2018 - 2019 Seminars

All seminars take place on Wednesdays at 1315 with student session at 1300, Room 54/8031 unless otherwise indicated

Schedule:

- 03/10/18 - Rainer Weiss (MIT) ***STAG LECTURE*** Room: Turner Sims, Time: 14:30

- 10/10/18 - Elias Kiritsis (Crete University and APC Paris)

- 17/10/18 - Max Guillen (ICTP-SAFIR IFT)

- 24/10/18 - Yago Bea (Barcelona)

- 31/10/18 - Ines Aniceto (Southampton)

- 07/11/18 - Masanori Hanada (Southampton)

- 14/11/18 - Sanjaye Ramgoolam (Queen Mary)

- 21/11/18 - Carl Turner (Cambridge) [CANCELLED]

- 28/11/18 - Stefanos Katmadas (KU Leuven)

- 05/12/18 - Christian Northe (Würzburg)

- 12/12/18 - Nina Miekley (Würzburg)

[Winter break: 17 December 2018 through 4 January 2019]

30/01/19 - Alessandra Gnecchi (Geneva)

06/02/19 - Carl Turner (Cambridge)

13/02/19 - Arthur Lipstein (Durham)

20/02/19 - Tomás Ortín (Madrid, IFT)

27/02/19 - Cyril Closset (Oxford)

06/03/19 - Bruno Le Floch (École Normale Supérieure)

13/03/19 - Yang-Hui He (City)

20/03/19 - Marco Meineri (EPFL Lausanne)

27/03/19 - Ignacio Reyes (AEI Potsdam)

[Spring break: 1 April 2019 through 26 April 2019]

01/05/19 - Pedro Jorge Martinez

08/05/19 -

15/05/19 - Jacopo Sisti (Trieste)

22/05/19 - Nadav Drukker (KCL)

29/05/19 - Christiana Pantelidou (Durham)

Titles and Abstracts:

10/10/18 Elias Kiritsis (Crete University and APC Paris)

Holographic RG flows on curved manifolds and the $F$-theorem

We study the $F$-theorem in the context of field theories on $S^3$ using gauge-gravity duality, with the radius of $S^3$ playing the role of a RG scale. We show that the on-shell action, evaluated over a set of holographic RG flow solutions, can be used to define good $F$-functions, which decrease monotonically along the RG flow from the UV to the IR. If the operator perturbing the UV CFT has dimension $\Delta > 3/2$ these $F$-functions correspond to an appropriately renormalised free energy. However, if the perturbing operator has dimension $\Delta < 3/2$ it is the quantum effective potential, i.e.~the Legendre transform of the free energy, which gives rise to good $F$-functions. We check that these observations hold beyond holography for the case of a free fermion on $S^3$ ($\Delta=2$) and the free boson on $S^3$ ($\Delta=1$), solving the puzzle regarding the non-monotonicity of the free energy for the free boson first observed in \cite{1105.4598}. We also show that for a particular choice of entangling surface, we can define good $F$-functions from an entanglement entropy, which coincide with certain $F$-functions obtained from the on-shell action.

10/17/18 Max Guillen (ICTP-SAFIR IFT)

Dynamics from pure spinor master actions

Using non-minimal pure spinor superspace, Cederwall has constructed BRST-invariant actions for D=10 super-Born-Infeld and D=11 supergravity which are quartic in the superfields. But since the superfields have explicit dependence on the non-minimal pure spinor variables, it is non-trivial to show these actions correctly describe super-Born-Infeld and supergravity. In this talk, I will expand solutions to the equations of motion from the pure spinor action for D=10 abelian super Born-Infeld to leading order around the linearized solutions and show that they correctly describe the interactions expected. If I have time, I will explain how to generalize these ideas to D=11 supergravity.

24/10/18 Yago Bea (Barcalona)

Holographic collisions across a phase transition [Slides]

We use holography to mimic heavy ion collisions and obtain new qualitative insights possibly relevant for QCD. Our studies are motivated by the extensive experimental efforts devoted to the search of the conjectured critical point in the QCD phase diagram. Holographically, we perform collisions in strongly-coupled gauge theories with thermal phase transitions. We find that near a second order phase transition almost all the energy of the projectiles is deposited into a long-lived, quasi-static blob of energy. Moreover, hydrodynamics describes this blob only if we consider a formulation different from the Muller-Israel-Stewart formulation that is almost universally used in the numerical codes describing the quark-gluon plasma formed at RHIC and LHC.

31/10/18 Ines Aniceto (Southampton)

A resurgent transseries for N=4 supersymmetric Yang-Mills plasma [Slides]

The non-equilibrium hydrodynamic phenomena in strongly coupled quantum systems such as the N=4 supersymmetric Yang-Mills plasma, can be addressed with the tools of the AdS/CFT correspondence, where calculations in the gravity side of the duality can be performed through a large proper-time expansion. This expansion is asymptotic and encodes a spectrum of non-hydrodynamic modes reflecting the frequencies of AdS black-brane quasinormal modes. The emerging structure for the description of physical quantities such as the energy density is of a resurgent transseries.

The theory of resurgence perfectly captures this perturbative/non-perturbative connection and its consequences. Moreover, it allows us to construct a full non-perturbative solution from perturbative data. In this talk, I will analyse essential role of resurgence theory, coupled to exponentially accurate numerical methods, to go beyond the perturbative results and obtain non-perturbative data. I will use these techniques to thoroughly study the resurgent transseries for the energy density of the N=4 SYM plasma.

07/11/18 Masanori Hanada (Southampton)

What is chaos?

How can we characterize the classical and quantum chaos? Typically, the sensitivity to small perturbations is regarded as the defining property. This characterization is reasonable for some cases, but fails in many cases. Even for classical systems, there is a subtlety when we relate them to quantum theory. For quantum chaos, the "universal" behavior of the energy spectrum is widely used, but it is not clear how it is related to the classical theory. In this talk we suggest other characterizations based on certain spectrum defined from two- and four-point functions. Along the way we see some "strongly" universal aspects of quantum gravitational systems.

14/11/18 Sanjaye Ramgoolam (QMUL)

Tensor Models, Algebras, and Topological Holography

The counting of observables in tensor models has a rich structure which is organised by underlying algebras related to permutations. This structure also organises the correlators of general observables in Gaussian tensor models. For both counting and correlators, these algebras lead to a topological holography based on lattice topological field theories of permutation groups, and generalizing branched covers of surfaces which play a central role in the string theory of large N two-dimensional Yang Mills theory. The talk is based on https://arxiv.org/abs/1307.6490 and https://arxiv.org/abs/1708.03524.

21/11/18 Carl Turner (Cambridge) [CANCELLED]

Z_2 Gauge Theory and Duality

I will try and clarify the surprisingly obscure story underlying some of the simplest non-trivial dualities in theoretical physics: the self-duality of the Ising CFT, and bosonization, both in 2d. After explaining some important, oft-overlooked subtleties in these stories, I will discuss how they help us understand the relationship between various different dualities, and see where they crop up in analyzing 2d field theories.

28/11/18 Stefanos Katmadas

BPS black holes in AdS4

In this talk I consider supersymmetric asymptotically AdS$_4$ black hole solutions in Fayet-Iliopoulos gauged theories. I will review the constructions of such solutions achieved in recent years, focusing on static solutions with a magnetic flux through the horizon that realises a topological twist. Subsequently, I will present the recent extension of this class to the rotating case, which is characterised by a squashed horizon whose curvature is again locked to the magnetic flux. Finally, I will discuss the BPS entropy function in the rotating case and its expected connection to the twisted index in the field theory dual.

05/12/18 Christian Northe (Würzburg)

Boundary RG flows, Kondo and AdS/CFT

Not only has the Kondo model played a major role in condensed matter physics, but it has also sharpened our techniques for dealing with boundaries and interfaces in CFT and string theory. Within CFT, the Kondo effect is described via branes which acquire additional dimensions. Starting from the NS5/F1 system, we have found the BPS solutions to the DBI system describing RG flows between D1- and D3-brane solutions and worked out their g-factors. Using a class of half BPS solutions provided we find corresponding backreacted supergravity interface solutions for both types of branes and work out their central charges and g-factors. Our approach provides an explicit example of a Kondo-like CFT defect, with an explicit gravitational dual.

12/12/18 Nina Miekley (Würzburg)

Complexity change under conformal transformations in AdS3/CFT2

In this talk, I will discuss the recent paper with Mario Flory (1811.03097) where we use the Complexity=Volume proposal to compute the change of complexity of holographic states caused by a small conformal transformation in AdS3/CFT2. This computation is done perturbatively to second order. We give a general result and discuss some of its properties. As operators generating such conformal transformations can be explicitly constructed in CFT terms, these results allow for a comparison between holographic methods of defining and computing computational complexity and purely field-theoretic proposals.

29/01/19 Alessandra Gnecchi (CERN)

Holographic RG flows from 7D gauged Supergravity

After reviewing properties and examples of holographic renormalization-group (RG) flows, I will present explicit solutions interpolating between AdS vacua of 7-dimensional gauged Supergravity. These vacua are characterized by two nilpotent elements, and are naturally interpreted as type IIA AdS7 solutions. I will show how they allow to test the conjecture that the hierarchy of the RG flow among a class of 6D SCFTs is determined by a partial ordering of nilpotent elements of the gauge group G that defines the theory.

06/02/19 Carl Turner (Cambridge)

Z_2 Gauge Theory and Duality

I will try and clarify the surprisingly obscure story underlying some of the simplest non-trivial dualities in theoretical physics: the self-duality of the Ising CFT, and bosonization, both in 2d. We will begin by understanding the basics of Z_2 gauge theory and spin connections, and then using this technology to explain some important, oft-overlooked subtleties in these stories. I will finish by presenting a simple but elegant duality web in two dimensions.

13/02/19 Arthur Lipstein (Durham)

Recursion Relations for Anomalous Dimensions of the 6d (2,0) Theory

I will describe recursion relations for the anomalous dimensions of double-trace operators occurring in the conformal partial wave expansion of four-point stress tensor correlators in the 6d (2,0) theory, which encode higher-derivative corrections to supergravity in AdS7×S4 arising from M-theory.

20/02/19 Tomás Ortín (Madrid, IFT)

Recent results on alpha prime corrections of stringy black holes

I will review some recent results on the explicit computation of the first-order alpha prime corrections of static, supersymmetric 4- and 5-dimensional stringy black holes. We will use the framework of the Heterotic Superstring effective action and we will study the corrections to entropy of those black holes and we will compare it to the microscopic computations in the literature. We will also show how some singular solutions are regularized by the alpha prime corrections in d=4 giving rise to globally-regular black-hole spacetimes and how to get regular supersymmetric, rotating black holes in 4 dimensions.

27/02/19 Cyril Closset (Oxford)

Gauge theories phases of 5d SCFTs: an M-theory/type IIA perpective

I will revisit the well-known construction of 5d SCFTs from M-theory on a CY3 singularity. Upon massive deformation, such 5d SCFTs are often expected to have 5d N=1 supersymmetric gauge theory descriptions at low energy. I will present a new way to study these 5d ``gauge theory phases'' systematically using type-IIA string theory, and I will comment on the phenomenon of "UV duality." Along the way, I will discuss some slightly subtle properties of the 5d N=1 Coulomb branch prepotential.

06/03/19 Bruno Le Floch (École Normale Supérieure)

Intersections of M2 and M5 branes and dualities

M-theory features M2 branes and M5 branes of dimensions 2+1and 5+1, respectively, which break (at least) half of the supersymmetry each. M2 branes can end supersymmetrically on M5 branes and the intersection is described by a 2-dimensional gauge theory with four supersymmetries. I will explain how to find this gauge theory by compactifying the brane setup on a Riemann surface C and using the AGT correspondence between 4d N=2 supersymmetric gauge theories and Liouville/Toda CFT on C. Symmetries of the CFT on C translate to gauge theory dualities. Time-permitting I will explain these Seiberg-like dualities, emphasizing the difference between 2-dimensional and higher-dimensional supersymmetric dualities.

13/03/19 Yang-Hui He (City)

Deep-learning the Landscape

We propose a paradigm to deep-learn the ever-expanding databases which have emerged in mathematical physics and particle phenomenology, as diverse as the statistics of string vacua or combinatorial and algebraic geometry. As concrete examples, we establish multi-layer neural networks as both classifiers and predictors and train them with a host of available data ranging from Calabi-Yau manifolds and vector bundles, to quiver representations for gauge theories. We find that even a relatively simple neural network can achieve astounding accuracy in a matter of minutes. This paradigm should prove useful in various investigations in landscapes in physics as well as pure mathematics.

20/03/19 Marco Meineri (Lausanne)

Reflection and transmission coefficients for conformal interfaces

We probe a generic two dimensional conformal interface via a collider experiment. We measure the energy and charges which are reflected and transmitted through the interface. We find that the average transmitted energy is independent of the way the state is constructed and determined by the central charge and a single piece of CFT data. We comment on the universality of the result and discuss some examples.

27/03/19 Ignacio Reyes (AEI Potsdam)

Images of entanglement

Much has been learned about entanglement due to conformal symmetry and the Rindler properties of the vacuum. In order to go beyond, we propose to use the method of images in QFT. We focus on the solvable yet non-trivial example of the 2d free fermion. After rederiving known results, we obtain the entanglement (modular) Hamiltonian on the torus, which provides the first example of a geometric flow not obtainable from Rindler space. We also comment on its applications to bulk reconstruction in higher spin holography.

01/05/19 Pedro Jorge Martinez (ICTP Safir)

Holographic excited states in AdS black holes

I will present and describe the properties of a set of excited states of a finite temperature CFT via holographic methods. These excited states can be non-pertubately defined in both sides of the duality and turn out to be coherent excitations over the TFD vacuum in the large N limit, for which we can compute its eigenvalues. First, I will review the problems arising from a pure real-time formulation of AdS/CFT and how these can be solved in a complex-time formulation of the duality over Schwinger-Keldysh paths. This framework, known as Skenderis and van Rees prescription, will render the prescription for excited states natural. I will then build a geometry dual to a particular Schwinger-Keldysh path for a finite temperature formalism made out of Euclidean and Lorentzian pieces of AdS-BHs, which is an interesting problem on its own, regarding the analyticity of fields inside the bulk. This allows both to observe a real time version of the Hawking Page transition as well as to study the excited states at finite temperature.

08/05/19 CANCELLED

15/05/19 Jacopo Sisti (SISSA)

Holographic Entanglement Entropy in AdS_4/BCFT_3 and the Willmore Functional

We study the holographic entanglement entropy of spatial regions with arbitrary shapes in the AdS_4/BCFT_3 correspondence focusing on the subleading term with respect to the area law term. An analytic expression, depending on the unit vector normal to the minimal surface, is obtained. This expression reduces to the Willmore functional with a proper boundary term when the bulk space-time is a part of AdS_4. Analytic expressions for some smooth domains are reproduced, including the one for a disk disjoint from the boundary. We also discuss the case in which the entangling curve intersects the boundary. In this case, a logarithmic divergence in the entanglement entropy occurs whose coefficient is determined by a corner function. Finally, a numerical approach based on Surface Evolver is employed to construct extremal surfaces anchored to entangling curves with arbitrary shape.

15/05/19 Nadav Drukker (KCL)

Supersymmetric surface operators in the six dimensional N=(2,0) theory

I will discuss the anomalies in the surface operators in six dimensions from three different points of view: From a free field theory calculation, from holography and using algebraic methods. This is a topic that has been studied many years ago, but by generalizing past calculations, I am able to find new results on these very interesting observables.