2016-2017 Seminars

All seminars take place on

Wednesdays at 1pm, Room B54/10037 (10B) -- unless otherwise indicated --

Schedule:

05/10/16 - Carlos Mafra (Southampton University)

12/10/16 - [to be announced at 1.45-2.45pm] Cindy Keeler (Niels Bohr Institute)

19/10/16 - Lata Kh. Joshi (Indian Institute of Tech.)

26/10/16 - David Gross (KITP, Santa Barbara)

02/11/16 - Sebastian Fischetti (Imperial College)

04/11/16 - Joao Caetano (P&A seminar)

09/11/16 - Marjorie Schillo (Leuven University)

16/11/16 - Neil Lambert (King’s College London)

23/11/16 - Miguel Tierz (Lisbon University)

30/11/16 - Miguel Paulos (CERN)

07/12/16 - Alex Arvanitakis (Cambdrige DAMTP)

14/12/16 - Oliver Schlotterer (Albert Einstein Institute)

11/01/17 - Erik Panzer (Oxford) - TBA

[ winter break ]

01/02/17 - Geoffrey Compere (Universite Libre de Bruxelles)

08/02/17 - Benjamin Basso (ENS)

15/02/17 - Ricardo Schiappa (Cern & IST Maths Lisbon)

22/02/17 - TBA - TBA

01/03/17 - Ioannis Papadimitriou (SISSA Trieste)

08/03/17 - Stephan Stieberger (Max Planck Inst)

15/03/17 - Jose Edelstein (Universidad Santiago de Compostela) - included in the String Theory and Gender meeting

22/03/17 - James Sparks (Oxford University, Math Inst)- TBA

[ spring break ]

27/04/17 - Nathan Berkovits - TBA

03/05/17 - Alessandra Gnecchi (Cern) -

09/05/17 - Richard Davison -TBA

17/05/17 - Prem Kumar (Swansey)- TBA

Talks Details:

Carlos Mafra, ``Computing iterated integrals using recursive field-theory methods"

I will begin by reviewing the computation of the superstring n-point disk integrand using the pure spinor formalism. Then I will show how the solution to the integrand contains a hidden key to unlock the computation of the alpha'-expansion of its associated iterated integrals using Berends-Giele recursion, a technique used to calculate scattering amplitudes in field theory. This indicates that there is a field theory governing the alpha'-expansion of these integrals, recently dubbed Z-theory.

Cindy Keeler, ``One-loop Effective Potentials via Quasinormal Modes"

We present a method of calculating one-loop effective potentials for quantum fluctuations from quasinormal modes. We will demonstrate the method for a harmonic oscillator and a scalar in De Sitter space. After briefly discussing its application in Anti-de Sitter space, we will speculate on the physical meaning of this mathematical trick as well as its extension to fields with higher spin, to product spaces, and to spaces with non-Dirichlet boundary conditions.

Lata Kh. Joshi, ``Time-dependent holographic spectral function"

Using state/geometry map, we have proposed a method to find retarded propagator in generic non-equilibrium states. We discuss the spectral function of a scalar in a class of non-equilibrium states. This class is given in terms of AdS-Vaidya geometry with an arbitrary parameter characterizing the dual state transition from one thermal state to another due to a homogenous quench. In this talk, I will discuss different routes and patterns of thermalization under various scalar momenta and quench durations.

Sebastian Fischetti, ``An Energy Bound on Renormalized Entanglement Entropy"

Despite its important role in fields ranging from condensed matter to QFT to quantum gravity, entanglement entropy remains difficult to understand explicitly. Useful formal statements can be made by defining a modular Hamiltonian, but this state-dependent operator is also typically intractable to study explicitly. I will show that in the context of AdS_4/CFT_3, it is possible to obtain a bound on the renormalized entanglement entropy of regions in general states of the CFT using a geometric flow called the inverse mean curvature flow. The bound constrains the entanglement entropy in terms of a state-dependent weighted energy density over the entangling region; in the case of spherical entangling regions in the vacuum state, this bound becomes the modular Hamiltonian, and we recover the first law of entanglement. I will comment on its validity away from the classical regime, possible connections to the complexity/volume duality, and extensions to higher dimensions.

Marjorie Schillo, ``A new top-down model of inflation"

I will review the mechanism of brane-flux annihilation in which D-brane charge in a flux background can dissolve and discharge Ramond-Ramond flux. A dynamical brane mediates the brane-flux annihilation as it moves across a compact internal cycle. This mechanism has recently been extended such that many units of flux can be discharged in a "flux cascade." The flux cascade can be used to reduce vacuum energy and give rise to 60 efolds of inflation.

Neil Lambert, ``M-branes and the (2,0) Superalgebra"

The dynamics of Dp-branes are given by actions which all share a common origin as reductions of ten-dimensional super-Yang-Mills gauge theory. Microscopically this can be viewed as a consequence of T-duality. M-branes are also related by T-duality along a 3-torus but with no microscopic description this is far from manifest. This talk will explore a formalism whereby the M2 and M5-brane dynamics arise as solutions to a single underlying six-dimensional (2,0) superalgebra.

Miguel Tierz ``Quantum phase transition in many-flavor supersymmetric QED in three dimensions"

We present a study of $\mathcal{N}=4$ supersymmetric QED in three dimensions, on a three-sphere, with 2N massive hypermultiplets and a Fayet-Iliopoulos parameter. We identify the exact partition function of the theory with a conical (Mehler) function. This implies a number of analytical formulas, including a recurrence relation and a second-order differential equation. In the large N limit, the theory undergoes a second-order phase transition on a critical line in the parameter space. We will discuss the critical behavior and compute the two-point correlation function of a gauge invariant mass operator.

Miguel Paulos, "The revenge of the S-matrix"

In this talk I will describe recent work aiming to reinvigorate the 50 year old S-matrix program, which aims to constrain scattering of massive particles non-perturbatively. I will begin by considering quantum fields in anti-de Sitter space and show that one can extract information about the S-matrix by considering correlators in conformally invariant theories. The latter can be studied with "bootstrap" techniques, which allow us to constrain the S-matrix. In particular, in 1+1D one obtains bounds which are saturated by known integrable models. I will also show that it is also possible to directly constrain the S-matrix, without using the CFT crutch, by using crossing symmetry and unitarity. This alternative method is simpler and gives results in agreement with the previous approach. Both techniques are generalizable to higher dimensions.

Alex Arvanitakis " Twistor variables for Anti-de Sitter (super)particles, strings and branes"

We will show how (super)twistors for AdS_{d+1} spacetimes simplify the dynamics of (super)particles, strings and branes propagating on those geometries. An AdS_{d+1} twistor is a pair of Penrose twistors for d-dimensional Minkowski space (modulo certain identifications). We will see how the construction works for superparticles on AdS, yielding a 1st-quantized description of graviton supermultiplets, and outline the generalisation to tensionless p-branes. We will also indicate how this construction could be relevant to AdS/CFT and the ``gravity=Yang-Mills squared'' proposal

Oliver Schlotterer "Loop-level KLT, BCJ and EYM amplitude relations"

In this talk, I will extend the tree-level Kawai-Lewellen-Tye (KLT) and Bern-Carrasco-Johansson (BCJ) amplitude relations to loop integrands of gauge theory and gravity. By rearranging the propagators of gauge and gravity loop integrands, the first manifestly gauge- and diffeomorphism invariant formulation of their double-copy relations is proposed. The one-loop KLT formula expresses gravity integrands in terms of more basic gauge invariant building blocks for gauge-theory amplitudes, dubbed partial integrands. The latter obey a one-loop analogue of the BCJ relations, and both KLT and BCJ relations are universal to bosons and fermions in any number of spacetime dimensions and independent on the amount of supersymmetry. Also, one-loop integrands of Einstein-Yang-Mills (EYM) theory are related to partial integrands of pure gauge theories.

Geoffrey Compère "The E7 black hole entropy"

I will first review the construction of the most general non-extremal single center black hole of N=8 supergravity. Its seed is a black hole with 5 independent electromagnetic charges. The appropriate setting to describe the seed solution is the STU model with SO(4,4) hidden symmetries. Some properties of the black hole will be described such as the existence of two regular extremal limits, the presence of a conformal Killing tensor and mysterious thermodynamic formulae. I will then describe the construction of an E7 invariant which allows to express the black hole entropy.

Benjamin Basso "Hexagons and Three-Point Functions"

I will present a framework for computing correlators of three single trace operators in planar N=4 SYM theory that uses hexagonal patches as building blocks. This approach allows one to exploit the integrability of the theory and derive all loop predictions for its structure constants. After presenting the main ideas and results, I will discuss some perturbative tests and open problems. If time permits, I will discuss the recent exciting idea about "hexagonalizing" the higher correlators (4pt functions, etc) of the theory.

Ioannis Papadimitriou "Supercurrent anomalies from holographic renormalization of N=2 gauged supergravity"

Supersymmetric localization is a powerful technique that permits certain observables in supersymmetric quantum field theories to be computed analytically, and has led to a plethora of new possible precision tests of the AdS/CFT correspondence. These tests, however, require a careful definition of the holographic dictionary in the presence of arbitrary sources for gauge invariant local operators, preserving supersymmetry. After briefly reviewing recent progress in determining the holographic supersymmetric renormalization scheme and the related open problems, I will present a systematic algorithm for computing supersymmetric observables holographically in N=2 gauged supergravity, both in four and five dimensions.

Stephan Stieberger "New amplitude relations in field and string theory"

Amplitude relations in field and string theory are not only of practical use to simplify computations but also shed light on their underlying quantum structure. As a consequence such relations very often allow to reveal hidden symmetries and give rise to new ideas and concepts of reformulating the original theory. We will exhibit relations between gauge and gravity amplitudes in field and string theory, which point into a deeper connection between gauge and gravity amplitudes. Furthermore, the properties of scattering amplitudes in both gauge and gravity theories suggest a deeper understanding from string theory. Studying the geometric properties of the string world–sheet proves to be very fruitful for a better understanding of the structure and symmetries of amplitudes. In particular, monodromy properties of the string world–sheet provide interesting mathematical identities and physical constraints. In this talk we shall elucidate some of these relations, connections and symmetries.

Jose Edelstein "Causality constraints in gravitational theories"

I will consider higher derivative corrections to the graviton 3-point coupling within a weakly coupled theory of gravity. Lorentz invariance allows further structures beyond that of Einstein’s theory. I will argue that these structures are constrained by causality, and show that the problem cannot be fixed by adding conventional particles with low spin but adding an infinite tower of massive particles with higher spins. Implications of this result in the context of AdS/CFT, quantum gravity in asymptotically at space-times, and non-Gaussianity features of primordial gravitational waves are discussed. The case of 3D gravity is briefly discussed.

James Sparks "Supersymmetric holography, and exact results in AdS/CFT from localization"

Alessandra Gnecchi "Aspects of black holes and holography in gauged Supergravity"

After reviewing properties of BPS black holes, I will present their supersymmetric flow in N=2 four dimensional gauged Supergravity, with particular emphasis on electric-magnetic duality properties. I will then describe the holographic renormalization needed to define physical quantities for black holes and black branes in AdS at finite temperature, in presence of scalar and vector fields. The last part of the talk with discuss thermodynamic properties and phase space of black branes in AdS, with the identification of a quantum critical point and its possible holographic interpretation.

Richard Davison "Diffusion and chaos in holographic systems at non-zero density"

Recent work has uncovered relations between the rate at which chaotic behaviour spreads in strongly interacting quantum systems, and the diffusivities of certain processes in these systems. Focusing mainly on holographic examples, I will explore the extent to which these relations hold in states at non-zero density, where the diffusion of charge and energy are no longer independent processes.

Prem Kumar "Higher spins, entanglement and time dependence in CFT_2/AdS_3"

We review some universal aspects of Renyi entropies of entanglement, in equilibrium and with local quenches, in 2d CFTs with W-symmetry,in the presence of chemical potentials for higher spin charge. We explain how these results are nontrivially reproduced by bulk higher spin black holes in the Chern-Simons formulation of higher spins on AdS_3. We use the bulk formulation to show that bounds on the scrambling time are violated in the presence of higher spin charge, and present some CFT results on scrambling time in the presence of higher spin chemical potentials.