimensional point of view, using (p,q)-webs of 5-branes in Type IIB string theory. This point of view makes many properties of the theories explicit. We will also touch on the AGT correspondence, that associates a 2-dimensional CFT, similar to the Liouville CFT, to the protected sector of four dimensional N=2 models.
Mon, 08 Feb 2010

12:00 - 13:00
L3

Holographic Superconductors in M-Theory

Jerome Gauntlett
(Imperial College)
Abstract
By constructing black hole solutions of D=11 supergravity we analyse the phase diagram of a certain class of three dimensional conformal field theories at finite temperature and finite charge density. The system exhibits superconductivity at lotemperatures and furthermore at zero tmeperature and finite charge density the system exhibits an emergent quantum critical behaviour with conformal symmetry. The construction of the black hole solutions rely on a new understanding of Kaluza-Klein reductions on seven dimensional Sasaki-Einstein manifolds.
Mon, 01 Feb 2010

12:00 - 13:00
L3

Twistor-Strings, Grassmannians and Leading Singularities

Lionel Mason
(Oxford)
Abstract
A systematic procedure is derived for obtaining an explicit, L-loop leading singularities of planar N=4 super Yang-Mills scattering amplitudes in twistor space directly from their momentum space channel diagrams. The expressions are given as integrals over the moduli of connected, nodal curves in twistor space whose degree and genus matches expectations from twistor-string theory. We propose that a twistor-string theory for pure N=4 super Yang-Mills, if it exists, is determined by the condition that these leading singularity formulae arise as residues when an unphysical contour for the path integral is used, by analogy with the momentum space leading singularity conjecture. We go on to show that the genus g twistor-string moduli space for g-loop N^{k-2}MHV amplitudes may be mapped into the Grassmannian G(k,n). Restricting to a leading singularity, the image of this map is a 2(n-2)-dimensional subcycle of G(k,n) of exactly the type found from the Grassmannian residue formula of Arkani-Hamed, Cachazo, Cheung and Kaplan. Based on this correspondence and the Grassmannian conjecture, we deduce restrictions on the possible leading singularities of multi-loop N^pMHV amplitudes. In particular, we argue that no new leading singularities can arise beyond 3p loops.
Mon, 25 Jan 2010

12:00 - 13:00
L3

Scanning through Heterotic Vacua

Yang-Hui He
(Oxford)
Abstract
We discuss some recent progress in obtaining the exact spectrum of the MSSM from a generalized embedding of the heterotic string. Utilizing current developments in algebraic geometry, especially algorithmic, we search through the landscape of vector bundles over Calabi-Yau manifolds for a special corner wherein such exact models may be found.
Mon, 18 Jan 2010

12:00 - 13:00
L3

T-Duality Invariant String Theory at the Quantum Level

Daniel Thompson
(Queen Mary, UL)
Abstract

In this talk I will be discussing some reformulations of string theory which promote T-duality to the level of a manifest symmetry namely Hull's Doubled Formalism and Klimcik and Severa's  Poisson-Lie T-duality.   Such formalisms double the number of fields but also incorporate some chirality-like constraint. Invoking this constraint leads one to consider sigma-models which, though duality invariant, do not possess manifest Lorentz Invariance.   Whilst such formalisms make sense at a classical level their quantum validity is less obvious.  I address this issue by examining the renormalization of these duality invariant sigma models.  This talk is based upon both forthcoming work and recent work in arXiv:0910.1345 [hep-th] and its antecedents arXiv:0708.2267, arXiv:0712.1121.

Mon, 30 Nov 2009

12:00 - 13:00
L3

Computational Challenges in Calabi-Yau and String Phenomenology

Maximillian Kreuzer
(Technische Universitaet Wien)
Abstract
I discuss some theorems and algorithms that we use for enumerating reflexive polytopes and related objects, as well as problems and examples that are of interest in both algebraic geometry and string phenomenology. I would also like to exchange ideas about possible synergies between the numerous current computational activities in the field.
Mon, 16 Nov 2009

12:00 - 13:00
L3

M2-branes at hypersurface singularities and their deformations

James Sparks
(Oxford)
Abstract
I will introduce a family of supersymmetric Chern-Simons-matter theories in d=2+1 dimensions, labelled by a positive integer n, and argue that these describe the low-energy worldvolume theory of M2-branes at a corresponding family of four-fold hypersurface singularities. There are dual descriptions in Type IIA involving a family of three-fold hypersurface singularities, and also a Type IIB dual of Hanany-Witten type involving D3-branes suspended between 5-branes. The n=1 theory has manifest N=6 superconformal symmetry and is the Aharony-Bergman-Jafferis-Maldacena theory on an M2-brane in flat spacetime. The n>1 theories are not conformal: however, the n>2 theories are all argued to flow to the same superconformal IR fixed point, while the n=2 theory flows to a theory that is AdS/CFT dual to a certain homogeneous Sasaki-Einstein 7-manifold. This is the base of the four-fold "conifold" singularity, and the smooth deformation of this singularity is interpreted as a particular mass deformation in the field theory. The IR theory of this deformation is conjecturally confining.
Mon, 09 Nov 2009

12:00 - 13:00
L3

On the classification of Brane Tilings

Amihay Hanany
(Imperial College)
Abstract
Brane Tilings give a large class of SCFT's in 3+1 and 2+1 dimensions. In this talk I will discuss several attempt to classify all such models. Statistical properties of these models can be derived using some techniques in number theory.
Mon, 02 Nov 2009

12:00 - 13:00
L3

Dynamical Vacuum Selection and Supersymmetry Breaking in String Theory

Jock McOrist
(Cambridge)
Abstract
Intersecting brane models in string theory have proven a useful tool for studying the dynamics of quantum field theories. I will describe how certain brane models may be used to shed light on the phenomenon of supersymmetry breaking and vacuum selection in a cosmological context.
Mon, 26 Oct 2009

12:00 - 13:00
L3

Gauge Threshold Corrections for Local String Models

Joe Conlon
(Oxford)
Abstract
Local string models are those where Standard Model degrees of freedom arise on a small region inside a large bulk volume. I study threshold effects on gauge coupling running for such models. The Kaplunovsky-Louis formula for locally supersymmetric gauge theories predicts the unification scale should be the bulk winding mode scale, parametrically large than the string scale where divergences are naively cut off. Analysis of explicit string models on orbifold/orientifold geometries confirms this; the winding mode scale arises from the presence of tadpoles uncancelled in the local model. I briefly discuss phenomenological applications to supersymmetry breaking and gauge coupling unification.
Mon, 19 Oct 2009

12:00 - 13:00
L3

A CY Manifold with 3 Generations and Small Hodge Numbers

Philip Candelas
(Oxford)
Abstract
I will discuss a Calabi-Yau manifold which admits free actions by Abelian and non-Abelian groups of order 12. The quotient manifolds have Euler number -6 and Hodge numbers (h^{11}, h^{21}) = (1,4). Apart from the various presentations of the Yau Manifold, that have Hodge numbers (6,9), this is the only other complete intersection CY manifold to admit a free quotient with Euler number -6 and hence three generations of particles with the standard embedding. I will discuss the spectrum of light particles and the possibility of a transgression to a heterotic vacuum on a manifold with Hodge numbers (2,2).
Mon, 12 Oct 2009

12:00 - 13:00
L3

CANCELLED

Marni Sheppeard
(Oxford)
Mon, 15 Jun 2009

12:00 - 13:00
L3

String Axiverse

Sergei Dubovsky
(Stanford)
Abstract

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