## Directions

Imperial College has its own detailed information on general directions and on getting to the theoretical physics group. The College is located on Prince Consort Road, south of Hyde Park (map). The most convenient access is via tube (South Kensington, Gloucester Road) or buses. The Theoretical Physics group resides on the 5th floor of the Huxley Building. The group also possesses its own description.

## Seminars at Imperial College

Found at least 20 result(s)

### 15.02.2017 (Wednesday)

#### The holographic Weyl semi-metal

Regular Seminar Karl Landsteiner (Madrid, IFT)

 at: 14:00 ICroom H503 abstract: Weyl semi-metals are an exciting new class of 3D materials with exotic transport properties. They are characterised by point like singularities in the Brillouin zone at which conduction and valence bands touch. Around these points the electronic quasi-particle excitations can be described by either left- or right-handed Weyl spinors. This makes high energy physics to be the low energy electronics of these materials. Holographic models might be useful in two ways: first some Weyl semi-metals might be strongly interacting and second holography is our best tool to understand and discover new exotic transport properties related to the chiral nature of the charge carriers, especially to chiral anomalies. I will review the construction of a holographic model supporting a topological quantum phase transition between a topological Weyl semi-metal phase and a trivial phase and use it to predict the existence of a new dissipation less form of viscosity (Hall viscosity) in the quantum critical region.

### 08.02.2017 (Wednesday)

#### Semiclassics, Goldstone Bosons and CFT data

Regular Seminar Alexander Monin (Ecole Polytechnique, Lausanne)

 at: 14:00 ICroom H503 abstract: In a generic CFT the spectrum of operators carrying a large U(1) charge can be analyzed semiclassically in an expansion in inverse powers of the charge. The key is the operator state correspondence by which such operators are associated with a finite density superfluid phase for the theory quantized on the cylinder. The dynamics is dominated by the corresponding Goldstone hydrodynamic mode and the derivative expansion coincides with the inverse charge expansion. I will illustrate this situation by first considering simple quantum mechanical analogues and then will systematize the approach by employing the coset construction for non-linearly realized space-time symmetries. Focussing on CFT3 I will illustrate that the three point function coefficients turn out to satisfy universal scaling laws and correlations as the charge and spin are varied.

### 02.02.2017 (Thursday)

#### Localised Black Holes and Precision Holography

Regular Seminar Jorge Santos (DAMTP)

 at: 13:00 ICroom H503 abstract: We numerically construct asymptotically global AdS_5 x S^5 black holes that are localised on the S^5. These are solutions to type IIB supergravity with S^8 horizon topology that dominate the microcanonical ensemble at small energies. At higher energies, there is a first-order phase transition to AdS_5-Schwarzschild x S^5. By the Anti-de Sitter/Conformal Field Theory (AdS/CFT) correspondence, this transition is dual to spontaneously breaking the SO(6) R-symmetry of N=4 supersymmetric Yang-Mills down to SO(5). We extrapolate the location of this phase transition and compute the expectation value of a scalar operator in the low energy phase. In addition, we discuss the construction of localised black holes in type IIA, which are dual (via T-duality) to the low temperature phase of thermal 1+1 dimensional supersymmetric Yang-Mills theory on a circle.

### 25.01.2017 (Wednesday)

#### Universal Formula for the Holographic Speed of Sound

Regular Seminar Andres Anabalon (Adolfo Ibanez U.)

 at: 14:00 ICroom H503 abstract: We consider planar hairy black holes in five dimensions with a real scalar field in the Breitenlohner-Freedman window. We show that is possible to derive a universal formula for the speed of sound for any theory. As an example we find all the planar black holes of the single scalar field consistent truncation of type IIB that preserves the SO(3)×SO(3) R-symmetry group of the gauge theory. We find the speed of sound for different values of the induced vacuum expectation value when a double trace deformation is induced in the gauge theory. For any theory, we show that the speed of sound is not bounded by any special value.

### 19.01.2017 (Thursday)

#### 6d Dual Conformal Symmetry and Minimal Volumes in AdS

Regular Seminar Arthur Lipstein (Durham)

 at: 13:00 ICroom H503 abstract: The planar scattering amplitudes of 4d N = 4 super Yang-Mills theory and 3d N=6 Chern-Simons theory exhibit a remarkable property known as dual conformal symmetry. Motivated by this, we investigate the consequences of dual conformal symmetry in six dimensions. We find that 6d dual conformal symmetry fixes the structure of the one-loop 4-point amplitude and suggests a Lagrangian with more than two derivatives. Moreover, we obtain a similar structure by generalizing the Alday-Maldacena solution for a string in Anti-de Sitter space to a 2-brane ending on a pillow-shaped surface in the boundary whose seams correspond to a null-polygon, which suggests an all-loop formula for the 4-point amplitude similar to that of N=4 super Yang-Mills theory.

### 14.12.2016 (Wednesday)

#### On branes and instantons

Regular Seminar Ruben Minasian (IPhT Saclay)

 at: 14:00 ICroom H503 abstract: I’ll review old and new aspects of deformed instanton equations derived from D-branes.

### 08.12.2016 (Thursday)

#### Classification of surface defects from Little Strings

Regular Seminar Nathan Haouzi (UC Berkeley)

 at: 13:00 ICroom H503 abstract: The so-called 6d (2,0) conformal field theory in six dimensions, labeled by an ADE Lie algebra, has become of great interest in recent years. Most notably, it gave new insights into lower dimensional supersymmetric field theories, for instance in four dimensions, after compactification. In this talk, I will talk about a deformation of this CFT, the six-dimensional (2,0) little string theory: its origin lies in type IIB string theory, compactified on an ADE singularity. We further compactify the 6d little string on a Riemann surface with punctures. The resulting defects are D-branes that wrap the 2-cycles of the singularity. This construction has many applications, and I will focus on one: I will provide the little string origin of the classification of surface defects of the 6d (2,0) CFT, for ADE Lie algebras. Furthermore, I will give the physical realization of the so-called Bala-Carter labels that classify nilpotent orbits of these Lie algebras.

### 01.12.2016 (Thursday)

#### Localised Black Holes and Precision Holography

Regular Seminar Jorge Santos (Cambridge)

 at: 13:00 ICroom H503 abstract: We numerically construct asymptotically global AdS_5xS^5 black holes that are localised on the S^5. These are solutions to type IIB supergravity with S^8horizon topology that dominate the microcanonical ensemble at small energies. At higher energies, there is a first-order phase transition to AdS_5-Schwarzschild x S^5. By the Anti-de Sitter/Conformal Field Theory (AdS/CFT) correspondence, this transition is dual to spontaneously breaking the SO(6) R-symmetry of N=4 supersymmetric Yang-Mills down to SO(5). We extrapolate the location of this phase transition and compute the expectation value of a scalar operator in the low energy phase. In addition, we discuss the construction of localised black holes in type IIA, which are dual (via T-duality) to the low temperature phase of thermal 1+1 dimensional supersymmetric Yang-Mills theory on a circle.

### 23.11.2016 (Wednesday)

#### SEMINAR CANCELLED

Regular Seminar Paul Townsend (Cambridge)

 at: 14:00 ICroom H503 abstract: The N-extended massless superparticle in a 4D Minkowski background has a worldline time-reversal invariance that becomes CPT in the quantum theory. This symmetry is anomalous for odd N, which explains why there is no CPT-self-conjugate massless supermultiplet for N=1. In the supertwistor formulation of superparticle mechanics, the anomaly is a close cousin of the parity anomaly of 3D gauge theories. CPT is not anomalous if N is even but if 2N is odd there is a Kramers degeneracy, which explains why the N=2 hypermultiplet is a doubled version of the CPT-self-conjugate supermultiplet of helicities (-1/2,0,0,1/2).

### 21.11.2016 (Monday)

#### Tinkertoys for the Exceptional (2,0) Theories

Exceptional Seminar Jacques Distler (Texas U.)

 at: 13:00 ICroom H503 abstract: I'll give an introduction to N=2, D=4 SCFTs of "Class-S" and a survey of our programme for classifying such theories. I will put special emphasis on the theories arising from the Exceptional (2,0) SCFTs in D=6.

### 17.11.2016 (Thursday)

#### Recent Advances in Heterotic Moduli

Regular Seminar Eirik Eik Svanes (Paris, LPTHE)

 at: 13:00 ICroom H503 abstract: I will discuss recent advances in addressing the heterotic moduli problem in six and seven dimensions. To map out the reduction to a lower dimensional effective theory, and in a quest to complete our understanding of the generically torsional geometries that appear, it is important to understand the moduli of these geometries. I will briefly review the infinitesimal moduli space in six dimensions, and comment on some recent advances in understanding its geometric properties. I will then explain how a similar story is emerging in seven dimensional compactifications on more exotic manifolds with G2-structure, giving hope that similar tools can be applied to understand moduli in other areas of string theory, with possible applications to phenomenology and AdS/CFT. If time, I will comment on some recent advances is studying finite deformations and obstructions of the moduli space.

### 10.11.2016 (Thursday)

#### A magnetically induced quantum critical point in holography

Exceptional Seminar Olga Papadoulaki (Utrecht)

 at: 13:00 ICroom H503 abstract: Abstract: We are using holography to study quantum phase transitions in a 2+1 D strongly coupled non conformal gauge theory at finite chemical potential and magnetic field B. Our gravity dual is 4D N = 2 Fayet-Iliopoulos gauged supergravity. We found a line of quantum critical points at B= Bc between extremal dyonic black branes and extremal magnetically charged "thermal gas" solutions. We will introduce the two types of solutions, their thermodynamics and their phase diagram. Then we will discuss the physics of the critical point and we will point out qualitative similarities between our field theory and Nambu- Jona- Lasinio model in 2+1 D under the presence of an external magnetic field. We will conclude with some open questions both on the gravity and field theory side and we will discuss future directions.

### 09.11.2016 (Wednesday)

#### Integrable deformations of AdS superstrings

Regular Seminar Ben Hoare (Zurich, ETH)

 at: 14:00 ICroom H503 abstract: In this talk I will describe recent progress towards understanding integrable deformations of superstrings in AdS backgrounds. One well-known example of such a deformation is the TsT transformation, which involves a T-duality, a shift introducing the deformation parameter, and a reverse T-duality. After introducing this deformation, I will give two alternative constructions: (i) as a particular non-abelian duality, and (ii) as a particular Yang-Baxter deformation. In the second part of the talk I will outline how this extends to a general relationship between non-abelian duality and Yang-Baxter deformations, and discuss the consequences for the Weyl invariance of the deformed models.

### 02.11.2016 (Wednesday)

#### Unitarity and positivity constraints for CFT at large N

Triangular Seminar L. Fernando Alday (Oxford)

 at: 15:00 ICroom Blackett LT2 abstract: We consider the four-point correlator of the stress tensor multiplet in N = 4 SYM in the limit of large c=N^2, but finite \lambda=g^2 N. For finite values of \lambda single-trace intermediate operators arise at order 1/c and this leads to specific poles in the Mellin representation of the correlator. The sign of the residue at these poles is fixed by unitarity. We consider solutions consistent with crossing symmetry and this pole structure. We show that in a certain regime all solutions result in a negative contribution to the anomalous dimension of twist four operators. This positivity condition can also be proven by assuming the correct Regge behaviour for the Mellin amplitude. The positivity constraints arising from CFT for the Mellin amplitude take a very similar form to the causality constraint for the forward limit of the S-matrix.

### 02.11.2016 (Wednesday)

#### Towards the string dual of a color superconductor

Triangular Seminar David Mateos (Barcelona)

 at: 16:30 ICroom Blackett LT2 abstract: At low temperature and high quark chemical potential, QCD exhibits color superconducting phases, namely phases in which the color gauge symmetry is spontaneously broken. I will describe work in progress aimed at constructing string duals of gauge theories that exhibit these types of phases.

### 28.10.2016 (Friday)

#### Non-Relativistic Scale Anomalies and Geometry

Exceptional Seminar Igal Arav (Tel Aviv U)

 at: 12:00 ICroom H503 abstract: I will discuss the coupling of non-relativistic field theories to curved spacetime, and develop a framework for analyzing the possible structure of non-relativistic (Lifshitz) scale anomalies using a cohomological formulation of the Wess-Zumino consistency condition. I will compare between cases with or without Galilean boost symmetry, and between cases with or without an equal time foliation of spacetime. In 2+1 dimensions with a dynamical critical exponent of z=2, the absence of a foliation structure allows for an A-type anomaly in the Galilean case, but also introduces the possibility of an infinite set of B-type anomalies. I will also derive Ward identities for flat space correlation functions in Lifshitz field theories, and develop a method for calculating Lifshitz anomaly coefficients from these correlation functions using split dimensional regularization.

### 26.10.2016 (Wednesday)

#### On universality of transport phenomena in holography

Regular Seminar Nick Poovuttikul (Leiden)

 at: 14:00 ICroom H503 abstract: In this talk, I will discuss transport phenomena in two classes of theories with holographic dual. In the first part, I will discuss systems where U(1) current is non-conserved due to anomaly and illustrate how one can show that the anomalous conductivities are non-renormalised in a large class of holographic RG flow. The holographic RG flow we considered is generated by arbitrary dilaton potentials and arbitrary higher derivative terms that do not break global symmetries, incorporating coupling constant corrections to the boundary theory in an expansions around infinite coupling. In the second part, I will focus on systems where translational symmetry is broken by slowly varying scalar fields and show that the bound shear viscosity/entropy density is violate. I will also discuss how to understand the violation in the language of forced fluid dynamics.

### 20.10.2016 (Thursday)

#### Conformal Higher Spin Theory and Twistor Space Actions

Regular Seminar Philipp Haehnel (Trinity College Dublin)

 at: 13:00 ICroom H503 abstract: We give an introduction to twistor theory and construct a twistor action for the self-dual part of conformal higher spin theories by treating the integrability condition for the holomorphic structure of a complex deformation of flat twistor space as the equations of motion of those higher spin fields. Analogous to the embedding of Einstein gravity with cosmological constant in Weyl gravity, we identify a ghost-free subsector which generates the unique three-point anti-MHV amplitude consistent with Poincaré invariance and helicity constraints.

### 20.10.2016 (Thursday)

#### Hexagon form factors in ABJM

Regular Seminar Raul Pereira (Uppsala)

 at: 13:30 ICroom H503 abstract: In this talk I will start by reviewing the hexagon bootstrap program for three-point functions in N=4. Then I will explain how to extend those methods to ABJM theory by considering three-point functions whose vacua preserve SU(1|2)xU(1) symmetry. I will show that this symmetry fully constraints the two-particle hexagon form factor and comment on how to construct the multi-particle hexagon. Finally, I will conclude with a summary of the difficulties in implementing this program for ABJM.

### 18.10.2016 (Tuesday)

#### On the quantization of a string with endpoint particles and the decay width of the hadronic string

Exceptional Seminar Cobi Sonnenschien (Tel Aviv)

 at: 13:30 ICroom H503 abstract: Holography inspired stringy hadrons (HISH) is a set of models that describe hadrons: mesons, baryons, glueballs and exotic hadrons as strings in four dimensional at space-time. The models are based on a "map" from stringy hadrons of curved holographic confining backgrounds. In the first part of the talk I will review the "derivation" of the models. I will start with a brief reminder of the passage from the original AdS/CFT correspondence to the string/gauge duality of certain favored confining holographic models. I will then describe the string configurations in these holographic backgrounds that correspond to Wilson lines, mesons, baryons, glueballs and exotics. Key ingredients of the four dimensional picture of hadrons are the "string end-point mass" and the "baryonic string vertex". I will determine the classical trajectories of the HISH spectra. I will review the current understanding of the quantization of these hadronic strings. The computation of HISH decay width of hadrons will be described. In the last part of the talk I will summarize the comparison of the outcome of the HISH models with the PDG data about mesons and baryons. I will present the values of the tension, masses and intercepts extracted from best fits to hadron spectra and write down certain predictions for higher excited hadrons. I will present attempts to identify glueballs. The decay width of certain hadrons will be compared with the theoretical calculation. I will suggest a window to the landscape of tetra-quarks and other exotic hadrons.