Theoretical Particle Physics

Joint seminars academic year: 2008-2009

Recent developments in 3D gravity

Speaker :Niklas Johansson (Uppsala University)
Date: Wednesday, October 1, 2008 - 12:30
Location: VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
A consistent quantization of three-dimensional AdS gravity would be of great interest, since this theory contains black holes. Recently, a chiral version of 3D topologically massive gravity was constructed. This theory has many appealing features. Its CFT dual was conjectured to be purely right moving, and therefore holomorphic. In this talk we review this construction, and the sometimes heated debate regarding its properties.


2008 Solvay chair in physics: inaugural lecture

Speaker: David Gross (Santa Barbara)
Date: Friday, October 3, 2008 - 16:00
Location: VUB/ULB, Campus Plaine, Forum A

The programme of the 2008 Solvay chair in physics.


Amsterdam-Brussels-Paris Doctoral School

Speaker: Brussels - September 22 - October 10
Date:Friday, October 10, 2008 (All day)

    String theory, Conformal Field Theory - Angel Uranga (CERN)
    Advanced Quantum Field Theory - Adel Bilal (ENS)
    General Relativity, Cosmology and Black Holes - Veronika Hubeny (Durham)
    QCD and the Large N Limit - David Gross (Santa Barbara - 2008 Solvay chair in physics)

Visit the website.


Beta-deformation for the matrix model of M-Theory

Speaker: Hidehiko Shimade (AEI Potsdam)
Date: Wednesday, October 15, 2008 - 10:30
Location: VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
I will discuss a new deformation of the matrix model of M-theory. It is motivated by the so-called β-deformation of D=4 N=4 super Yang-Mills theory, which preserves the conformal symmetry. I will show that the deformed matrix model can be considered as the matrix model for a certain eleven-dimensional supergravity background (the pp-wave background with a certain non-constant four-form flux). The deformed model admits stable solutions corresponding to membranes with torus topology. I will discuss some properties of these stable solutions.


Statistical predictions from field theory landscapes

Speaker: Vijay Balasubramanian (University of Pennsylvania)
Date: Wednesday, October 15, 2008 - 12:45
Location: VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay


A Stress Tensor for Asymptotically Flat Gravity

Speaker: Amitabh Virmani (ULB)
Date: Wednesday, October 22, 2008 - 13:00
Location: Leuven, Celestijnenlaan 200D 06.32
In this talk I will show that the addition of an appropriate boundary term to the gravitational action yields a well-defined variational principle for asymptotically flat spacetimes, and thus leads to a natural definition of conserved quantities at spatial infinity. I will connect such results to other formalisms by showing that i) the canonical form of the action is the ADM action, and ii) that the conserved quantities defined by stress-tensor methods agree with the Ashtekar-Hansen conserved quantities.


The Dual Graviton and E11

Speaker: Olaf Hohm (University of Groningen)
Date: Wednesday, October 29, 2008 - 10:30
Location: Leuven, Celestijnenlaan 200D 06.34
In this talk I review work done in collaboration with N. Boulanger on the construction of a parent action for the dual graviton. This is inspired by the conjectured E11 structure underlying supergravity theories, which predicts the presence of a dual graviton. A parent action is determined, which contains the ordinary graviton together with its dual and which is classically equivalent to Einstein gravity. I discuss the resulting gauge symmetries and comment on their meaning with regard to the E11 proposal.


Compactification of Higher Derivate Corrections and Automorphic Forms

Speaker:Daniel Persson (ULB)
Date:Wednesday, October 29, 2008 - 13:00
Location:Leuven, Celestijnenlaan 200D 06.32
Toroidal compactification of supergravity reveals a hidden global symmetry G(R) in three dimensions. In this talk I will discuss how discrete U-duality subgroups G(Z) in D=3 can be used to analyze non-perturbative contributions to the effective action. In particular, we consider the compactification to three dimensions of higher derivative corrections, and show through general arguments that the symmetry group G(R) is explicitly broken in the resulting action. I will then argue that invariance under a discrete subgroup G(Z) may be restored upon including non-perturbative effects. This "non- perturbative completion" requires the introduction of G(Z)-invariant automorphic forms. After this general discussion we consider the explicit example of quartic derivative corrections to the universal sector of IIA Calabi-Yau compactifications, which at the two- derivative level exhibits a global SU(2,1)-symmetry upon reduction to D=3. In this setup one may explicitly construct an automorphic form invariant under a discrete subgroup of SU(2,1), which incorporates the contribution from expected D2- and NS5-brane instanton effects.


Operator Mixing and the AdS/CFT correspondence

Speaker:Rodolfo Russo (Queen Mary)
Date:Wednesday, November 5, 2008 - 10:30
VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
I will present a direct prescription for computing the mixing among gauge invariant operators in N=4 SYM. This approach is based on the action of the superalgebra on the states of the theory and thus it can be also applied to resolve the mixing in the dual string description. I will present some explicit examples and discuss the relevance of these results for the computation of the 3-point correlators.


Strings (etcetera) in singular gravitational waves

Speaker:Oleg Evnin (VUB)
Date:Wednesday, November 5, 2008 - 13:00
Location:VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
I'll discuss how free strings (and some other objects) may or may not propagate consistently across singularities in certain classes of plane gravitational waves. These singularities arise as Penrose limits of Friedmann-like cosmologies (and other familiar singular space-times), and they provide a rare example of relatively tractable singular time-dependent backgrounds in a number of approaches to quantum gravity.


Gauged N=2 supergravity from generalized geometry

Speaker:Davide Cassani (ENS)
Date:Wednesday, November 12, 2008 - 10:30
Location:VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
In this talk we discuss dimensional reductions of type II supergravity on SU(3)xSU(3) structure backgrounds with general fluxes. We describe a procedure for truncating type II theories to four dimensional N=2 gauged supergravity. Then we analyse the features of this truncated theory, showing how its data are related to the generalized geometry formalism. In particular, we give a geometric expression for the full N=2 scalar potential, as well as for the period matrices associated with the N=2 special Kaehler geometry. Finally we establish a precise correspondence between the N=1 vacuum conditions obtained from this 4d N=2 action and the equations characterizing supersymmetric backgrounds at the ten dimensional level.


The elliptic genus from split flow trees and Donaldson-Thomas invariants

Speaker:Thomas Wyder (KUL)
Date:Wednesday, November 12, 2008 - 13:00
Location:VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay


Black holes, instantons and twistors

Speaker:Boris Pioline (LPHTE Paris)
Date:Wednesday, November 19, 2008 - 10:30
Location:Leuven, Celestijnenlaan 200D 06.34
Precision counting of BPS black holes in D=4, N=2 string theory remains an outstanding challenge, especially in view of the moduli dependence of the black hole spectrum. For this purpose, it would be very desirable to have a globally defined object, continuous across the lines of marginal stability, and sensitive to the black hole degeneracies within each connected domain in moduli space. I will argue that the quaternionic- Kahler metric on the vector moduli space after compactification to 3 dimensions is ideally suited for this purpose. Moreover, it is identical to the hypermultiplet moduli space in D=4 for the T-dual theory. I will describe recent progress in computing instanton corrections to this metric, based on twistorial techniques, and a suggestive relation to the wall-crossing formula of Kontsevich and Soibelman.


Black holes and Fermi surfaces

Speaker:Michael Berkooz (Weizmann Institute)
Date:Wednesday, November 19, 2008 - 13:00
Location:Leuven, Celestijnenlaan 200D 06.32
The talk will discuss recent progress in understanding a new class of N=4 operators and their possible AdS duals. The operators are Fermi surface operators, which are not accessible in the standard large N limit. Such operators appear to be only weakly renormalizeable in perturbation theory. Their AdS duals are related to black holes with 1/16 SUSY in AdS.


World-sheet duality for supersphere sigma-models

Speaker:Thomas Quella (University of Amsterdam)
Date:Wednesday, November 26, 2008 - 10:30
Location:Leuven, Celestijnenlaan 200D 06.34
Sigma models on superspheres have been proposed as a compact toy model for the string propagation on AdS spaces by Mann and Polchinski. These sigma models form a one parameter family of CFTs which cannot be addressed using standard techniques. However, recently a strong-weak coupling duality has been conjectured by Candu and Saleur which allows to describe the supersphere sigma models in terms of a conformally invariant Gross-Neveu model with OSP(2S+2|2S) symmetry. We describe how the special properties of the OSP(2S+2|2S) supersymmetry can be used to derive the full spectrum of a specific D-brane in these Gross-Neveu models for all values of the coupling constant, capturing BPS as well as non-BPS contributions. By comparing the resulting spectrum with a semi-classical partition function for the supersphere sigma models, our results provide highly non-trivial additional evidence in favor of the proposed duality. Similar techniques can be used to determine brane spectra in AdS(3)xS(3) with a mixture of Ramond-Ramond and Neveu-Schwarz fluxes, and might turn out to be useful even in more complicated AdS-spaces. Based on arXiv:0809.1046 (with V. Mitev and V. Schomerus).


Dual superconformal symmetry of amplitudes in N=4 SYM

Speaker:James Drummond (LAPP Annecy)
Date:Wednesday, November 26, 2008 - 13:00
Location:Leuven, Celestijnenlaan 200D 06.32
I will discuss some recent advances in the study of scattering amplitudes in N=4 SYM. In particular I will highlight a duality between the simplest (MHV) amplitudes and Wilson loops. This duality implies a novel symmetry of amplitudes, dual conformal symmetry, which is distinct from the ordinary conformal symmetry of N=4 SYM. I will show that the symmetry can be naturally extended to a full dual superconformal symmetry of all scattering amplitudes in N=4 SYM and I will describe explicity how it is realised on the amplitudes, with direct confirmation at tree-level and one loop.


The N=2 cascade revisited

Speaker:Cyril Closset (ULB)
Date:Wednesday, December 3, 2008 - 10:30
Location:VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay


First-order flow equations for extramal and non-extremal black holes

Speaker:Bert Vercnocke (KUL)
Date:Wednesday, December 3, 2008 - 13:00
Location:VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
For both extremal and non-extremal, spherically symmetric, static black holes in theories with massless scalars and vectors coupled to gravity we discuss conditions for the existence of first-order flow equations. We apply the analysis to theories that have a symmetric moduli space after a dimensional reduction over the timelike direction.


Overview of higher-dimensional black holes

Speaker:Roberto Emparan (UB)
Date:Wednesday, December 10, 2008 - 10:30
Location:VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
In recent years we have discovered that black holes in higher dimensions exhibit much richer dynamics than four-dimensional ones. Great progress has been made in five dimensions, while larger dimensions remain largely unexplored territory. I will attempt to provide a bird's-eye view of the current status of the subject.



Speaker:Roberto Emparan (UB)
Date:Wednesday, December 10, 2008 - 13:00
Location:VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
A main novel feature of higher-dimensional black holes, compared to four-dimensional ones, is that their horizons can have two characteristic lengths of very different size. This calls for qualitatively new techniques to deal with their dynamics. We have developed a long-distance effective theory that captures their dynamics at scales much larger than the short scale. This approach not only reveals several new kinds of black holes: it also changes our perspective on what should be the aim of the study of higher-dimensional black holes.


Stanislav Kuperstein (VUB)

Date:Wednesday, December 17, 2008 - 10:30
Location:Leuven, Celestijnenlaan 200D 06.34


Maximally Supersymmetric Theories on the Light-Cone

Speaker:Sung-Soo Kim (Chalmers)
Date:Wednesday, December 17, 2008 - 13:00
Location:Leuven, Celestijnenlaan 200D 06.34
I will talk about how maximally supersymmetric theories can be described in the light-cone superspace. In this talk, I will focus on the realization of E7(7) symmetry for N=8 Supergravity, and show (i) all physical degrees of freedom transform under the coset E7(7)/ SU(8) and (ii) how this symmetry is used to construct higher-order interaction terms in the light-cone formulation.


Gravity and hydrodynamics

Speaker:Mukund Rangamani (University of Durham)
Date:Wednesday, February 4, 2009 - 10:30
Location:Leuven, Celestijnenlaan 200D 06.32
I will review the fluid-gravity correspondence, which relates classical gravitational dynamics to the dynamics of fluids living on the boundary of the spacetime. In particular, we will show how generic long wavelength solutions in a quantum field theory map to regular inhomogeneous black hole solutions. We will also discuss some applications of this correspondence to black hole physics.


Brownian motion in AdS/CFT

Speaker:Mukund Rangamani (University of Durham)
Date:Wednesday, February 4, 2009 - 13:00
Location:Leuven, Celestijnenlaan 200D 06.32
We will discuss Brownian motion and the associated Langevin equation in AdS/CFT. The probe particle undergoing stochastic Brownian motion will be modeled by a fundamental string in an asymptotically AdS black hole spacetime. We will discuss the origins of the friction force and the random force in the Langevin equation from a gravitational perspective and describe a gravitational derivation of the fluctuation-dissipation theorem.


Adding massive dynamical quarks to the gauge/string correspondence

Speaker:Franceso Bigazzi (ULB)
Date:Wednesday, February 18, 2009 - 10:30
Location:VUB/ULB, Campus Plaine, building NO, 9th floor, Salle des Professeurs
Differently from pure Yang-Mills, QCD is not strictly confining. The dynamical quarks screen the color charges and their pair production cause the chromoelectric flux tubes to break. At present, it is extremely hard to account for these effects on the lattice. The string/gauge theory correspondence offers a set of simple tools to overcome this limit, at least for certain strongly coupled supersymmetric extensions of planar QCD. The gauge theories can be mapped to classical supergravity solutions accounting for the full backreaction of a large number of "color" and smeared "flavor" D-branes. Remarkably, if the flavors are massive these solutions can be regular at the origin and can thus be suitably employed to explore the IR field theory regime. I will show, mainly considering D3-D7 setups on the conifold, how the above mentioned screening effects are accounted for by the dual supergravity solutions. Moreover, the latter predict the occurrence of novel quantum phase transitions in the static quark-antiquark potential, for planar gauge theories having at least two independent physical scales. I will discuss the universal properties of these transitions.


The return of the phoenix universe

Speaker:Jean-Luc Lehners (Princeton)
Date:Wednesday, February 18, 2009 - 13:00
Location:VUB/ULB, Campus Plaine, building NO, 9th floor, Salle des Professeurs
In cyclic universe models based on a single scalar field (e.g. the radion determining the distance between branes in M-theory), virtually the entire universe makes it through the ekpyrotic smoothing and flattening phase, bounces, and enters a new epoch of expansion and cooling. This stable evolution cannot occur, however, if scale-invariant curvature perturbations are produced by the entropic mechanism because it requires two scalar fields (e.g. the radion and the Calabi-Yau dilaton) evolving along an unstable classical trajectory. In fact, I will discuss how an overwhelming fraction of the universe fails to make it through the ekpyrotic phase; nevertheless, a sufficient volume survives and cycling continues forever provided the dark energy phase of the cycle lasts long enough, of order a trillion years. Two consequences are a new role for dark energy and a global structure of the universe radically different from that of eternal inflation.


Kellog Stelle (Imperial College, London)

Date:Wednesday, February 25, 2009 - 10:30
Location:Leuven, Celestijnenlaan 200E 01.209


Holographic gluon plasma from 5d dilaton-gravity

Speaker:Liuba Mazzanti
Date:Wednesday, February 25, 2009 - 13:00
Location:Leuven, Celestijnenlaan 200E 01.209
I will describe black-hole solutions in five-dimensional gravity with a negative cosmological constant and a self-interacting scalar field. The geometry is asymptotically logarithmically anti-de Sitter in the UV. A phase transition occurs if an only if the IR behavior of the dilaton potential is associated to a confining gauge theory. I will illustrate the properties of the black-hole solutions and of the phase transition for a generic monotonic dilaton potential. The transition is holographically dual to the confining/deconfining phase transition of a 4d theory closely matching pure Yang-Mills, both at zero and finite temperature.


Deformed gauge theories and their string duals

Speaker:Emiliano Imeroni (ULB)
Date:Wednesday, March 4, 2009 - 10:30
Location:VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Gauge theories with deformed products of fields in the lagrangian constitute an interesting generalization of the gauge/string duality. We review a systematic procedure to find the string duals of such theories, called the TsT transformation, and illustrate its properties by means of a few examples.


Conformal current algebra in two dimensions

Speaker:Raphael Benichou (ENS, Paris)
Date:Wednesday, March 4, 2009 - 13:00
Location:VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
We construct a non-chiral current algebra in two dimensions consistent with conformal invariance. We show that this algebra is realized in non-linear sigma models on some supergroup manifold. These models are relevant for the description of perturbative string theory in AdS backgrounds with Ramond-Ramond fluxes.


Cosmological pertubations in non-local SFT cosmological models

Speaker:Alexay Koshelev (VUB)
Date:Wednesday, March 11, 2009 - 10:30
Location:Leuven, Celestijnenlaan 200E 01.209
In this talk I review the scalar field theory models arising from String Field Theory (SFT) description of the tachyon condensation and consider their coupling to Einstein gravity. Having sketched the classical solutions to equations of motion the cosmological perturbations of such models are considered and distinguishes from the behavior of perturbations in standard local cosmological scenarios are emphasized.


Quantizing N=2 Multicenter Solutions

Speaker:Sheer El-Showk (UvA)
Date:Wednesday, March 11, 2009 - 13:00
Location:Leuven, Celestijnenlaan 200E 01.209
N=2 supergravity in four dimensions, or equivalently N=1 supergravity in five dimensions, has an interesting set of BPS solutions that each correspond to a number of charged centers. This set contains black holes, black rings and their bound states, as well as many smooth solutions. Moduli spaces of such solutions carry a natural symplectic form which we determine, and which allows us to study their quantization. By counting the resulting wavefunctions we come to an independent derivation of some of the wall-crossing formulae. Knowledge of the explicit form of these wavefunctions allows us to find quantum resolutions to some apparent classical paradoxes such as solutions with barely bound centers and those with an infinitely deep throat. We show that quantum effects seem to cap off the throat at a finite depth and we give an estimate for the corresponding mass gap in the dual CFT. This is an interesting example of a system where quantum effects cannot be neglected at macroscopic scales even though the curvature is everywhere small.


N=1 supergravity with Chern-Simons terms and anomaly cancellation

Speaker:Jan De Rydt (KUL)
Date:Wednesday, March 18, 2009 - 10:30
Location:VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
The general actions of matter-coupled N=1 supergravity have Peccei-Quinn terms and generalized Chern-Simons terms that may violate gauge and supersymmetry invariance. We discuss their ability to cancel quantum anomalies and exhibit conditions for the consistency of this Green-Schwarz mechanism. The mechanism can also be generalized to an electromagnetic covariant framework, which requires the introduction of the embedding tensor. In the presence of quantum anomalies, we show that consistency requires a modification of the linear constraint on the embedding tensor.


Orientiholes : black hole technology applied to flux vacua

Speaker:Mboyo Esole (Harvard)
Date:Wednesday, March 18, 2009 - 13:00
Location:VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
By T-dualizing space-filling D-branes, we obtain black hole bound states living in a universe with a gauged spatial reflection symmetry. We call these objects "orientiholes". The gravitational entropy of orientihole configurations provides an estimate of the number of vacua in various sectors of the IIB landscape. Basic physical properties of orientiholes provide a useful alternative picture on a number of issues arising in D-brane model building. More generally, we give orientihole generalizations of recently derived wall crossing formulae, and conjecture a relation to the topological string analogous to the OSV conjecture, but with a linear rather than a quadratic identification of partition functions.


Top anomalous couplings in single top production

Speaker:Priscila De Aquino (KUL)
Date:Wednesday, March 25, 2009 - 10:30
Location:Leuven, Celestijnenlaan 200E 01.209
In the Standard Model (SM), most of the top quarks are produced in pairs, via the strong interaction. However, a sizeable fraction is also produced singly, via the weak interaction, through an exchange of a virtual W boson. These processes are particularly sensitive to FCNC and anomalous couplings to the top. In this context, I will discuss single top production in association with a photon at the LHC as a test of the SM and to search for new physics.


Galois symmetries in Super Yang-Mills Theories

Speaker:Frank Ferrari (ULB)
Date:Wednesday, March 25, 2009 - 13:00
Location:Leuven, Celestijnenlaan 200E 01.209
Classifying the phases of gauge theories is hindered by the lack of local order parameters. In particular, the standard Wilson's and 't Hooft's non-local order parameters are known to be insufficient to explain the existence of the plethora of phases that are found in supersymmetric gauge theories. Motivated by these observations, we reanalyze the concept of gauge symmetry breaking using Galois theory. Unlike the ordinary classical notion of unbroken gauge group, the Galois symmetry makes sense in the full quantum theory and must be a phase invariant. The algebraic structure underlying the space of vacua of supersymmetric gauge theories, that we have developed recently, is precisely designed to allow a rigorous mathematical implementation of these ideas.


Gravitational instantons, ricco flows and integrable structures

Speaker:Marios Petropoulos (CPHT-Polytechnique)
Date:Wednesday, April 1, 2009 - 10:30
Location:VUB/ULB, Campus Plaine, building NO, 9th floor, Salle des Professeurs
Four-dimensional gravitational instantons appear as solutions of self-dual Einstein's equations. The latter turn out to be equivalent to the first-order equations describing the evolution of homogeneous spaces under Ricci flow. This relationship is quite general and might be relevant for the search of cosmological backgrounds (in string theory in particular). It will be exposed in the simple case of Bianchi IX homogeneous spaces, where the equations exhibit remarkable integrability properties.


Simplicity in the Structure of N=8 Supergravity Amplitudes

Speaker:Pierre Vanhove (CEA-Saclay and IHES)
Date:Wednesday, April 1, 2009 - 13:00
Location:VUB/ULB, Campus Plaine, building NO, 9th floor, Salle des Professeurs
Recently, there have been remarkable progresses on the analysis of the multi-loop amplitudes in supergravity in various dimensions using string theory and in field theory methods. There are strong indications that four-dimensional maximal supergravity theories have a better ultraviolet behaviour than expected from a conservative implementation of on-shell superspace. The pure spinor formalism allowed us to make concrete predictions for the structure of higher-loop four-graviton amplitudes that have been confirmed so far by all explicit field theoretic amplitude computations. As well beside the constraints from supersymmetry, the amplitudes in colorless gauge theories have a much more simple structure than guessed from a traditional approach based on a Lagrangian and Feynman graphs formalism. We will explain how a string based approach makes obvious fundamental cancellations in colorless gauge amplitudes, leading to the no-triangle property of N=8 supergravity one-loop amplitude.


Conformally supersymmetric Wilson loops in N=4 super Yang-Mills

Speaker:Vasily Pestun (Harvard)
Date:Wednesday, April 22, 2009 - 10:30
Location:VUB/ULB, Campus Plaine, building NO, 9th floor, Salle des Professeurs
We define conformally supersymmetric Wilson operators in N=4 super Yang-Mills gauge theory and give a complete classification of all such operators with minimal amount of supersymmetry.


Large D-instanton effects in string theory

Speaker:Stefan Vandoren (Utrecht)
Date:Wednesday, April 22, 2009 - 13:00
Location:VUB/ULB, Campus Plaine, building NO, 9th floor, Salle des Professeurs
By reduction along the time direction, black holes in 4 dimensions yield instantons in 3 dimensions. Each of these instantons contributes individually at order $\exp(-|Q|/g_s)$ to certain protected couplings in the three-dimensional effective action, but the number of distinct instantons is expected to be equal (or comparable) to the number of black hole micro-states, i.e. of order $\exp(Q^2)$. The same phenomenon also occurs for certain protected couplings in four dimensions, such as the hypermultiplet metric in type II string theories compactified on a Calabi-Yau threefold. In either case, the D-instanton series is therefore asymptotic, much like the perturbative expansion in any quantum field theory. By using a Borel-type resummation method, adapted to the Gaussian growth of the D-instanton series, we find that the total D-instanton sum has an inherent ambiguity of order $\exp(-1/g_s^2)$. We further suggest that this ambiguity can be lifted by including Kaluza-Klein monopole or NS5-brane instantons.


Hidden symmetries as a black hole solution generating technique

Speaker:Ella Jamsin (ULB)
Date:Wednesday, April 29, 2009 - 10:30
Location:Leuven, Celestijnenlaan 200E 01.209
Upon reduction on a (D-3)-dimensional torus, some supergravities in D dimensions reveal a global symmetry under a finite Lie group G, which is larger than the expected GL(D-3,R) isometry of the torus. This hidden symmetry can be used to construct new solutions of the D-dimensional theory. In this talk, I will focus on the case of five-dimensional minimal supergravity, which, in the presence of two comuting Killing vectors, reduces to three-dimensional gravity coupled to a non-linear sigma model which is invariant under the exceptional Lie group G_2. I will show how this provides a powerful method to construct black hole solutions in five dimensions.


D-brane Inflation in String Theory

Speaker:Anatoly Dymarsky (Stanford)
Date:Wednesday, April 29, 2009 - 13:00
Location:Leuven, Celestijnenlaan 200E 01.209
I will discuss some aspects of the stringy models of inflation based on the KKLMMT proposal. In this case one can use the gauge/gravity duality to explicitly calculate the potential for the mobile D3-brane and estimate the compactification effects.


On-shell methods in gauge theory and gravity

Speaker:Andreas Brandhuber (Queen Mary, London)
Date:Wednesday, May 6, 2009 - 10:30
Location:Leuven, Celestijnenlaan 200D 06.34
In this talk I will give an overview of modern, twistor inspired methods to calculate multi-particle scattering amplitudes. In particular I will describe on-shell recursion relations and the generalised unitarity method for loop amplitudes. The most remarkable feature of these methods is that they only require on-shell information as input which makes them extremely efficient tools for practical calculations. Furthermore, these methods can be applied to a wide range of quantum field theories including N=4 super Yang-Mills, N=8 supergravity and QCD.


Hidden structures in gauge theory and gravity

Speaker:Andreas Brandhuber (Queen Mary, London)
Date:Wednesday, May 6, 2009 - 13:00
Location:Leuven, Celestijnenlaan 200D 06.34
In recent years several unexpected structures have been uncovered in maximally supersymmetric quantum field theories by studying multileg or multiloop scattering amplitudes. In my talk I will describe three examples of such novel structures: 1) cross order relations in higher loop S-matrix elements in N=4 SYM and N=8 SUGRA 2) a novel duality between Wilson loops and amplitudes in N=4 SYM which has intriguing relations to the AdS/CFT correspondence 3) supersymmetric on-shell recursion relations and dual superconformal symmetry.


Lattice simulation of thermal D0-branes

Speaker:Toby Wiseman (Imperial College)
Date:Wednesday, May 20, 2009 (All day)
Location:VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
The AdS-CFT correspondence and its generalizations provide a fascinating gauge theory description that is supposedly dual to certain gravitational theories. At finite temperature these contain black holes, and in principle solving the gauge theory would give a microscopic quantum description of these black holes. However since these theories are strongly coupled they remains analytically intractable. I will describe the use of numerical lattice simulation to solve a promising example at finite temperature with the aim to extract the dual black hole thermodynamics from the gauge theory.


Dark matter interpretation of the Pamela and Fermi results : theoretical trends

Speaker:Thomas Hambye (ULB)
Date:Wednesday, May 20, 2009 - 10:30Location:
VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
The recently observed excess of cosmic high energy positrons by the Pamela experiment has been the origin of a huge series of works proposing various possible explanations, astrophysical ones (pulsar) or particle physics ones (dark matter annihilation or decay). After a brief review on the experimental situation, which includes the last week result of the Fermi experiment on the total flux of high energy electrons and positrons, will be presented the various constraints any dark matter model must satisfy to reproduce the data: magnitude of cross sections needed, types of annihilations and decays, mechanism to enhance the annihilation, etc. A few models which can satisfy these constraints will be singled out: light mediator models, hidden sector models, exotic susy models. Special emphasize will be put on what is probably the most exciting possibility: that this data is the manifestation of GUT physics violating the global symmetry which at low energy is responsible for dark matter stability. Two example models of this kind will be discussed briefly, including the hidden vector dark matter model.


Donaldson-Thomas partitions and refined bound state indices

Speaker: Walter Van Herck (KUL)
Date:Wednesday, May 27, 2009 - 10:30
Location:Leuven, Celestijnenlaan 200D 06.34
In this seminar, I demonstrate for a class of examples, that Donaldson- Thomas invariants are naturally split up into partitions, and that this leads to refined indices enumerating BPS bound states in type IIA compactifications on a Calabi-Yau 3-fold. More concretely, I will give a worked out example of the enumeration of BPS D-particles in a compactification of type IIA on a hypersurface in a weighted projective space. These models strenghten the strong split attractor flow conjecture.


Susy and the brane: A model story

Speaker: Wieland Staessens
Date: Wednesday, May 27, 2009 - 13:00
Location: Leuven, Celestijnenlaan 200D 06.34
A Type II  string propagating in general background fields (with exclusion of RR-fluxes)  can be described using the RNS-formalism. Imposing N=(2,2) supersymmetry on the  world-sheet constrains the target space geometry to be bihermitian/Generalized  Kahler. When dealing with an open string one should introduce a boundary which  breaks the N=(2,2) world-sheet supersymmetry to an N=2 boundary supersymmetry.  The boundary conditions for the open string describe the embedding of the  D-brane to which the open string is attached. When using an N=2 boundary  superspace formulation, one can obtain a clear geometrical characterization of  D-branes wrapping subspaces of Generalized Kahler Geometries. Moreover,  T-duality in the boundary superspace formulation becomes a useful tool to  construct non-trivial D-brane configurations. This talk reviews some of the  results in 0709.3733, 0809.3659 and unpublished work.


Black Holes and the Structure of Spacetime

Speaker: aulik Parikh (IUCAA, India)
Date: Wednesday, June 3, 2009 - 10:30
Location: VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Black holes  have taught us a variety of surprising lessons about the structure of gravity  and spacetime. I will describe two of these: a paradox for the holographic dual  of de Sitter space, as well as the mysterious manner in which classical gravity  appears as the thermodynamics of spacetime itself.


Testing String Theory with Cosmological Observations

Speaker: Robert Brandenberger (McGill)
Date: Wednesday, June 3, 2009 - 13:00
Location: VUB/ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
In spite of  its phenomenological successes, inflationary universe cosmology suffers from  important conceptual problems. These motivate the search for a new paradigm  based on improved fundamental physics. I will present a toy model of early  universe cosmology constructed from some basic principles of superstring  theory. This model can explain current observations and makes specific  predictions which will allow the test of certain aspects of string theory in  cosmological observations.