# Group seminars

Besides the joint ULB-VUB-KUL-UMons seminars, the group organizes local seminars.

Tuesday, May 20 2014, 14h00

Amitabh Virmani (Institute of Physics, Sachivalaya Marg, Bhubaneswar)

### Integrability and Inverse Scattering in Supergravity

STU supergravity becomes an integrable system for solutions that effectively only depend on two variables. This class of solutions includes the Kerr solution and its charged generalizations that have been studied in the literature. We present an inverse scattering method that allows to systematically construct solutions of this integrable system. The method is similar to the one of Belinski and Zakharov for pure gravity but uses a different linear system due to Breitenlohner and Maison and here requires some technical modifications. We illustrate this method by constructing a four-charge rotating solution from flat space. A generalization to other set-ups is also discussed.

Campus Plaine, NO.6.10 (6th floor, NO Building)

Monday, May 12 2014, 14h30

Tomas Prochazka (Academy of Sciences of the Czech Republic, Prague)

### W infinity and triality

I review some properties of W algebras which are extensions of the Virasoro algebra in 2D conformal field theory by higher spin currents. Recently they have been shown to play a role in the Gaberdiel-Gopakumar duality between 3D higher spin theory and 2D CFT. After reviewing various methods to construct these algebras I use the conformal bootstrap to find the structure constants of one such W algebra, the so called W[1+infinity]. Finally, I make some comments on an interesting discrete triality symmetry of this algebra.

Campus Plaine, 2N6.110 (6th floor, NO Building)

Tuesday, May 6 2014, 14h00

Joris Raeymaekers (Academy of Sciences of the Czech Republic, Prague)

### Black hole deconstruction and 5D supergravity with hypermultiplets

I will briefly review the black hole deconstruction proposal, which describes black hole microstates in terms of brane configurations consisting of fluxed D6-anti-D6 pairs surrounded by ellipsoidal D2-branes. I will then report on some progress in constructing the corresponding backreacted microstate geometries in 5-dimensional supergravity, which involve exciting the universal hypermultiplet. After discussing the general structure of hypermultiplet solutions, I will classify susy solutions with isometries, extending earlier results of Bellorin, Meessen and Ortin. Of special importance are solutions with a toric Kahler base, which include a supersymmetric version of the Godel universe as well as new solutions relevant for black hole deconstruction.

Campus Plaine, NO.6.10 (6th floor, NO Building)

Tuesday, April 29 2014, 14h00

Arunabha Saha (Harish-Chandra Res. Inst., Allahabad)

### Phase Structure of Higher Spin Black Holes

We revisit the study of the phase structure of higher spin black holes carried out in arXiv:1210.0284 using the "canonical formalism". In particular we study the low as well as high temperature regimes. We show that the Hawking-Page transition takes place in the low temperature regime. The thermodynamically favoured phase changes from conical surplus to black holes and then again to conical surplus as we increase temperature. We then show that in the high temperature regime the diagonal embedding gives the appropriate description. We also give a map between the parameters of the theory near the IR and UV fixed points. This makes the "good" solutions near one end map to the "bad" solutions near the other end and vice versa.

Campus Plaine, NO.6.10 (6th floor, NO Building)

Thursday, April 24 2014, 11h30

Alan Garbarz (U. Nacional de la Plata)

### Virasoro group and AdS3 gravity

We revisit the description of the space of asymptotically AdS3 solutions of pure gravity in three dimensions with a negative cosmological constant as a collection of coadjoint orbits of the Virasoro group. Each orbit corresponds to a set of metrics related by diffeomorphisms which do not approach the identity fast enough at the boundary. Orbits contain more than a single element and this fact manifests the global degrees of freedom of AdS3 gravity, being each element of an orbit what we call boundary graviton. We show how this setup allows to learn features about the classical phase space that otherwise would be quite difficult. Most important are the proof of energy bounds and the characterization of boundary gravitons unrelated to BTZs and AdS3 . In addition, it makes manifest the underlying mathematical structure of the space of solutions close to infinity. Notably, because of the existence of a symplectic form in each orbit, being this related with the usual Dirac bracket of the asymptotic charges, this approach is a natural starting point for the quantization of different sectors of AdS3 gravity. We finally discuss previous attempts to quantize coadjoint orbits of the Virasoro group and how this is relevant for the formulation of AdS3 quantum gravity.

Campus Plaine, NO.6.10 (6th floor, NO Building)

Monday, February 24 2014, 14h00

Rodrigo Olea (U. Andrés Bello, Santiago)

### On renormalized AdS action and Critical Gravity

It can be shown that the renormalized action for AdS gravity in even spacetime dimensions is equivalent --on shell-- to a polynomial of the Weyl tensor, whose first non-vanishing term is Weyl^2. Remarkably enough, the coupling of this last term coincides with the one that appears in Critical Gravity.

Campus Plaine, Solvay room (5th floor, NO Building)

Monday, February 17 2014, 14h00

Hideki Maeda (CECs, Valdivia)

### Throat quantization of the Schwarzschild-Tangherlini(-AdS) black hole

Adopting the throat quantization pioneered by Louko and Makela, we derive the mass and area spectra for the Schwarzschild-Tangherlini black hole and its anti-de Sitter (AdS) generalization in arbitrary dimensions. We obtain exact spectra in three special cases: the three-dimensional BTZ black hole, toroidal black holes in any dimension, and five-dimensional Schwarzshild-Tangherlini(-AdS) black holes. For the remaining cases the spectra are obtained for large mass using the WKB approximation. For asymptotically flat black holes, the area/entropy has an equally spaced spectrum, as expected from previous work. In the asymptotically AdS case on the other hand, it is the mass spectrum that is equally spaced. Our exact results for the BTZ black hole with Dirichlet boundary conditions agree with those obtained previously via completely different methods.

Campus Plaine, Solvay room (5th floor, NO Building)

Monday, ferbruary 10 2014, 14h00

Miguel Pino (Universidad de Santiago de Chile)

### The Boulware-Deser mode in Zwei-Dreibein gravity

Massive gravity in three dimensions accepts several different formulations. Recently, the 3-dimensional bigravity dRGT model in first order form, Zwei-Dreibein gravity, was considered by Bergshoeff et al. and it was argued that the Boulware-Deser mode is killed by extra constraints. We revisit this assertion and conclude that there are sectors on the space of initial conditions, or subsets of the most general such model, where this mode is absent. But, generically, the theory does carry 3 degrees of freedom and thus the Boulware-Deser mode is still active. Our results also sheds light on the equivalence between metric and vierbein formulations of dRGT model.

Campus Plaine, NO.6.10 (6th floor, NO Building)

Wednesday, January 22 2014, 14h00

Malte Henkel (University of Nancy)

### Non-relativistic conformal invariance, its logarithmic extensions and physical ageing

The slow relaxations which may occur in many-body systems when quenched to a co-existence regime below their critical point can be characterised by the breaking of time-translation-invariance and dynamical scaling, with a non-trivial value of the dynamical exponent z. It is possible to formulate variants of conformal invariance for a given value of z, which can be different from 1. The best-known examples are the Schroedinger (z=2) and the conformal Galilean (z=1) algebras. The requirement of co-variance of the non-equilibrium two-time response functions under these transformations permits to predict the form of their universal scaling f unctions, in relatively good agreement with simulational data in specific models. Here, we present recent results on decomposable, but not irreducible representations of these algebras, which generalise known results from logarithmic conformal invariance. Applications include the directed percolation/Reggeon field-theory universality class and interface growth in the Kardar-Parisi-Zhang universality class.

Campus Plaine, Solvay room (5th floor, NO Building)

Friday, December 13 2013, 11h00

Georgios Itsios (U. of Patras)

### Addition of fermionic impurities to the unquenched ABJM

In this talk I shall describe the addition of fermionic impurities to the unquenched ABJM Chern–Simons-matter theory using the holographic approach. In this approach the impurities are represented by probe D6-branes extending along the radial direction and wrapping a five-dimensional submanifold inside a squashed CP^3. Using this setup, I will analyze the straight flux tube embeddings and the corresponding fluctuation modes of the D6-branes.

Campus Plaine, Solvay room (5th floor, NO Building)

Tuesday, December 10 2013, 14h00

Monica Guica (Pennsylvania U.)

### Gravitation from entanglement in holographic CFTs

Entanglement entropy obeys a first law', much like the ordinary first law of thermodynamics. In any CFT with a semiclassical holographic dual, we show that the first law of entanglement entropy for ball-shaped regions in the CFT is equivalent to the linearized gravitational equation of motion in the bulk, including arbitrary higher curvature corrections. The bulk equations of motion imply the first law of entanglement in the dual CFT, and conversely, the first law can be used to reconstruct the linearized bulk geometry solely from knowledge of the entropy function. The usual Ryu-Takayanagi formula $S={\cal A} /(4\Gn)$ leads to the linearized Einstein equations, whereas the Wald entropy functional leads to the equations for higher curvature gravity. Additionally, the first law can be used to derive the holographic dictionary for the stress tensor. In particular, this approach provides a simple alternative to holographic renormalization for computing its expectation value in arbitrary higher derivative gravitational theories. It also carries holographic information about the additional operators in the boundary theory which couple to the bulk metric in higher curvature gravity.

Campus Plaine, NO610 (6th floor, NO Building)

Monday, December 2 2013, 14h00

Karapet Mkrtchyan (Scuola Normale Superiore, Pisa)

### On Higher Spin Algebras in different dimensions

We discuss general properties of Higher Spin algebras in different space-time dimensions. We compute explicitly the Traces, Bilinear Forms and Structure Constants for these algebras. We discuss finite dimensional truncations of these algebras, that exist in 2, 3 and 5 dimensions.

Campus Plaine, OF2066 (Forum)

Thursday, November 28 2013, 14h00

Yun-Long Zhang (U. of Southampton & Chinese Academy of Sciences)

### Petrov type I Condition and Dual Fluid Dynamics?

Recently Lysov and Strominger [arXiv:1104.5502] showed that imposing Petrov type I condition on a (p + 1)-dimensional timelike hypersurface embedded in a (p + 2)-dimensional vacuum Einstein gravity reduces the degrees of freedom in the extrinsic curvature of the hypersurface to that of a fluid on the hypersurface, and that the leading-order Einstein constraint equations in terms of the mean curvature of the embedding give the incompressible Navier–Stokes equations of the dual fluid. In this paper we show that the non-relativistic fluid dual to vacuum Einstein gravity does not satisfy the Petrov type I condition at next order, unless additional constraint such as the irrotational condition is added. In addition, we show that this procedure can be inversed to derive the non-relativistic hydrodynamics with higher order corrections through imposing the Petrov type I condition, and that some second order transport coefficients can be extracted, but the dual “Petrov type I fluid” does not match the dual fluid constructed from the geometry of vacuum Einstein gravity in the non-relativistic limit. We discuss the procedure both on the finite cutoff surface via the non-relativistic hydrodynamic expansion and on the highly accelerated surface via the near horizon expansion.

Campus Plaine, O.9.906 (9th floor, NO Building)

Thursday, November 28 2013, 11h00

Gian Francesco Giudice (CERN)

### What did we learn from the BEH mass?

The discovery of the Brout-Englert-Higgs (BEH) boson has been a major milestone in physics. I will discuss what we learned from the measurement of the mass of this particle. In particular, I will consider the implications for the Standard Model, for theories beyond the Standard Model (such as supersymmetry and composite models), and for the stability of the scalar potential.

Joint seminar with the ULB PhysTh group

Campus Plaine, Solvay room (5th floor, NO Building)

Tuesday, November 26 2013, 11h00

Dario Rosa (U. Milano-Bicocca)

### All AdS_7 solutions of type II supergravity

In M-theory, the only AdS_7 supersymmetric solutions are AdS_7 x S^4 and its orbifolds. We find and classify new supersymmetric solutions of the type AdS_7 x M_3 in type II supergravity. While in IIB none exist, in IIA with Romans mass (which does not lift to M-theory) there are many new ones. We use a pure spinor approach reminiscent of generalized complex geometry. Without the need for any Ansatz, the system determines uniquely the form of the metric and fluxes, up to solving a system of ODEs. Namely, the metric on M_3 is that of an S^2 fibered over an interval; this is consistent with the Sp(1) R-symmetry of the holographically dual (1,0) theory. By including D8 brane sources, one can numerically obtain regular solutions, where topologically M_3 = S^3.

Campus Plaine, 2N6.110 (6th floor, NO Building)

Tuesday, October 29 2013, 11h00

Thomas Vanel (LPTHE & LPT ENS, Paris)

### Dressing the electron star in a holographic superconductor

Over the last five years, the gauge/gravity correspondence has been applied to describe quantum critical systems at finite density. The simplest model to consider is Einstein-Maxwell gravity, and the ground state of the system is described by Reissner-Nordstrom black hole where all the charge is carried by the black hole. However, it turns out that this solution is unstable to the formation of both fermionic and bosonic matter, corresponding in the dual field theory to the creation of a Fermi surface and the onset of superconductivity, respec- tively. We consider Einstein-Maxwell system coupled to a perfect fluid of charged fermions and a charged scalar field. In addition to the black hole, electron star and holographic super- conductor solutions, we find new asymptotically AdS4 solutions, dual to 2+1 CFTs at zero temperature and finite chemical potential, which contain both scalar hair and an electron star. We compute the free energy and show that these new solutions are thermodynami- cally favoured when they exist. Moreover, we find evidence for a continuous phase transition between the holographic superconductor and the new solutions.

Campus Plaine, 2N6.110 (6th floor, NO Building)

Tuesday, October 22 2013, 11h00

Gautier Solard (LPTHE, Paris)

### AdS vacua in IIB vacua, scale separation and SU(2) torsion

In this talk we study AdS_4 with N=1 supersymmetry in type IIB supergravity. Using Generalised Geometry it is easy to see that the internal manifold has to have SU(2) structure. We will then focus on coset and group manifolds and we will look for examples allowing for scale separation. We will also show that, for constant warp factor, it is not possible to have sourceless solutions.

Campus Plaine, 2N6.110 (6th floor, NO Building)

Monday, October 21 2013, 14h00

Giulio Bonelli (SISSA, Trieste)

### N=1 Geometries via M-theory

We provide an M-theory geometric set-up to describe four-dimensional N=1 gauge theories. This is realized by a generalization of Hitchin's equation. This framework encompasses a rich class of theories including superconformal and confining ones. We show how the spectral data of the generalized Hitchin's system encode the infrared properties of the gauge theory in terms of N=1 curves. For N=1 deformations of N=2 theories in class S, we show how the superpotential is encoded in an appropriate choice of boundary conditions at the marked points in different S-duality frames. We elucidate our approach in a number of cases -- including Argyres-Douglas points, confining phases and gaugings of T_N theories -- and display new results for linear and generalized quivers.

Campus Plaine, Salle des Professeurs (9th floor, NO Building)

Tuesday, September 17 2013, 14h00

Wolfgang Mueck (INFN Napoli)

### How many photons are bound by an electric charge?

The question of how many photons are bound by an electric charge may sound like discussing the sex of angels. However, recent proposals, in particular the mechanism of classicalization in non-renormalizable theories and the portrait of a black hole as a Bose-Einstein condensate of bound gravitons, with both involve the notion of an occupation number associated with some source, have made it actually an interesting question to be asked. In this seminar, I will argue that canonical quantization of the electromagnetic field in the presence of a classical current gives a definite and finite answer to this question. The answer supports the proposed occupation number formulae. The generalization to gravity is also discussed.

Campus Plaine, Solvay Room (5th floor , NO Building)

Monday, September 09 2013, 17h00

Mauricio Valenzuela (University Austral Chile, Valdivia)

### Three-dimensional fractional higher spin gravity

We construct a Chern-Simons theory consisting of fractional spin gauge fields coupled to Lorentz tensorial higher spin gauge fields and internal gauge fields.

Campus Plaine, NO.6.10 Room (6th floor , NO Building)

Tuesday, July 02 2013, 15h00

Teake Nutma (Max Planck Institute for Gravitational Physics, Potsdam)

### Higher spins in 3D

One of the simplest setups for higher spins is perhaps a spin-3 field coupled to 3D gravity. In the Chern-Simons formulation this theory looks deceitfully simple, but in the metric-like formulation things become very complicated, very fast. In this talk I will discuss some of the features and issues of the metric-like formulation.

Campus Plaine, NO.6.10 Room (6th floor , NO Building)

Tuesday, June 18 2013, 14h00

Maxim Grigoriev (Lebedev Physical Institute, Moscow)

### Higher order singletons, partially massless fields and their boundary values in the ambient approach

Using ambient space we develop a fully gauge and o(d,2) covariant approach to boundary values of AdS(d+1) gauge fields. It is applied to the study of (partially) massless fields in the bulk and (higher-order) conformal scalars, i.e. singletons, as well as (higher-depth) conformal gauge fields on the boundary. In particular, we identify the corresponding Fradkin-Tseytlin equations as obstructions to the extension of the off-shell boundary value to the bulk, generalizing the usual considerations for the holographic anomalies to the partially massless fields. We also relate the background fields for the higher-order singleton to the boundary values of partially massless fields and prove the appropriate generalization of the Flato-Fronsdal theorem, which is in agreement with the known structure of symmetries for the higher-order wave operator. All these facts support the following generalization of the higher-spin holographic duality: the O(N) model at a multicritical isotropic Lifshitz point should be dual to the theory of partially massless symmetric tensor fields described by the Vasiliev equations based on the higher-order singleton symmetry algebra.

Campus Plaine, Solvay Room (5th floor , NO Building)

Tuesday, June 11 2013, 14h00

Marios Petropoulos (Ecole Polytechnique, Paris)

### Holographic fluids, nut charge and the Cotton tensor

In view of the recent interest in reproducing holographically various properties of conformal fluids, we analyze the emergence of rotation and vortices in the framework of AdS/CFT. The bulk nut charge appears as a source of boundary vorticity with non-vanishing Cotton tensor while the nut - mass duality translates into a Cotton tensor - stress tensor duality. A class of boundary geometries appear in this way that satisfy the topological massive gravity equations with consequences on the transport coefficients of the corresponding fluids.

Campus Plaine, Room NO.6.10 (6th floor , NO Building)

Thursday, June 6 2013, 14h00

Anastasios Petkou

### Singleton deformation of AdS4 higher-spin theory and the phase structure of the 3d O(N) vector model

We consider a singleton deformation of the AdS4 higher-spin theory dual to the three-dimensional O(N) vector model. The singleton couples to the higher-spin multiplet only through a marginal boundary interaction. We argue that the effect of such a deformation is to shift N -> N+ 1 in both sides of the holographic correspondence and we show how the gap equations of the three-dimensional O(N) vector model arise from the higher-spin theory. The singleton deformation breaks higher-spin symmetry and gives rise to the well-known 1/N anomalous dimensions of the boundary theory

Campus Plaine, Solvay Room (5th floor, NO Building)

Tuesday, June 4 2013, 11h00

Simon Knapen (Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854, USA)

### Higgs Mediation

I will review the mu/Bmu and the A/mH problems in the context of Higgs mediated supersymmetry breaking and present a general framework for Higgs-Messenger interactions, as well as two examples. This approach proves to be particularly powerful for solutions that rely on strong dynamics in the supersymmetry breaking sector.

Campus Plaine, Solvay Room (5th floor, NO Building)

Thursday, May 23 2013, 11h00

M. Sivakumar

### Exact Bosanisation in 3D

An exact equivalence of U(1)scalar field with level-1 CS gauge field to free fermions will be discussed. Fermion operator is constructed out of bosonic fields. Extension to interacting theory will also be discussed. No low energy or long wave length approximations used.

Campus Plaine, Room NO.6.10 (6th floor, NO Building)

Tuesday, May 14 2013, 14h00

Andrea Mezzalira

### Fermionic Corrections to Fluid Dynamics from BTZ Black Holes

We reconstruct the complete fermionic orbit of BTZ black hole by acting with finite supersymmetry transformations. The solution satisfies the exact supergravity equations of motion to all orders in the fermonic expansion and the final result is given in terms of fermionic bilinears. By fluid/gravity correspondence, we derive linearized Navier-Stokes equations and a set of new differential equations from Rarita-Schwinger equation. We compute the boundary energy-momentum tensor and we interpret the result as a perfect fluid with a modified definition of fluid velocity. Finally, we derive the modified expression for the entropy of the black hole in terms of the fermionic bilinears. This talk is based mainly on arXiv:1302.5060

Campus Plaine, NO.6.10 (6th floor, NO Building)

Tuesday, April 23 2013, 11h00

Joan Simon (School of Mathematics, Edinburgh University)

### A strongly coupled zig-zag transition

The zig-zag symmetry transition is a phase transition in 1D quantum wires, in which a Wigner lattice of electrons transitions to two staggered lattices. Previous studies model this transition by a Luttinger liquid coupled to a Majorana fermion. They exhibit interesting RG flows, involving quenching of velocities in subsectors of the theory. We suggest an extension of this model by replacing the Majorana fermion by a more general CFT, which includes an experimentally realizable case with two Majorana fermions. We analyse its RG flow both in field theory and using AdS/CFT techniques in the large c limit of the CFT. The model has a rich phase structure with new qualitative features, already in the two Majorana fermion case. The AdS/CFT calculation involves considering back reaction in space-time to capture subleading effects.

Campus Plaine, Solvay Room (5th floor, NO Building)

Tuesday, February 19 2013, 11h30

Igor Khavkine

### Covariant phase space symplectic form and Peierls inversion formula in the presence of constraints and gauge

It is well known that a Poisson bracket may be constructed for a field theory directly from its Lagrangian using the Peierls formula. However the Peierls formula requires the existence of some retarded and advance Green functions, which is not automatic. It is also well known that the canonical symplectic form may also be be constructed directly from the Lagrangian using the covariant phase space method. An elegant argument showing that these two formulas are mutually inverse in the case of scalar field theory with normally hyperbolic equations of motion was given by Forger & Romero [arXiv:math-ph/0408008]. I generalize their argument to arbitrary field theories, which may have constraints and be gauge invariant, provided certain sufficient conditions hold. These conditions include symmetric hyperbolizability of the equations of motion and the vanishing of certain cohomologies of elliptic complexes that extend the constraints and the gauge generators. Based on p art of [arXiv:1211.1914].

Campus Plaine, Solvay Room (5th floor, NO Building)

Tuesday, February 19 2013, 14h00

Jakob Palmkvist

### Lie superalgebras and supergravity

I will explain two different ways of extending the U-duality algebra of maximal supergravity in D dimensions to an infinite-dimensional Lie superalgebra with a corresponding level decomposition, such that the dynamical p-form fields transform in the representations at level p. One way, which leads to a Borcherds algebra, is to add an odd null root to the set of simple roots of the U-duality algebra. The other way is to consider the embedding tensor, which describes gauge deformations of the theory, as the components of an element at level -1. This leads to a new Lie superalgebra with a non-triangular structure, which gives exactly the tensor hierarchy of representations arising in the embedding tensor formalism. I will prove that these representations are always contained in those coming from the Borcherds algebra, and we explain why some of the latter are not included in the tensor hierarchy. Furthermore, I will show that the different Borcherds algebras for different D can be unified into a single one. The talk is based on 1301.1346 (with A. Kleinschmidt) and work in progress.

Campus Plaine, Solvay Room (5th floor, NO Building)

Thursday, June 21 2012, 11h00

Euihun Joung (Pisa SNS)

### Cubic interactions of higher spins in (A)dS

Cubic interactions of massive and partially-massless totally-symmetric higher-spin fields in any constant-curvature background of dimension greater than three are investigated. Making use of the ambient-space formalism, the consistency condition for the traceless and transverse parts of the parity-invariant interactions is recast into a system of partial differential equations. The latter can be explicitly solved for given s_1-s_2-s_3 couplings and the 2-2-2 and 3-3-2 examples are provided in detail for general choices of the masses. On the other hand, the general solutions for the interactions involving massive and massless fields are expressed in a compact form as generating functions of all the consistent couplings. The St\'uckelberg formulation of the cubic interactions as well as their massless limits are also analyzed.

Campus Plaine, Solvay Room (5th floor, NO Building)

Monday, May 7 2012, 11h00

Gonzalo Torroba (Stanford)

### Uplifting AdS/CFT to cosmology

In this talk we will present concrete steps towards a complete holographic duality for de Sitter and certain FRW cosmologies. The basic strategy is to start from AdS/CFT dual pairs and introduce uplifting' ingredients which produce cosmological solutions. de Sitter space is realized in terms of a metastable brane/antibrane system, providing a microscopic interpretation of the Gibbons-Hawking entropy. Its decay into open FRW gives rise to a time-dependent warped metric, and we analyze the basic features of the holographic dual. At finite times, this theory has a finite cutoff and dynamical gravity, but at late times the entropy goes off to infinity, suggesting a precise field theory dual.

Campus Plaine, Solvay Room (5th floor, NO Building)

Tuesday, April 24 2012, 14h00

Robert Berman

### Kähler-Einstein metrics emerging from free fermions and statistical mechanics

From a statistical mechanical point of view it is natural to view differential geometry as an emergent phenomena: the smooth shapes that we see are emergent effects of some underlying microscopic model, as the number of particles tends to infinity. On the other hand, from a mathematical point of view it is also natural to view differential geometry as a limit of algebraic geometry, as the "degree" tends to infinity. Naively, this just amounts to the fact that any smooth curve can be approximated by a polynomial curve, but, in fact, this idea goes much deeper and is related to the fundamental "Yau-Tian-Donaldson conjecture" concerning Kähler-Einstein metrics on projective algebraic varities. In this talk I will explain how these two different points of view on differential geometry can be merged, leading to a new statistical mechanics approach to Kähler-Einstein metrics. It turns out that - from a physical point of view - the underlying microscopic theory consists of a gas of free fermions subject to a non-standard “beta-deformation”, making it back-ground free. Time permitting I will also point out some connections to quantum gravity and speculate on possible relations to the recent work of Ferrari-Klevtsov-Zelditch on random Kähler metrics.

Campus Plaine, Solvay Room (5th floor, NO Building)

Tuesday, April 24 2012, 11h00

Alfredo Perez

### Regularized action for higher spin gravity in 3D: black holes, global charges and thermodynamics.

A finite action principle for higher spin gravity in three dimensions is proposed and explicitly tested for a class of spin 3 black holes that have been recently found. Our results turn out to agree with the ones obtained through the canonical (Regge-Teitelboim) approach.

Campus Plaine, Room N0.6.10

Friday, February 24 2012, 14h00

Alexander Belavin

### Instantons and 2d Coset CFT.

A few years a Alday,Gaiotto and Tachikawa dicovered a remarkable correspondence between $N=2$ SUSY gauge theory in $4$ dimensions and $2$-dimensional Conformal field theory. We consider the generalization this correspondence to the case 4-dimensional $N=2$ SUSY $SU(r)$ gauge theory on the $R_4/Z_p$ and Coset 2d Conformal field theory with the chiral algebra $A{r,p}=frac{widehat{**mathfrak{gl}](n)_r}>{widehat{mathfrak{gl}}(n-p)_**r}$. The algebraic reason of the correspondence is the existence of a natural action of this chiral algebra on cohomologies of Instanton Moduli space. Basis of the cohomologies space consists of the torus fixed points (Nekrasov's instantons). From 2d point of view it is a special basis in the Algebra of Local Fields in the corresponding CFT. Structure constants of OPE in this basis are given by explicit and simple expressions. I describe the construction of such basis for case $r=2$, $p=2$ .

Campus Plaine, Room N0.6.10

Tuesday, February 21 2012, 11h00

Jesper Greitz

### Borcherds algebras from superspace

The positive roots of certain Borcherds algebras have been known for some time to encode the form fields of maximal supergravities. The apparent discrepancy between the finite number of form fields and the infinitely many roots of Borcherds algerbas is naturally resolved in a superspace framework. The degree of forms in superspace do not have a limit, and therefore there is no need to truncate the positive roots of the Borcherds algebra. In particular the field strength form fields of degree \geq D+1 allowed by supersymmetry agree with the predictions of the relevant Borcherds algebras in type IIA and IIB supergravity. All though the superforms of degree \geq D+2 are zero in supergravity we show, using the example of half-maximal theories, that they need not be zero when string corrections are included. At the end of the seminar I will discuss how the consistent and soluble Bianchi identities for the field strengths define Lie super co-algebras. This hints towards an explanation of why Borcherds algebras appear in supergravities.

Campus Plaine, Solvay Room (5th floor, NO Building)

Friday, January 20 2012, 14h00

George Marinescu

### Equidistribution of zeros of holomorphic sections of high tensor powers of line bundles

We present some equidistribution results for sequences of random sections of high tensor powers of positive line bundles over non-compact manifolds (e.g. Riemann surfaces with cusps, arithmetic quotients or, more generally, quasi-projective manifolds). We also examine the equidistribution of sections of big line bundles endowed with singular Hermitian metrics.

Campus Plaine, Salle des professeurs (Ninth floor, NO building)

Thursday, January 17 2012, 14h00

Juan Maldacena

### Constraining theories with higher spin symmetry

We consider conformal field theories with exactly conserved currents of spin s> 2. We show that such theories are essentially free theories. This is an extension of the Coleman Mandula theorem to the case of conformal field theories. We also discuss the case where the higher spin symmetry is slightly broken.
Based on papers by JM and A. Zhiboedov, *arXiv:1112.1016& to appear.

Campus Plaine, Solvay Room (5th floor, NO Building)

Monday, October 17 2011, 10h30

Frank Ferrari

### Two Dimensional Quantum Gravity Revisited

I'll describe in elementary terms a new approach to the sum over Kähler metrics which provides in particular a rigorous definition of the quantum gravity path integral in two dimensions. The method relies on rather recent developments in complex geometry by Tian, Yau, Donaldson and others. If I have enough time, I'll also briefly talk about new gravitation effective actions (beyond the standard Liouville case) that show up naturally in this context.

Campus Plaine, Room NO.6.10 (Sixth floor, NO building)

Monday, September 12 2011, 10h30

Hamid R.Afshar

### Holograms of Conformal Chern-Simons Gravity

We show that conformal Chern-Simons gravity in three dimensions has various holographic descriptions. They depend on the boundary conditions on the conformal equivalence class and the Weyl factor, even when the former is restricted to asymptotic Anti-deSitter behavior. For constant or fixed Weyl factor our results agree with a suitable scaling limit of topologically massive gravity results. For varying Weyl factor we find an enhancement of the asymptotic symmetry group, the details of which depend on certain choices. We focus on a particular example where an affine u(1) algebra related to holomorphic Weyl rescalings shifts one of the central charges by 1. The Weyl factor then behaves as a free chiral boson in the dual conformal field theory.

Campus Plaine, Salle des professeurs (Ninth floor, NO building)

Wednesday, September 07 2011, 14h00

Adam Schwimmer (Weizmann Institute of Science, Israel)

### Trace Anomaly Matching and the a-Theorem

The a-Theorem gives a characterization of RG flows for unitary theories in d=4. We will discuss a proof of the theorem based on the matching of conformal anomalies and the special features of the dilaton effective action in spontaneously broken conformal theories.

Campus Plaine, Solvay Room (Fifth floor, NO building)

Thursday, June 30 2011, 14h00

Bernard L. Julia (Ecole Normale Supérieure)

### The supermagic triangle is 30

TBA

Campus Plaine, Salle des professeurs (Ninth floor, NO building)

Thursday, June 30 2011, 11h00

Eric A. Bergshoeff (University of Groningen)

### New Results on Supersymmetric Solitons in String Theory

We probe doubled geometry with dual fundamental branes, i.e. solitons. Restricting ourselves first to solitonic branes with more than two transverse directions we find that the doubled geometry requires an effective wrapping rule for the solitonic branes which is dual to the wrapping rule for fundamental branes. This dual wrapping rule can be understood by the presence of Kaluza-Klein monopoles. Extending our analysis to supersymmetric solitonic branes with less than or equal to two transverse directions we show that such solitons are precisely obtained by applying the same dual wrapping rule to these cases as well. This extended wrapping rule can not be explained by the standard Kaluza-Klein monopole alone. Instead, it suggests the existence of a class of generalized Kaluza-Klein monopoles in ten-dimensional string theory.

Campus Plaine, Salle des professeurs (Ninth floor, NO building)

Wednesday, June 1 2011, 14h00

Xavier Bekaert (Tours U., CNRS)

### Effective action in a higher-spin background

The regularised effective action, obtained after integrating out a scalar field with the most general (quasi-)local quadratic action, is computed perturbatively in the external fields. The latter fields are interpreted as an infinite tower of background higher-spin gauge fields. The non-Abelian group of higher-spin symmetries and anomalies, as well as the structure of the various (divergent or finite) terms, is determined. The relation of these results with the holographic correspondence and a generalized induced gravity programme (à la Sakharov) is discussed.

Campus Plaine, Solvay Room (Fifth floor, NO building)

Tuesday, May 31 2011, 11h00

Alan Garbarz (University of Buenos Aires)

### Topics on 3d gravity and AdS/CFT

In this talk we will discuss some aspects of three dimensional gravity and the recent attempts to construct its quantum version in a consistent manner via AdS/CFT correspondence. We will start reviewing seminal results in the field, such as Brown-Henneaux´s work on asymptotic symmetries, which will play a main role in the discussion. The main idea of Witten´s work of 2007 will also be commented due to its importance in triggering the recent study of others (than GR) gravitational theories in three dimensions. Among these theories, we will discuss the so-called Chiral Gravity, its cousin dubbed Log Gravity, and the more recent New Massive Gravity. These theories are intended to give alternatives for a possible quantum theory of gravity in three dimensions, with notable differences in comparison with GR, such as novel black holes solutions and deformed AdS asymptotics. In addition, possible physically sensible contributions to the partition function will be mentioned, like particle-like singularities and space-times with torsion.

Campus Plaine, Room NO.6. 10 (Sixth floor, NO building)

Monday, May 16 2011, 11h00

Dmitry Melnikov (Tel Aviv University)

### "High Density Holographic Baryons"

Large N hadronic matter must possess a nuclear crystal phase. In my talk I will discuss a holographic realization of nuclear crystals. In particular I will be interested in the behavior of the crystals at very high densities, where the phase transition to a quark liquid phase is expected.

Campus Plaine, Solvay Room (Fifth floor, NO building)

Monday, April 11 2011, 14h00

Oscar Bedoya (Sao Paulo)

### "Superstring Sigma Models Computation Using the Pure Spinor Formalism"

In this talk I will show how using one-loop computations in the Superstring Sigma model with pure spinors, one can extract useful information about the space-time. I will consider three examples: The space-time equations of motion for the the type II Superstring, the Yang-Mills Chern Simons corrections, and finally, the quantum current algebra for the superstring in the $AdS_5 \times S^5$ background.

Campus Plaine, Solvay Room (Fifth floor, NO building)

Thursday, February 17 2011, 14h00

Rakibur Rahman (Scuola Normale Superiore)

### "String Theory and The Velo-Zwanziger Problem"

The Velo-Zwanziger problem is the loss of (causal) propagation of a charged massive field with spin > 1, when minimally coupled to a constant electromagnetic background. String theory, originally proposed as a theory of massive high-spin particles, should spell out appropriate non-minimal terms to give a consistent description of such a system. We show that while string theory does achieve this feat, only the leading Regge trajectory fields can be exposed in isolation. We comment on the roles of the critical dimension and of the gyromagnetic ratio, and prescribe some consistent dimensional reduction that can produce interesting models.

Campus Plaine, Room NO.6. 10 (Sixth floor, building NO)

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Updated February 2011 - Comments regarding this webpage can be addressed to delphine.vantighem AT ulb.ac.be