Past seminars

Academic year: September 2004 - September 2005

Within the context of SUSY GUTs, cosmic strings are generically formed at the end of hybrid inflation. However, the WMAP CMB measurements strongly constrain the possible cosmic strings contribution to the angular power spectrum of anisotropies. We investigate the parameter space of SUSY hybrid (F- and D- term) inflation, to get the conditions under which theoretical predictions are in agreement with data.

Place:  VUB, Oefenplein, building G, 1G.003
Time:   Wednesday, July 6, 2005 at 13:00

In this talk, I will discuss how, if seriously taken, a quite different picture of 4-dimensional spacetime for massless chiral fermions (i.e. the standard model) within the context of intersecting branes arises. These arguments build on the fact that the open and closed string sector of string theory are different by nature: the closed string sector is purely determined by the spacetime background, the open string sector by strings that stretch in between branes. All these arguments can be made extremely precise in a simple OmegaR-Orientifold of type IIA containing intersecting D6-branes on the torus. Some prospects for cosmology and particle physics are discussed as well.

Place:  VUB, Oefenplein, building G, 1G.003
Time:   Wednesday, July 6, 2005 at 10:30

(Supersymmetric) gauge theories can be realized by open strings on suitable systems of branes. Including simple closed string backgrounds can lead to interesting modifications of these gauge theories, such as non-commutative or non-anti-commutative deformations which are connected, respectively, to a constant NS-NS B field or a constant R-R field strength. We show how the effect of closed backgrounds can be explicitly derived by computing standard string amplitudes in the NSR formalism. We also take a look at the corresponding deformations of the instantonic solutions and their moduli spaces.

Place:  Leuven, Celestijnenlaan 200 D 06.34
Time:   Wednesday, June 29, 2005 at 13:00

We discuss how open strings stretched between a system of N D-branes emerge in Vacuum String Field Theory. We show how higgsing is generated when the branes are dis
Placed: the shift in the mass formula is due to a twist anomaly, a contribution that formally violates associativity at the midpoint in the matter sector.

Place:  Leuven, Celestijnenlaan 200 D 06.32
Time:   Wednesday, June 15, 2005 at 13:00

We consider some theories of gravity coupled to matter fields. These "maximally oxidised theories" are characterised by a non-linearly realised symmetry G, where G is a simple group, emerging in the dimensional reduction down to three dimensions. Particular cases of these theories are: the bosonic sector of 11 dimensional supergravity, the low energy effective action of the bosonic string and pure gravity. There are indications that these theories are characterised by an infinite dimensional Kac-Moody symmetry G+++. To investigate this, theories explicitely invariant under non-linear transformations of G+++ have been considered. This construction will be explained in details. We first focus on the non-linear realisation of finite dimensional simple Lie algebras. Then, explain how to manage with infinite dimensional Kac-Moody algebras. We will explain briefly how to make the link with the "maximally oxidised theories" by two inequivalent truncations of these theories. One of the truncation is relevant in the context of cosmological billiard and the other admits exact solutions, identical to those of the maximally oxidised field theories which describe intersecting extremal branes smeared in all directions but one. Dualities expressed as Weyl reflections extend to all G+++ theories. We discuss the significance of the G+++ symmetries and their implications for gravity theories.

Place:  Leuven, Celestijnenlaan 200 D 06.32
Time:   Wednesday, June 15, 2005 at 10:30

This Colloquium deals with Neutrinos, the most elusive constituents of the Universe, and of the Scientists who hunt them. Their detection in the most recent underground experiments have led to the conclusion that different neutrinos ``morph" into one another, leading to yet another paradigm shift in our understanding of the underlying theory of Nature. Neutrinos, the only messengers from the Sun's inner core, traverse the Earth with ease, and reassure us that the Sun is shining even at night.
[NOTE: not a usual seminar!]

Place:  ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Wednesday, June 8, 2005 at 16:00

Place:  Leuven, Celestijnenlaan 200 D 06.32
Time:   Wednesday, June 1, 2005 at 13:00

We will see how generalized Calabi-Yau manifolds as defined by Hitchin emerge from supersymmetry equations in type II theories in the presence of fluxes. To that end, we will first review the formalism of G-structures, which is central in the context of compactification with fluxes. We will then see briefly the needed ingredients of generalized complex geometry, to show how supersymmetry equations impose a generalized Calabi-Yau condition on the internal manifold.

Place:  Leuven, Celestijnenlaan 200 D 06.32
Time:   Wednesday, June 1, 2005 at 10:30

Visit the website.

Place:  Leuven
Time:   Monday, May 30, 2005
            Tuesday, May 31, 2005

Place:  VUB
Time:   Wednesday, May 25, 2005

The Universite Libre de Bruxelles, the Vrije Universiteit Brussel, the International Solvay Institutes for Physics and Chemistry Brussels and the EC network MRTN-CT-2004-005104 "Constituents, Fundamental Forces and Symmetries of the Universe" announce the workshop "Dark Matters". The aim of the workshop is to confront the cosmological dark matter and dark energy problems to the latest attempts to solve them, including new ideas on dark matter and on modifications of gravity.

Visit the website.

Place:  ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Wednesday, May 18, 2005
            Thursday, May 19, 2005
            Friday, May 20, 2005

In these lectures I shall discuss recent progress in relating the degeneracy of BPS elementary string states to the entropy of black holes carrying the same quantum numbers. The topics include review of old scaling argument, the role of higher derivative terms in the computation, explicit calculation of the entropy based on higher derivative action and attempts to go beyond the large charge limit.
A link to the lectures can be found here: http://itf.fys.kuleuven.be/~toine/ASenLect1.ps

Place:  Leuven, Celestijnenlaan 200 D 06.34
            Leuven, Celestijnenlaan 200 D 06.32
            Leuven, Celestijnenlaan 200 D 06.32
            Leuven, Celestijnenlaan 200 D 06.32
Time:   Tuesday, May 10, 2005 at 10:30
            Wednesday, May 11, 2005 at 10:30
            Wednesday, May 11, 2005 at 13:30
            Thursday, May 12, 2005 at 13:30

We will introduce the basics aspects of the non-linear realization for space-time symmetries. The role of central charges in the symmetry algebra will be discussed. Branes in a target space-time break some of the symmetries of the background. The low energy action of branes can be constructed from the non-linear realization method. The method will be applied to branes in several space-times.

Place:  Leuven, Celestijnenlaan 200 D 06.32
Time:   Wednesday, April 27, 2005 at 13:00

We discuss on the bubbling for the D5-D1 system, trying to some bring some light to the physical meaning of such construction. A connection to the framework championed by Mathur on the much studied D5-D1 system is also suggested.

Place:  Leuven, Celestijnenlaan 200 D 06.32
Time:   Wednesday, April 27, 2005 at 10:30

We will introduce the basics aspects of the non-linear realization for space-time symmetries. The role of central charges in the symmetry algebra will be discussed. Branes in a target space-time break some of the symmetries of the background. The low energy action of branes can be constructed from the non-linear realization method. The method will be applied to branes in several space-times.

Place:  ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Wednesday, April 20, 2005 at 13:00

Place:  ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Wednesday, April 20, 2005 at 10:30

We will introduce the basics aspects of the non-linear realization for space-time symmetries. The role of central charges in the symmetry algebra will be discussed. Branes in a target space-time break some of the symmetries of the background. The low energy action of branes can be constructed from the non-linear realization method. The method will be applied to branes in several space-times.

Place:  Leuven, Celestijnenlaan 200 D 06.32
Time:   Wednesday, April 13, 2005 at 13:00

Abstract: we discuss expected decay modes of large strings and indicate string configurations which are not easily broken. By a precise quantum analysis which confirms the expectation's we find superstring states which have a lifetime growing like a power of the mass. These states dominantly decay by emitting a soft radiation exactly like a gravitational antenna.

Place:  Leuven, Celestijnenlaan 200 D 06.32
Time:   Wednesday, April 13, 2005 at 10:30

We will introduce the basics aspects of the non-linear realization for space-time symmetries. The role of central charges in the symmetry algebra will be discussed. Branes in a target space-time break some of the symmetries of the background. The low energy action of branes can be constructed from the non-linear realization method. The method will be applied to branes in several space-times.

Place:  ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Wednesday, April 6, 2005 at 13:00

The superembedding technique is used to derive a simple form of the Dirac equations and of the corresponding quadratic action for the fermions on the worldvolumes of the M5 and M2 brane in a generic D=11 supergravity background with bulk and worldvolume fluxes. The knowledge of the explicit dependence on fluxes in the fermionic action on the M5 brane can be useful for the analysis of moduli stabilization in search for phenomenologically relevant implications of string theory.

Place:  ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Wednesday, April 6, 2005 at 10:30

See: hep/th0501114

Place:  ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Wednesday, March 23, 2005 at 13:00

See: hep/th0501114

Place:  ULB, Campus Plaine, Building NO, 5th floor, Salle Solvay
Time:   Wednesday, March 23, 2005 at 10:30

see hep-th/0501053

Place:  K.U. Leuven, Celestijnenlaan 200 D 06.32
Time:   Wednesday, March 16, 2005 at 10:30

Place:  ULB, campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Wednesday, March 9, 2005 at 13:00

I will review a recently proposed method for solving the Killing spinor equations in arbitrary dimensions. The efficiency of the method will be illustrated by recent progress on the classification of the supersymmetric solutions of 11D and IIB supergravity.

Place:  ULB, Campus Plaine, Building NO, 5th floor, Salle Solvay
Time:   Wednesday, March 9, 2005 at 10:30

I will discuss the stability of braneworld spacetimes bounded by a positive and a negative tension brane. These play a major role in the duality between M-theory and the heterotic string, but raise obvious questions about stability owing to the presence of the negative tension brane. I will show why the system is nonetheless stable against worldvolume localised perturbations.

Place:  K.U. Leuven, Celestijnenlaan 200 D 06.32
Time:   Wednesday, March 2, 2005 at 13:00

I will discuss the recent revival of interest in cosmic strings and superstrings and some of the developments behind it.

Place:  K.U. Leuven, Celestijnenlaan 200 D 06.32
Time:   Wednesday, March 2, 2005 at 10:30

Place:  ULB, Campus Plaine, building NO, 5th floor, Salle Solvay
Time:   Wednesday, February 23, 2005 at 13:00

We will argue that the cigar conformal field theory SL(2,R)/U(1) is a prime example of a non-rational conformal field theory and we exhibit some of its characteristic features that might generalize to other non-rational conformal field theories. We discuss how this particular non-rational conformal field theory (with or without boundary) arises in string theory backgrounds with NS5-branes, and how the technical advances reached in the conformal field theory illuminate aspects of string theory and holography in these backgrounds.

Place:  ULB, Campus Plaine, Building NO, 5th floor, Salle Solvay
Time:   Wednesday, February 23, 2005 at 10:30

Non-relativistic string theory is a particular limit of string theory that could be useful to find a new sector of the theory where dualities like AdS/CFT can be tested. I will show how the non relativistic limit can be taken for string theory and I will discuss some classical solutions to describe some peculiarity of the theory.

Place:  K.U. Leuven, Celestijnenlaan 200 D 05.34
Time:   Wednesday, February 16, 2005 at 13:00

We review recent developements in understanding string theory vacua when non-trivial fluxes are considered. We focus on the moduli stabilization problem and on obtaining realistic vacua with positive cosmological constant. We show how this can be addressed in the context of the effective 4-dimensional theories which are obtained through compactifications on twisted tori.

Place:  K.U. Leuven, Celestijnenlaan 200 D 05.34
Time:   Wednesday, February 16, 2005 at 10:30

During the last years, the application of dilatation operator techniques to determine anomalous scaling dimensions in N=4 SYM attracted a lot of attention, as these allow strong quantitative tests of the dynamical AdS/CFT correspondence in the BMN limit. After a general introduction to the subject, we will have a look at new results from a four-loop calculation in a dimensionally reduced toy model that exhibit a violation of BMN scaling in a very special and easily understandable way. Furthermore, first explicit data about wrapping interactions are provided, which are inaccessible by usual techniques but play a key role on the gauge theory side of the correspondence.

Place:  ULB, Campus Plaine, Building NO, 5th floor, Salle Solvay
Time:   Wednesday, February 9, 2005 at 13:00

Abstract: Recently, there has been a renewed interest in the consequences of higher curvature terms for the entropy of extremal black holes. In this talk we discuss old and recent results on this topic. Specifically, we review the construction of supersymmetric stationary solutions of matter-coupled N=2 supergravity. For static single-centered (black hole) solutions, the entropy agrees, when calculated according to prescription given by R. Wald, with the asymptotic state degeneracy of wrapped D-branes or M-branes in a suitable background. Then we discuss recent results, where we compare the (Wald) entropy of dyonic extremal black holes in N=4 string compactifications to a state counting formula proposed by R. Dijkgraaf, E. Verlinde and H. Verlinde some time ago.

Place:  ULB, Campus Plaine, Building NO, 5th floor, Salle Solvay
Time:   Wednesday, February 9, 2005 at 10:30

(and Walter's Birthday)
Time:   Thursday, February 3, 2005
Place:  K.U. Leuven, Celestijnenlaan 200 D 05.34
Time:   Thursday, January 27, 2005 at 13:00

In order to fix the moduli, non-trivial fluxes might the essential input. We summarize different aspects of compactifications in the presence of fluxes, as there is the relation to generalized Scherk-Schwarz reductions and gauged supergravity but also the description of flux-deformed geometries in terms of G-structures and intrinsic torsion.

Place:  K.U. Leuven, Celestijnenlaan 200 D 05.34
Time:   Thursday, January 27, 2005 at 10:30

Topological string amplitudes satisfy holomorphic anomaly equations, which are related to the special geometry of Calabi-Yau compactifications. In this talk, I will elaborate on the world-sheet description of the holomorphic anomaly in terms of the non-decoupling of certain BRST-exact states.

Place:  ULB, Campus Plaine, Building NO, 5th floor, Salle Solvay
Time:   Thursday, January 20, 2005 at 13:00

Place:  ULB, Campus Plaine, Building NO, 5th floor, Salle Solvay
Time:   Thursday, January 20, 2005 at 10:30
Place:  K.U. Leuven, Celestijnenlaan 200 D 05.34
Time:   Wednesday, January 12, 2005 at 13:00
Place:  K.U. Leuven, Celestijnenlaan 200 D 05.34
Time:   Wednesday, January 12, 2005 at 10:30
Place:  ULB, Campus Plaine, Building NO, 5th floor, Salle Solvay
Time:   Wednesday, January 5, 2005 at 13:00

String theory has an enormous amount of vacua, and statistical methods seem to be appropriate to explore this vast landscape. In this approach, rather than to try and construct individual examples of vacua, one tries to derive their distribution over parameter space. In this talk I will present a fully explicit model with fixed moduli which we recently constructed, explain why this is pretty but doesn't help us much in understanding the universe, and contrast this to what one can expect from the statistical approach. I will sketch how statistical computations are done in practice and present new results on distributions of supersymmetry breaking vacua in string theory.

Place:  ULB, Campus Plaine, Building NO, 5th floor, Salle Solvay
Time:   Wednesday, January 5, 2005 at 10:30

When multiple D-branes coincide, their coordinates become matrix valued. I will first review a formalism implementing general coordinate invariance in this context for the transversal coordinates. It is based on the introduction of a vector field with a simple covariant transformation law (this in contrast with the coordinates, which will have a very complex transformation law). The most interesting and the hardest coordinate transformations however seem to be the ones mixing longitudinal and transversal coordinates. Those impose the most stringent conditions on the action. For simplicity we focus on the case of Lorentz transformations of D0-branes. I will describe three ways to go about this problem: the static gauge approach, the "naive" covariant description and the covariant description based on a vector field. This is still work in progress, so there will be no definitive answers.

Place:  K.U. Leuven, Celestijnenlaan 200 D 05.34
Time:   Thursday, December 23, 2004 at 13:00

The talk will be about a large class of scalar cosmologies which are often obtained from higher dimensional theories, namely those cosmologies driven by multiple exponential potentials. We will describe in very general terms how solutions of these theories look like and how they can be obtained. This will involve a discussion about autonomous dynamical systems. The solutions obtained will be power law FRW universes and de Sitter universes.

Place:  K.U. Leuven, Celestijnenlaan 200 D 05.34
Time:    Thursday, December 23, 2004 at 10:30

Place:  ULB, Campus Plaine, 2.N6.110
Time:   Thursday, December 16, 2004 at 13:00

Place:  ULB, Campus Plaine, 2.N6.110
Time:   Thursday, December 16, 2004 at 10:30

Place:  K.U. Leuven, Celestijnenlaan 200 D 05.34
Time:   Thursday, December 9, 2004 at 13:00
Place:  K.U. Leuven, Celestijnenlaan 200 D 05.34
Time:   Thursday, December 9, 2004 at 10:30
Time:   Thursday, December 2, 2004

The aim of the journal club is to explain the ideas, motivations and developments that have led to the conjecture that perturbative gauge theory has an alternative formulation as a string theory in twistor space. Continuing on hep-th/0312171, we complete the translation of Yang-Mills amplitudes to twistor space, and sketch why these results suggest a description in terms of a (topological) string theory.

Place:  ULB, Campus Plaine, 2.NO.6.110
Time:   Thursday, November 25, 2004 at 13:00

Place:  VUB, Oefenplein, building G, 1G.003
Time:   Thursday, November 25, 2004 at 10:30

The aim of the journal club is to explain the ideas, motivations and developments that have led to the conjecture that perturbative gauge theory has an alternative formulation as a string theory in twistor space. I will start with an attempt to go through (the first half) of Wittens article hep-th/0312171, and explain bits and pieces on spinor calculus, helicity amplitudes in Yang-Mills theory and twistor space.

Place:  K.U. Leuven, Celestijnenlaan 200 D 05.34
Time:   Thursday, November 18, 2004 at 13:00

First Alexander Wijns will discuss the counting of BPS string modes on a circle. Then we will continue the discussion of hep-th/0202072 and hep-th/0401115 in group and where we want to go in the next few weeks.

Place:  K.U. Leuven, Celestijnenlaan 200 D 05.34
Time:   Thursday, November 18, 2004 at 10:30

Series of lectures consisting of four lectures:
1) Conformal symmetry in QCD
2) QCD on the light-cone and anomalous dimensions of Wilson operators
3) Integrability of the one-loop dilatation operator in QCD and relation to spin chains
4) Extension to super Yang-Mills theories and relation to super spin chains

Place:  ULB, Campus Plaine, 2.NO.6.10
Time:   Monday, November 8, 2004 at 14:00
            Tuesday, November 9, 2004 at 10:00
            Tuesday, November 9, 2004 at 14:00
            Wednesday, November 10, 2004 at 10:00

Place:   ULB, Campus Plaine, 2.NO.6.110
Time:   Thursday, November 4, 2004 at 13:00

We will take another look at the old problem of separating the zero mode in Liouville field theory.

Place:  VUB, Oefenplein, building G, 1G.003
Time:   Thursday, November 4, 2004 at 10:30

We discuss relevant projectors in the star algebra of Open String Field Theory that can represent D-branes of any codimension. The cohomology around such solutions is shown to correspond to the expected open string spectrum. We present other kind of projectors that exhibits localized time dependence and are interpreted as brane-decay towards the closed string vacuum.

Place:  K.U. Leuven, Celestijnenlaan 200C - 01.K
Time:   Thursday, October 28, 2004 at 10:30

Everybody wishing to attend is kindly asked to try and read hep-th/0202072 and hep-th/0401115. From there on we will discuss in group what we understood and where we want to go in the next few weeks.

Place:  ULB, Campus Plaine, Salle Solvay, building NO, 5th floor
Time:   Thursday, October 21, 2004 at 13:00

Place:  VUB, Oefenplein, building G, 1G.003
Time:   Thursday, October 21, 2004 at 10:30

The Equivalence Principle (EP) has historically played an important role in the development of gravitation theory.
As early as the 16th century Galileo understood that all bodies, whatever their mass and composition, fall with the same acceleration in the gravitational field (Universality of Free Fall - UFF) and for the first time he made some experiments to provide experimental evidence for it. Later, Newton introduced the concept of inertial and gravitational mass and his formulation of the law of gravitational attraction showed how the UFF and the EP -for which the gravitational mass must be the same as the inertial mass- are linked.
But the birth of General Relativity, at the beginning of 20th century, has put experiments on the EP in a new perspective. In 1907 Einstein formulated a stronger equivalence principle (Einstein Equivalence Principle - EPP) stating that there is a complete physical equivalence between a gravitational field and an accelerated reference frame. EEP is in fact the heart of General Relativity, because if the EEP is valid then gravitation must be a curved spacetime, and the only theories of gravity that can embody EEP are those that satisfy the postulates of metric theories of gravity. Since that moment, testing the EP has become an important check to probe the foundations of General Relativity itself.
In this lecture we shall present a brief introduction on EP, summarize the current status of experiments and consider the novelty of testing the EP in space. In particular, we will describe and compare three missions currently under investigation by independent laboratories around the world: MICROSCOPE (France), STEP (USA) and GG (Italy).

Place:  VUB - Campus Oefenplein, Building G, level 1 - Room 1G-003
Time:   Friday, October 15, 2004 at 14:30

Place:  K.U. Leuven, Celestijnenlaan 200 D 05.34
Time:   Thursday, October 14, 2004 at 13:00

I shall review the Randall-Sundrum domain wall solution arising from a massive Kaluza-Klein reduction of Type IIB supergravity in ten dimensions. I will discuss the stability of such an object and also reconsider its interpretation as a soliton of the Type IIB theory, conlcuding with some remarks on other such domain walls in general dimensions.

Place:  K.U. Leuven, Celestijnenlaan 200 D 05.34
Time:   Thursday, October 14, 2004 at 10:30

Place:  ULB, Campus Plaine, Salle Solvay (5th floor, NO building),
Time:   Thursday, October 7, 2004 at 13:30
Place:  ULB, Campus Plaine, 2.NO.707
Time:   Thursday, October 7, 2004 at 10:30

Informal seminar. Meaning a blackboard talk and interactions with the audience are expected.

Place:  ULB, Campus Plaine, 2.NO.6.10
Time:   Tuesday, September 28, 2004 at 14:00

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