2017-2018 String Theory Seminars of the STAG Research Centre, University of Southampton

All seminars take place on

Wednesdays at 15:00, Room 54/7033 (7C) -- unless otherwise indicated --

Schedule:

- 26/09/17 - Arjun Bagchi (Indian Institute of Technology, Kanpur)

- 04/10/17 - Olga Papadoulaki (Southampton University)

- 11/10/17 - Brandon Robinson (Southampton University)

- 17/10/17 - Toby Wiseman (Imperial College) ***Tuesday seminar at 1pm, Room 34/4005***

- 24/10/17 - David Turton (Southampton university) ***Tuesday seminar at 1pm, Room 34/4005***

- 25/10/17 - Benjamin Doyon (Kings College)

- 31/10/17 - Panos Betzios (Crete University) ***Tuesday seminar at 1pm, Room 34/4005***

- 01/11/17 - Nikola Gushterov (University of Oxford)

- 08/11/17 - J. Bell Burnell (University of Oxford) ***STAG LECTURE***

- 15/11/17 - Juan Pedraza (Amsterdam University)

- 21/11/17 - Kazem Bitaghsir Fadafan (Shahrood University of Technology) ***Tuesday seminar at 1pm, Room 34/4005***

- 22/11/17 - Vasilis Niarchos (Durham University) *** ROOM: 07 / 3027 (L/R F1), TIME: 13:00-15:00***

- 29/11/17 - Natalia Pinzani Fokeeva (Leuven University)

- 06/12/17 - Andreas Braun (University of Oxford)

- 13/12/17 - Saso Grozdanov (MIT)

[ winter break Sunday 17 December 2017 - Sunday 7 January 2018 ]

- 10/01/18 - Benjo Fraser (Chulalongkorn University)

- 31/01/18 - Christopher Beem (Oxford University)

- 07/02/18 - David Vegh (Queen Mary)

- 14/02/18 - Michela Petrini (UPMC)

- 21/02/18 - David Berman (Queen Mary)

- 27/02/18 - Arpan Bhattacharyya (Yukawa Inst., Kyoto U.) ***Tuesday seminar at 13:45, Room 54/8031***

- 28/02/18 - Benjamin Withers (Geneva University)

- 07/03/18 - Dionysis Anninos (Amsterdam University)

- 14/03/18 - Frank Ferrari (ULB)

[ spring break Sunday 18 March 2018 - Sunday 15 April 2018 ]

- 17/04/18 - Georgios Papathanasiou (DESY) ***Tuesday seminar at 13:45, Room 54/8031***

- 18/04/18 - Lilia Anguelova (Inst. for Nucl. Research and Nucl. Energy, Sofia)

- 24/04/18 - Ioannis Papadimitriou (KIAS) ***Tuesday seminar at 13:45, Room 54/8031***

- 25/04/18 - Chris Herzog (King's College)

- 02/05/18 - Andy ***Journal Club/Postgraduate seminar***

- 08/05/18 - Oliver Schlotterer (AEI Potsdam) ***Tuesday seminar at 13:45, Room 54/8031***

- 09/05/18 - Adam Bzowski (CEA, Saclay)

- 16/05/18 - Emil Martinec (Chicago University)

Talks Details:

- Emil Martinec (Chicago University) - 16/05/18

"String Theory of Supertubes"

The internal structure of extremal and near-extremal black holes in string theory involves a variety of ingredients — strings and branes — that lie beyond supergravity, yet it is often difficult to achieve quantitative control over these ingredients in a regime where the state being described approximates a black hole. The supertube is a brane bound state that has been proposed as a paradigm for how string theory resolves black hole horizon structure. This talk will describe how the worldsheet dynamics of strings can be solved exactly in a wide variety of supertube backgrounds, opening up the study of stringy effects in states near the black hole transition.

- Adam Bzowski (CEA, Saclay) - 09/05/18

"Conformal field theory in momentum space"

I will present the momentum space approach to correlation functions in CFTs based on [1304.7760] and subsequent papers. These techniques lead to simple derivations and representations of conformal 3-point functions. Furthermore, I will show how they can be used to elucidate the structure of anomalies and beta functions.

- Oliver Schlotterer (AEI Potsdam) - 08/05/18

"Tree amplitudes of bosonic, heterotic and supersymmetric strings as field-theory double copies"

Tree-level amplitudes of type-I superstrings have been dramatically simplified by presenting them as double copies of field-theory amplitudes and scalar integrals over disk worldsheets. By the Kawai—Lewellen—Tye relations, such field-theory structures immediately propagate from open strings to closed-string amplitudes. In this talk, the double-copy construction will be extended to the tree-level amplitudes of bosonic and heterotic string theories. As a double-copy component of the open bosonic string, we identify a field theory involving dimension-six operators such as D^2 F^2 which has previously appeared in the double-copy construction of conformal supergravity.

- Chris Herzog (King's College) - 25/04/18

"Boundary Trace Anomalies and Boundary Conformal Field Theory"

I will discuss boundary contributions to the anomaly in the trace of the stress tensor in boundary conformal field theory. Unlike the case without a boundary, an anomaly can appear also in odd dimension. Four new central charges appear in dimensions three and four. While largely unstudied, these boundary charges hold out the tantalizing possibility of being as important in the classification of quantum field theory as the bulk central charges “a” and “c”. I will show how these charges can be computed from displacement operator correlation functions. I will also demonstrate a boundary conformal field theory in four dimensions with an exactly marginal coupling where these boundary charges depend on the marginal coupling. The talk is based on arXiv:1707.06224, arXiv:1709.07431, as well as work to appear shortly.

- Ioannis Papadimitriou (KIAS) - 24/04/18

"5D Rotating Black Holes and the nAdS2/nCFT1 Correspondence"

I will discuss a consistent Kaluza-Klein reduction of 5D Einstein gravity on warped S^2 x S^1 that includes the Myers-Perry or Kerr-AdS black holes with two equal angular momenta as special cases. The effective 2D theory admits an AdS2 vacuum corresponding to the extremal Kerr black hole. I will holographically derive the quantum effective action of the dual 1D theory in conformal perturbation theory around the extremal solution and will show how the presence of a conformal anomaly gives rise to the Schwarzian effective action. I will conclude with some comments on black hole thermodynamics.

- Lilia Anguelova (Inst. for Nucl. Research and Nucl. Energy, Sofia) - 18/04/18

"Exploring Holography (AdS/CFT ) via Tensor Network"

In this talk, I will discuss how various aspects of holography i.e AdS/CFT correspondence can emerge from tensor network description, for example, bulk operator reconstruction (HKLL), correspondence between boundary global symmetry and bulk isometry etc. Also, I will discuss how to compute correlation functions using the tensor network, reproducing essential features AdS/CFT and focus on a simple toy model based on p-adic version of AdS/CFT where all of these can be explicitly realized.

-David Berman (Queen Mary) - 21/02/18

"A review of double and exceptional field theory"

I will review the development and motivation of double and exceptional field theories

-Michela Petrini (UPMC) - 14/02/18

"CONSISTENT TRUNCATIONS AND EXCEPTIONAL GEOMETRY"

In this talk I will introduce some basic fact about generalised

geometry and show how this reformulation of supergravity can be

used to derive and classify consistent truncations

-David Vegh (Queen Mary) - 07/02/18

"Pair-production of cusps on a string in AdS_3"

The classical motion of a Nambu-Goto string in AdS_3 spacetime is governed by the generalized sinh-Gordon equation. It can locally be reduced to the sinh-Gordon, cosh-Gordon, or Liouville equation, depending on the value of the scalar curvature of the induced metric. In this talk, I examine solutions that contain both sinh-Gordon-type and cosh-Gordon-type regions. I show that near the boundaries of these regions (generalized) solitons can be classically pair-produced. The solitons move subluminally (superluminally) in the sinh-Gordon (cosh-Gordon) region on the worldsheet, and they correspond to cusps on the string. For the calculations, I use an exact discretization of the equation of motion. The solutions correspond to segmented strings.

- Christopher Beem (Oxford University) - 31/01/18

"The VOA/SCFT correspondence"

I will review a deep correspondence between N=2 superconformal field theories in four dimensions and vertex operator algebras. Recent work with L. Rastelli has identified the Higgs branch of a four-dimensional SCFT with a geometric invariant of the corresponding VOA: the “associated variety”. This connection implies surprising modularity properties for the superconformal index of the four-dimensional theory, and also hints at the possibility of reconstructing the VOA from moduli space data.

- Benjo Fraser (Chulalongkorn University) - 10/01/18

"Non-Abelian T-duality and coset CFT"

In the first part of this talk we will review Non-Abelian T-duality and the latest developments, which are mostly related to the supergravity limit of string theory. In the rest of the talk we will examine a large-level limit in an exact coset CFT, which is an equivalent formulation of the duality at the classical level. Taking the same limit in the partition function we hope to make statements about the quantum case.

- Saso Grozdanov (MIT) - 13/12/17

"Generalised global symmetries in field theory and holography"

In my talk, I will discuss the concept of generalized global symmetries and their applications. From the point of view of effective field theory, I will first present a recent, comprehensive reformulation of magnetohydrodynamics, which is a theory of low-energy excitations in plasmas. I will then discuss our recent construction of holographic duals to theories with generalised global symmetries. Of particular focus will be a five-dimensional bulk theory with a dynamical two-form gauge field, which is dual to a field theory in which magnetohydrodynamics captures the dynamics of its infra-red limit. In the final part of this talk, I argue that generalised global symmetries are a powerful tool for constructing theories with dynamical defects even in the absence of known microscopic origins of such symmetries. As an example, I will use the theory of viscoelastic states and formulate it both in effective field theory and holography.

- Andreas Braun (University of Oxford) - 06/12/17

"Compact G2 manifolds and Dualities between M-Theory, Heterotic String Theory and F-Theory"

M-theory on K3 surfaces and Heterotic Strings on T^3 give rise to dual theories in 7 dimensions. Applying this duality fibre-wise is expected to connect M-theory on G2 manifolds with Heterotic Strings on Calabi-Yau

threefolds (together with vector bundles). We make these ideas explicit for a class of G2 manifolds realized as twisted connected sums and prove the equivalence of the spectra of the dual theories.

- Natalia Pinzani Fokeeva (Leuven University) - 29/11/17

"Effective field theories for hydrodynamics"

Hydrodynamics has been recently recasted in an effective field theory language where the Schwinger-Keldysh formalism plays a central role. I will discuss the superspace implementation of this formalism which allows to efficiently impose the symmetries of the problem and to construct simple expressions for the effective action. I will also show how the entropy current with a positive definite divergence arises

as a Noether current in this setup.

- Vasilis Niarchos (Durham University) - 22/11/17

"Phases of QCD3 from string theory"

A scenario for the phases of vacuum QCD3 has been proposed recently by Komargodski and Seiberg. Three phases occur in this scenario: a phase of bosonization, a phase of spontaneous global symmetry breaking and a phase captured by some IR CFT. I will present a UV embedding of this scenario in a non-supersymmetric brane configuration in string theory where evidence for a non-supersymmetric Seiberg duality can be found. The magnetic dual explains naturally all the proposed phases, in particular, it explains bosonization and symmetry breaking as different patterns of magnetic squark condensation (or in string theory as different patterns of open string tachyon condensation and brane reconnection). It provides also a natural candidate for the IR CFT of the CFT phase.

- Kazem Bitaghsir Fadafan (Shahrood University of Technology) - 21/11/17

"On diffusivities bounds from holography"

Study of some quantum systems in condensed matter theory such as strange metals and cold atomic phase need to consider strongly interacting many-body quantum physics. To study such strongly interacting systems, the usual theoretical methods are not efficient and one should use new methods which are based on the non-perturbative approaches. Using the gauge-string correspondence is a new tool for studying the transport coefficients in a strongly interacting systems. One of the important observables is the electrical conductivity at finite density and disorder. We discuss the proposed bound of the electrical conductivity in strongly coupled disordered quantum field theories using holography. It is interesting that in the incoherent limit, the transport is given by the diffusion of charge and energy. In this limit, a universal bound for the diffusivities have been proposed from holography which is related to quantum chaos and butterfly velocity. We consider coupling of the Maxwell term to the bulk Weyl tensor and compute the butterfly velocity, charge and energy diffusion with broken translational symmetry.

- Juan Pedraza (Amsterdam University) - 15/11/17

"Chaotic strings in AdS/CFT"

Holographic theories with classical gravity duals are maximally chaotic, i.e. they saturate the universal bound on the rate of growth of chaos. It is interesting to ask whether this property is true only for leading large N correlators or if it can show up elsewhere. We consider the simplest setup to tackle this question: a Brownian particle coupled to a thermal ensemble. This system is dual to a long string stretching between the event horizon of an AdS black brane and its asymptotic boundary. We find that the four-point out-of-time-order correlator that diagnoses chaos initially grows at an exponential rate that saturates chaos bound, however, the scrambling time is parametrically smaller than for plasma excitations. Our result shows that, at least in certain cases, maximal chaos can be attained in the probe sector without the explicit need of gravitational degrees of freedom.

- Nikola Gushterov (University of Oxford) - 01/11/17

"Holographic Entanglement Density"

We will study the way in which the entanglement entropy of a holographic quantum system becomes the thermodynamic entropy, at finite temperature, as we vary the size of the entangling region. For this purpose, we will concentrate on the holographic entanglement entropy using the Ryu - Takyanagi prescription. We will study the holographic entanglement per unit volume, to understand how the Infrared degrees of freedom can affect the way in which the entanglement entropy becomes extensive. In particular, we will show that the holographic entanglement density will become over - extensive and develop a peak, for intermediate scales.

ref: https://arxiv.org/abs/1708.09376

- Panos Betzios (Crete University) - 31/10/17

"Fluctuations of the Meron Wormhole"

In this talk I will first give a brief introduction to instantons and merons. Then I will describe a Euclidean meron wormhole solution of Einstein - Yang-Mills. Its study is motivated by the antipodal mapping of 't Hooft in the context of black hole physics, that provides a possible understanding of the role of the two exteriors of the maximal Kruskal extension. Our wormhole background provides an easier context where this mapping can be further analyzed. I will then describe some preliminary efforts to compute the fluctuation spectrum of fields up to spin-2 in order to test the stability of the solution. I will conclude with a study of Green's functions of scalar fields and a possible analytic continuation to Lorentzian signature.

- Benjamin Doyon (Kings College) - 25/10/17

"Emergent hydrodynamics in integrable systems out of equilibrium"

The hydrodynamic approximation is an extremely powerful tool to describe the behavior of many-body systems such as gases. At the Euler scale (that is, when variations of densities and currents occur only on large space-time scales), the approximation is based on the idea of local thermodynamic equilibrium: locally, within fluid cells, the system is in a Galilean or relativistic boost of a Gibbs equilibrium state. This is expected to arise in conventional gases thanks to ergodicity and Gibbs thermalization, which in the quantum case is embodied by the eigenstate thermalization hypothesis. However, integrable systems are well known not to thermalize in the standard fashion. The presence of infinitely-many conservation laws preclude Gibbs thermalization, and instead generalized Gibbs ensembles emerge. In this talk I will introduce the associated theory of generalized hydrodynamics (GHD), which applies the hydrodynamic ideas to systems with infinitely-many conservation laws. It describes the dynamics from inhomogeneous states and in inhomogeneous force fields, and is valid both for quantum systems such as experimentally realized one-dimensional interacting Bose gases and quantum Heisenberg chains, and classical ones such as soliton gases and classical field theory. I will give an overview of what GHD is, how its main equations are derived, its relation to quantum and classical integrable systems, and some geometry that lies at its core. I will then explain how it reproduces the effects seen in the famous quantum Newton cradle experiment, and how it leads to exact results in transport problems such as Drude weights and non-equilibrium currents.

This is based on various collaborations with Alvise Bastianello, Olalla Castro Alvaredo, Jean-Sébastien Caux, Jérôme Dubail, Robert Konik, Herbert Spohn, Gerard Watts and my student Takato Yoshimura, and strongly inspired by previous collaborations with Denis Bernard, M. Joe Bhaseen, Andrew Lucas and Koenraad Schalm.

- David Turton (Southampton University) - 24/11/17

"Black Hole Microstates in String Theory"

The study of black hole internal structure in String Theory offers the potential to resolve the black hole information paradox. I will give an overview of recent work on constructing families of smooth horizonless supergravity solutions describing black hole microstates. Where applicable, I will present a holographic description of these solutions. I will also discuss the physics of an observer falling into a black hole.

- Toby Wiseman (Imperial College) - 17/10/17

"Geometrical constraints on holographic CFTs"

AdS-CFT rephrases certain questions in specific quantum field theories in terms of a dual geometric problem. This allows techniques in geometry to be applied which may yield new constraints and results governing the physics of these theories. We explore these ideas giving some examples, and emphasising their power in the case the field theory is put on a curved spacetime. We will discuss bounds on the energy gap when these theories are confining. In the 2+1 case, we will discuss a result that implies the Casimir energy is always non-positive when the theory is put on a curved space. And again in 2+1 I will describe a bound on entanglement entropy that applies in rather general situations. I will then discuss to what extent these results may constrain the behaviour of more general (non-holographic) theories.

- Brandon Robinson (Southampton University) - 11/10/17

"Supersymmetric Probe Branes and Localization in AdS/CFT"

Supersymmetric localization provides a powerful mechanism in gauge theories on curved backgrounds that allows for the exact, non-perturbative calculation of BPS observables. Kappa symmetry can be used in the D3/D5 and D3/D7 probe brane systems to holographically calculate similar BPS quantities. Confronting these two computations yields a novel precision test of the probe brane paradigm in the AdS/CFT correspondence.

- Olga Papadoulaki (Southampton University) - 04/10/17

"Black Holes, Matrix models and Holography"

In this talk, I will give an overview of the work I did during my PhD and what I am currently working on. I will start by presenting a description of Gerard 't Hooft's black hole S-matrix using a collection of inverted harmonic oscillators. Then I will give a very brief overview on how and why we are trying to compute the 1-loop corrections to a wormhole solution. In the second part, I will present an S^1/Z_2 orbifold in Liouville string theory through a dual matrix quantum mechanics description and I will explain how we performed a matching on the two sides of the duality. In the same context, I will give a brief overview on my current work that involves coupling to the gauged matrix quantum mechanics model fundamental and anti-fundamental fields. Finally, I will present how we modelled a quantum critical point in a 2+1 dimensional system at finite charge density under constant external magnetic field using applied holography.

- Arjun Bagchi (Indian Institute of Technology, Kanpur) - 26/09/17

"Constructing the BMS Bootstrap"

We elaborate on aspects of the recently introduced BMS bootstrap programme. We consider 2d field theories with BMS3 symmetry and extensively use highest weight representations to uncover the BMS version of crossing symmetry in 4-point functions that are constrained by symmetry. The BMS bootstrap equation is formulated and then analytic expressions for BMS blocks are constructed by looking at the limit of large central charges. These results are also applicable to 2d Galilean Conformal Field Theories through the isomorphism between the BMS3 and 2d Galilean Conformal Algebras. We recover our previously obtained results in the non-relativistic limit of the corresponding ones in 2d relativistic CFTs. This provides a comprehensive check of our previous analysis.