Theoretical Particle Physics

# Joint seminars academic year: 2014-2015

### Grassmannian Geometry of Scattering Amplitudes

Jaco Bourjaily (NBI, Copenhagen, Denmark)
Wednesday, May 20, 2015 - 13:00
ULB - Solvay Room

The past several years have been witness to an ongoing revolution in our understanding of (perturbative) quantum field theory. In particular, a concrete proposal now exists for how to reformulate any theory recursively—without any reference to virtual particles, gauge redundancies, or any of the other unphysical baggage that so complicates computations in the traditional formalism. In addition to greatly simplifying computations, the recursive reformulation provides an important connection between field theory and the geometry (and combinatorics) of certain subspaces of the Grassmannian—a connection that has proven extremely fruitful for both physics and mathematics in recent years. Because both sides of this connection are greatly simplified (and best understood) in the case of planar, maximally supersymmetric Yang-Mills, this will be the primary example discussed. I will provide a broad overview to these ideas, starting from basic principles of quantum mechanics.

### Minimal unitary representations, AdS/CFT dualities and higher spin algebras and their deformations

Murat Guaydin (Penn State USA)
Wednesday, May 20, 2015 - 10:30

I will first review the twistorial oscillator construction of unitary representations of space-time symmetry (super-) algebras that underlie AdS/CFT dualities. Then I will discuss the geometric quasiconformal realizations of these algebras whose quantization leads to their minimal unitary representations and their deformations. The enveloping algebras of the minimal unitary realizations of AdS_(d+1)/Conf_d (super)-algebras define the corresponding higher spin (super)-algebras in the respective dimensions. I will discuss how these novel realizations lead to a one parameter family of AdS_5/Conf_4 higher spin (super)-algebras labelled by four dimensional helicity. One finds a discrete infinite family of AdS_7/Conf_6 higher spin (super)-algebras labelled by the spin of an SU(2) symmetry which is the 6d analog of helicity. In AdS_6 there is a single deformation of the higher spin algebra and a unique higher spin superalgebra given by the enveloping algebra of the minimal unitary realization of the exceptional Lie superalgebra F(4) which is the unique superconformal algebra in five dimensions.

### Integrable $\lambda$-Deformations and AdS spacetimes

Dan Thompson (VUB & International Solvay Institutes)
Wednesday, May 6, 2015 - 13:00
ULB - Solvay Room

In this talk I will introduce a novel class on integrable 2d field theories that interpolate between (gauged)-WZW models and the non-Abelian T-dual of the principal chiral model describing strings on a group (coset) space. We will focus on these so-called lambda deformations applied to AdS_n \times S^n spacetimes and show how these deformations can be given a full SUGRA embedding. In the context of holography, these theories have been conjectured to encode quantum group deformations of string S-matrix for the case that the deformation parameter is a root of unity. Though integrability is in the title, it will be integrability-lite: the emphasis of this talk will be pedagogical and I will be sure to explain lots of useful background concepts.

### Probing the chiral and deconfinement transition in a magnetic field using soft wall QCD models

David Dudal (UGent & KULeuven)
Wednesday, May 6, 2015 - 10:30

5D wall models (with cut-off AdS space) are a popular tool to study nonperturbative aspects of the strong interaction, usually described by the Quantum Chromodynamics (QCD) Lagrangian. They are in particular useful to incorporate ---to some extent--- the key features of confinement and chiral symmetry breaking, based on which a rich QCD phenomenology can be probed, including the phase diagram in terms of temperature. Here, we discuss the additional effect of a magnetic field (presumably of relevance to heavy ion collision physics or early time cosmology) on both transitions. Firstly, we study in a soft wall setting the J/Psi charmonium bound state and its melting as the phenomenological messenger of deconfinement. We also have a look at some transport coefficients of the plasma. Secondly, we adopt a magnetic dual metric (due to d'Hoker and Kraus) as a modified background for the soft wall model and we explain why. For small values of the magnetic field, we hope to find first holographic evidence for the unexpected 'inverse magnetic catalysis' as reported by state-of-the-art lattice data concerning the chiral transition. We briefly point out improvements to be implemented in the future to come closer to genuine QCD, even at zero magnetic field. The talk is based on Phys.Rev. D91 (2015) 086002 (with T. Mertens) and work in progress (with T. Mertens and D.R. Granado).

### F-theory and the geometry of symmetries

Timon Weigand (U. Heidelberg, Germany)
Wednesday, April 29, 2015 - 13:00
VUB - D03.06

F-theory is a powerful framework to describe the dynamics of a large class of non-perturbative string vacua. Recent years have seen a lot of progress in our understanding of the dictionary between the effective quantum field theory associated with such F-theory vacua and the geometry and topology of the compactification space. We will describe some recent developments in this context and explain how objects of typical interest in arithmetic geometry acquire a direct physical meaning via F-theory. In particular we will exemplify how the emergence of discrete symmetries in the effective action relates to beautiful geometric concepts such as torsional homology, conifold transitions or even an object known to mathematicians as the Tate-Shafarevich group. The resulting structure explicitly realizes general conjectures concerning the nature of global symmetries in any theory of quantum gravity.

### A topologically twisted index for 3d SUSY gauge theories

Francesco Benini (Imperial, UK)
Wednesday, April 29, 2015 - 11:00

I will discuss an index, different from the superconformal index, obtained by placing supersymmetric gauge theories on S^2 x S^1 with topological twist on S^2. This index counts ground states on the sphere in the presence of magnetic fields, and has applications to branes wrapped on Riemann surfaces. I will present an exact formula, obtained with localization techniques, give examples and compare with known dualities. As a byproduct, I will obtain a formula for 2d theories on S^2 (and an alternative way to compute A-model topological amplitudes) and 4d theories on S^2 x T^2.

### Universal Properties of Supersymmetric Partition Functions

Lorenzo de Pietro (Weizmann Institute, Israel)
Wednesday, April 22, 2015 - 13:00
KUL - 200C 01.06

We consider 4d N=1 superconformal theories on a cylinder. The partition function on this geometry computes the superconformal index, and can be obtained via the path integral with time direction compactified on a circle and periodic conditions for fermions. We will describe universal formulas for the asymptotics of such partition functions in the limit of very large circle and of very small circle. These limits are completely fixed in terms of coefficients of the Weyl anomaly (a,c). We will explain why supersymmetry is a necessary condition in 4d to establish these higher dimensional analogues of classic results in 2d CFTs. Finally we will discuss some applications and the extension to 6d.

### Gauged supergravities: a window on (non) geometric string compactifications

Wednesday, April 22, 2015 - 10:30
KUL - 200C 01.06

I will review some recent discoveries that hugely enlarged the landscape of possible string effective theories. I will especially focus on the uplift of the vacua of maximal supergravity and on the surprising relations that emerge between some (non)-geometric reductions and gaugings.

### Higher spin dS/CFT

Arnaud Lepage-Jutier (ULB)
Wednesday, April 15, 2015 - 13:00
ULB - Solvay Room

I will present the Higher Spin Realization of the dS/CFT correspondence of Anninos, Hartman and Strominger, outlining the recent and less recent developments, as well as examining some interesting puzzles in the holographic understanding of toy cosmologies. Discussion is highly encouraged!

### Exact correlation functions in 4d N = 2 SCFTs

Marco Baggio (ETH Zurich, Switzerland)
Wednesday, April 15, 2015 - 10:30
ULB - Solvay Room

We describe a technique to compute exact correlation functions of chiral primary operators in N=2 superconformal field theories. These correlation functions are nontrivial, receiving both perturbative and non-perturbative corrections, and obey exact differential equations with respect to the coupling constants. We provide the complete solution for 2- and 3-point functions of chiral primaries in N=2 SU(2) superconformal QCD using input from supersymmetric localization, and we compare the results to a 2-loop perturbative computation. We also comment on extensions to more general setups and implications for generic N=2 SCFTs.

### Anisotropic Holographic Insulators and Homes' Relation

Rene Meyer (IPMU Japan)
Wednesday, April 1, 2015 - 13:00
VUB - K.2.Auditorium.2

We present results on a holographic s-wave superconductor in a helically symmetric Bianchi VII symmetric space-time, and discuss the validity of Homes' law in this system. We determine the phase diagram in terms of the helix parameters, and the AC conductivity of the different phases. We in particular show that Homes' relation holds for an regime of intermediary momentum relaxation strength. For both weak and very strong lattice perturbations, i.e. weak and strong momentum dissipation, Homes' relation is violated.

### The Fate of Antibranes

Fridrik Gautason (KULeuven)
Wednesday, April 1, 2015 - 10:30
VUB - K.2.Auditorium.2

I discuss the possible resolution mechanisms for the well known antibrane singularities. I will make a sharp distinction between antibranes with AdS worldvolume and compact internal space, and ones with flat worldvolume and noncompact internal space. The latter includes the insertion of anti-D3 branes in the KS throat which is at the heart of the KKLT construction. The resolution of the singularity occurs either through the Myers effect which effectively shields the singularity or the decay of the entire solution through a NS5 domain wall.

### Higher spin extension of cosmological spacetimes in three dimensions: asymptotic symmetries and thermodynamics

David Tempo (ULB & CEC Valdivia Chile)
Wednesday, March 25, 2015 - 13:00
KUL - 200C 1.06

A generalized set of asymptotic conditions for higher spin gravity without cosmological constant in three spacetime dimensions is constructed. They include the most general temporal components of the gauge fields that manifestly preserve the original asymptotic higher spin extension of the BMS3 algebra, with the same central charge. By virtue of a suitable permissible gauge choice, it is shown that this set can be directly recovered as a limit of the boundary conditions that have been recently constructed in the case of negative cosmological constant, whose asymptotic symmetries are spanned by two copies of the centrally-extended W3 algebra. Since the generalized asymptotic conditions allow to incorporate “chemical potentials” conjugated to the higher spin charges, a higher spin extension of locally flat cosmological spacetimes becomes naturally included within the set. It is shown that their thermodynamic properties can be successfully obtained exclusively in terms of gauge fields and the topology of the Euclidean manifold.

### Mellin amplitudes: the scattering amplitudes of Conformal Field Theory

Joao Penedones (U. Porto, Portugal)
Wednesday, March 25, 2015 - 10:30
KUL - 200C 01.06

Correlation functions of local operators are the most natural observables in a Conformal Field Theory (CFT). I will argue that writing these correlation functions using the Mellin representation makes their properties more transparent. In particular, it gives rise to a useful analogy between Mellin amplitudes and scattering amplitudes. I will review the definition and main properties of Mellin amplitudes. Namely, its factorization properties and its applications in the context of the AdS/CFT correspondence.

### Entwinement and the reconstruction of space

Vijay Balasubramanian (Pennsylvaina U., USA & VUB)
Wednesday, March 11, 2015 - 13:00
KUL - 200C 01.06

### Model building after LHC8

Diego Redigolo (LPTHE Paris)
Wednesday, March 11, 2015 - 10:30
KUL - 200C 01.06

As an introduction to the subject, I will first review in light of the first LHC run several model building proposals for Beyond the Standard Model physics. Special attention will then be devoted to low-energy supersymmetry breaking and especially to gauge mediation models. In this context I will discuss in details the striking consequences of imposing the Higgs mass constraint and give a glance to the LHC phenomenology.

### Unwinding Inflation and Brane Dynamics

Marjorie Schillo (KULeuven)
Wednesday, February 25, 2015 - 13:00
KUL - 200C 01.06

Higher-form flux that extends in all 3+1 dimensions of spacetime is a source of positive vacuum energy that can drive meta-stable eternal inflation. If the flux also threads compact extra dimensions, the spontaneous nucleation of a bubble of brane charged under the flux can trigger a classical cascade that steadily unwinds many units of flux, gradually decreasing the vacuum energy while inflating the bubble. The cascade will end in the self-annihilation of the brane into radiation. This provides a viable mechanism for inflation, with primordial curvature perturbations coming from both standard slow-roll fluctuations and also periodic open string production due to brane scattering. The central mechanism, the flux cascade, involves repeated brane-antibrane scattering events. Thus, the study of this model precipitated new results regarding the spectrum of open string production during scattering, and the probability of annihilation for brane-antibrane scattering at small impact parameter.

### The Thermal Scalar and Random Walks in Curved Spacetime

Thomas Mertens (UGent)
Wednesday - February 25, 2015 - 10:30
KUL - 200C 01.06

I will discuss the generalization of the random walk picture of the near-Hagedorn string gas for curved spacetimes. I start by (qualitatively) reviewing high-temperature string thermodynamics in flat space. Then, after introducing our approach based on a combination of the worldsheet path integral and the thermal scalar field theory, I consider the specific case of the AdS3 (and BTZ) WZW model. After that I apply our method to Rindler space, the results of which we interpret as providing a realization of Susskind’s picture of the long string surrounding the black hole event horizon. Based on arXiv:1305.7443, arXiv:1307.3491, arXiv:1402.2808, arXiv:1408.6999, arXiv:1408.7012 and arXiv:1410.8009 in collaboration with H. Verschelde and V. I. Zakharov.

### Ambitwistors and the Scattering Equations

David Skinner (Cambridge, UK)
Wednesday, February 18, 2015 - 13:00
ULB - Solvay Room

Scattering amplitudes have a rich hidden structure that is completely obscured by their traditional representation in terms of Feynman diagrams. Recently, Cachazo, He and Yuan have discovered new representations of tree amplitudes for the scattering of any number of particles in a wide variety of massless theories, including Yang-Mills, Gravity and Nonlinear Sigma Models. I'll explain what these CHY representations really mean, showing how they're intimately related to the geometry of the space of null geodesics, known as ambitwistor space.

### Extremal chiral ring states in AdS/CFT are described by free fermions

David Berenstein (UC Santa Barbara, USA)
Wednesday, February 18, 2015 - 10:30

Half BPS states (operators) in N=4 SYM are famously described by free fermions both at weak and strong coupling. I describe a set of conjectures for a preferred class of states in more general conformal field theories that can be tested in supergravity for when such a free fermion description might arise and some motivation for it applying generally. The states in question belong to the chiral ring of a supersymmetric conformal field theory that extremize an additional U(1) charge for fixed dimension and can be reduced to multi-traces of a composite matrix field, which is equivalent to using Young tableaux (Schur polynomials) as a basis. The main conjecture asserts that if the Young tableaux are orthogonal, then the set of extremal three point functions of traces to order 1/N are determined up to a single constant. The conjecture is extended further by providing an exact norm for the Schur basis and this norm arises from a set of free fermions for a generalized oscillator algebra.

### Consistently violating the non-Gaussian consistency relation

Gonzalo A. Palma (U. Chile, Santiago)
Wednesday, February 11, 2015 - 13:00
UMONS

Cosmic inflation explains the origin of primordial curvature perturbations, the cosmic seed needed to explain the Cosmic Microwave Background anisotropies, and the appearance of all structures in the universe. In this talk, I will review the so called Maldacena’s consistency relation, which tells us that the 3-point and 2-point functions of curvature perturbations are related in certain limit. This consistency relation is regarded as one of the most powerful tools available in cosmology to falsify a large variety of inflationary models. In addition, I will discuss certain class of models, called non-attractor models, that violate this consistency relation. I will show that in such models, the 3-point and 2-point functions are still related together, in a way that may be determined exclusively from symmetries.

### Quantum higher spins and AdS/CFT

Wednesday, February 11, 2015 - 10:30
UMONS

We will review recent work on computing one-loop corrections of quantum higher spins fields in AdS spaces and corresponding tests of vectorial AdS/CFT duality. Part of this talk will be based on arXiv:1410.3273.

### The Effective Field Theory of Large Scale Structures

Enrico Pajer (U. Utrecht, Nederland)
Wednesday, December 10, 2014 - 13:00
ULB - Solvay room

Large Scale Structures (LSS), namely the distribution of matter in the universe, are sensitive probes of all elements of our cosmological model, such as for example Dark Matter, Dark Energy and the mechanism that generates primordial perturbations. Given the high precision of future LSS data it is very important to obtain equally accurate theoretical predictions. Inhomogeneities on scales larger than about Mpc are small and can hence be described analytically. A consistent formulation of the problem requires the use of Effective Field Theory and provides highly accurate analytical predictions. These results will be useful in extracting information from upcoming cosmological surveys.

### A New Model for Elliptic Fibrations of Rank 1

Mboyo Esole (Harvard University, USA)
Wednesday, December 10, 2014 - 10:30
ULB -Solvay room

We introduce a new model for elliptic fibrations endowed with a Mordell-Weil group of rank one. We will use this model to determine the spectrum of singular fibers of an elliptic fibration of rank one and compute a generating function for its Euler characteristic. With a view toward string theory, we determine a semi-stable degeneration which is understood as a weak coupling limit in F-theory. We show that it satisfies a non-trivial topological relation at the level of homological Chern classes. This relation ensures that the D3 charge in F-theory is the same as the one in the weak coupling limit.

### Holography for N=2* on S^4

Nikolay Bobev (KULeuven)
Wednesday, December 3, 2014 - 13:00
MONS 0A.11 pentagon

I will discuss the gravitational dual of a mass deformation of N=4 SYM, called N=2* SYM, on S4. Using holographic techniques one can calculate the universal contribution to the corresponding free energy in the planar limit and at large 't Hooft coupling. The result matches the expression recently computed using supersymmetric localization in the field theory. This agreement represents a non-trivial precision test of holography in a non-conformal setting. I will also briefly discuss the extension of these results to mass deformations of N=4 SYM with N=1 supersymmetry.

### F-theory on singular spaces

Andrés Collinucci (ULB)
Wednesday, December 3, 2014 - 10:30
MONS 211 bat6

F-theory paints a beautiful picture that relates gauge theories to purely geometric information, whereby the Dynkin diagrams of gauge groups come to life as 2-cycles of internal spaces. A major difficulty arises, though: Such spaces are necessarily singular, and treating them sensibly requires resolving or deforming the singularities. These procedures artificially mutilate the spectrum arising from these theories, and entire Higgs branches are missed in the analysis.
Recently, my collaborator R. Savelli and I have proposed a new strategy that allows one to deal with singular spaces directly. It is based on Eisenbud’s so-called matrix factorisations. It is a remarkably simple concept that has very deep connections to the mathematics of singularities.
In this blackboard talk, I will review the notions needed to formulate the issues, assuming only basic knowledge of string theory. Then, after introducing our proposal, I will show examples of its applications, such as computations of chiral spectra for bound states of 7-branes.

### Holographic lattices, metals and Insulators

Jerome Gauntlett (Imperial, UK)
Wednesday, November 26, 2014 - 13:00
KUL 200N 00.01

The AdS/CFT correspondence provides a powerful framework for studying strongly coupled field theories using gravitational techniques. Over the past few years there has been intense activity in this area which has been motivated by possible applications to condensed matter systems. In particular many novel classes of black hole solutions have been discovered.
The talk will describe constructions of black holes that describe metals, (both "coherent" and "incoherent"), insulators and transitions between them, topics of great interest in condensed matter. A key ingredient is to break the translation invariance of the dual field theory using a lattice. On route we present some analytic results for the DC conductivity of the metals and insulators in terms of black hole horizon data, obtaining some universal results.

### Non-associative Geometry and Double Field Theory

Dieter Luest (Munich, Germany)
Wednesday, November 26, 2014 - 10:30
KUL 200N 00.01

In this talk I will discuss the emergence of non-commutativity and non-associativity within the context of non-geometric closed string backgrounds. It leads to a deformation of space which can be described by a non-trival 3-bracket among functions, leading to a kind of volume uncertainty. In addition the role of non-associativity in conformal field theory as well as in double field theory will be discussed.

### ENTANGLEMENT DAY @ VUB

Monday, November 24, 2014 - 09:30

A one day workshop on Entanglement Entropy and Holography at the VUB.

• 11:00-11.35: MERA & holography (Karel Van Acoleyen UGENT)
• 11:40-12:15: Continuous MERA (Jutho Haegeman UGENT)
• 12:20-12:55: Prospects of real-time evolution & thermal states with MERA (Matthias Bal UGENT)
• Lunch Break
• 14:00-14:45 An Introduction to Holographic entanglement entropy  (Nabil Iqbal UVA)
• Break
• 15:00-15:45 Holographic Entanglement Entropy & Gravitational Anomalies (Nabil Iqbal UVA)

### An introduction to resurgence in the principal chiral model

Daniele Dorigoni (Cambrige, UK)
Wednesday, November 19, 2014 - 13:00
ULB -Solvay room

Resurgence provides a systematic unification of semiclassical analysis for the perturbative and non-perturbative sectors. Expansions about different saddle points are quantitatively related to one another in a precise manner. I will illustrate the general framework by discussing various examples, from the double-well potential in QM, to asymptotically free QFTs such as the principal chiral model, where this resurgent approach yields a new semiclassical interpretation of IR renormalons.

### Recent Highlights (and more) in AdS/CFT Integrability

Thomas Klose (Humboldt University of Berlin, Germany)
Wednesday, November 19, 2014 - 10:30
ULB - Solvay room

This talk attempts to give an overview over the recent developments in the field of integrability in the AdS/CFT correspondence. The topics include the quantum spectral curve, the calculation of three-point correlation functions, and the bootstrapping of scattering amplitudes at finite coupling. As much as this is possible, the focus shall be on concepts rather than technicalities.

### The superconformal bootstrap program

Balt Van Rees (Cern & Durham University, UK)
Wednesday, November 12, 2014 - 13:00
KUL 200N 00.01

The idea of the bootstrap for conformal field theories (CFTs) is back in full swing. With novel techniques we can extract precise, quantitative results from the constraints of conformal invariance, unitarity and in particular crossing symmetry in general CFTs. I will discuss how these constraints can be very powerful in supersymmetric conformal theories (SCFTs) as well. In some cases, like N=2 SCFTs in four dimensions, there even exists a truncation of the crossing symmetry equations to a chiral algebra which can be analytically solved. Such results can then be used as input for a numerical 'solution' of general strongly coupled SCFTs.

### B-mode cosmology

Paolo Creminelli (ICTP Trieste, Italy)
Wednesday, November 12, 2014 - 10:30
KUL 200N 00.01

Experiments studying CMB B-mode polarization are entering an interesting regime for primordial gravitational waves. I will discuss the robustness of the inflationary predictions, the implications of a possible measurement and the theoretical and experimental expectations on the level of gravitational waves.

### Six-dimensional superconformal theories in string theory

Alessandro Tomasiello (Milan Bicoca, Italy)
Wednesday, November 5, 2014 - 13:00
ULB Salle de profs 2.NO9-06

I will review recent classification results for six-dimensional superconformal theories coming from string theory. I will first describe perturbative type II AdS7 vacua; I will show evidence that these vacua match with certain NS5-D6-D8 configurations. The field theories are certain strong-coupling limits of linear quivers. I will then switch to F-theory, where one can also describe non-perturbative branes; the linear quivers can now also contain exceptional gauge groups, and can be built from a relatively small number of 'building blocks' which generalize bifundamental matter. These also point to a new notion of 'fractional M5 branes'.

### Hovering black holes

Jorge Santos (Cambridge, UK)
Wednesday, October 29, 2014 - 13:00
VUB E.0.05.

We construct the holographic dual of an electrically charged localised defect in a conformal field theory at strong coupling. In doing so, we find that the theory sometimes flows to new IR fixed points, some of which we are able to find in a closed form. When the IR theory is either marginally connected to pure AdS, or is AdS itself, we find that a new gravitational phenomena occurs: for an impurity of a sufficiently large amplitude, a spherical extremal Reissner-Nordström black hole nucleates in the domain of outer communications - a hovering black hole. We construct this new phase, for many choices of boundary chemical potential profile, and study its properties. We find a universal curve for the entropy of the defect as a function of its amplitude, reminiscent of an adiabatic Choptuik critical scaling. We comment on the possible field theory implications of our results.

### Minimal massive 3D gravity: the Third Way

Paul Townsend (Cambridge, UK)
Wednesday, October 29, 2014 - 11:00
VUB E.0.12

Topologically Massive Gravity (TMG) is a parity-violating third-order extension of 3D GR that propagates a single massive graviton. In asymptotically anti-de Sitter spacetimes, the asymptotic symmetry algebra is the 2D conformal group but its central charges (left/right) are both positive only if the bulk graviton is a ghost, so TMG is not the classical limit of any unitary 3D quantum gravity theory. Minimal Massive Gravity (MMG) has identical bulk properties to TMG but resolves this bulk/boundary clash. Its field equations differ from those of TMG by the addition of a new tensor, which is such that consistency with Bianchi identies is achieved in a surprising new way, the third way'', which is (so far) unique to MMG. This talk will review TMG, explain the construction of MMG and present a new geometrical way to understand how it evades the bulk/boundary clash of TMG.

### Exploring New Ideas in AdS3 Quantum Gravity.

Shahin Sheikh Jabbari (IPM Tehran, Iran)
Wednesday, October 15, 2014 - 13:00
VUB G1.003

In this talk I will explore ideas in quantizing AdS3 Einstein gravity. We start with the most general solution to the 3d gravity theory which respects Brown-Henneaux boundary conditions. These solutions are specified by two holomorphic functions and satisfy simple superposition rule. These geometries generically have a bifurcate Killing horizon (with a noncompact bifurcation curve) which is not an event horizon of simply BTZ-type black hole. Nonetheless, there are superpositions of these geometries which have event horizon. We propose to view these geometries as semiclassical fuzzball microstates'' of BTZ black holes appearing as superposition of these geometries. We also comment on possible relevance of multi-BTZ black holes and details of quantization of these semiclassical microstates.

### Electromagnetic response of strongly coupled plasmas

Andrea Mezzalira (ULB)
Wednesday, October 8, 2014 - 13:00
KUL 200S 01.04

In this talk we present an analysis of the electromagnetic response of strongly coupled neutral plasmas described by the gauge/gravity correspondence. The coupling of the external electromagnetic field with the tower of quasi-normal modes of the plasmas supports the presence of various electromagnetic modes with different properties.
Among them we underline the existence of negative refraction with low dissipation for a transverse mode. Previous hydrodynamical approaches have shown the ubiquitous character of negative refraction in charged
plasmas and the absence thereof in neutral plasmas. Our results here extend the analysis for neutral plasmas beyond the hydrodynamical regime. As an application of these new insights we briefly discuss the case of the quark gluon plasma in the temperature dominated regime. The talk is based on arXiv:1404.4048

### The Coulomb branch formula for quiver indices.

Boris Pioline (Cern & Paris)
Wednesday, October 8, 2014 - 10:30
KUL 200S 00.04

Quiver quantum mechanics is a useful tool for describing BPS bound states in N=2 gauge theories and string vacua. In a recent series of works with J. Manschot and A. Sen,  we proposed a formula that expresses the BPS index of quiver quantum mechanics with generic superpotential in terms of a set of invariants associated to single-centered' or pure-Higgs' states. The distinguishing feature of these invariants is that they are insensitive to the Fayet-Iliopoulos parameters, in contrast to the BPS index which is discontinuous across walls of marginal stability. The formula suggests an effective Coulomb branch description in models where the Higgs and Coulomb branches do not decouple.