## 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)

### 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)

### 12.10.2016 (Wednesday)

#### N=2 super Yang-Mills and the Geometry of Magnetic Monopoles

Regular Seminar Andy Royston (Texas A-M)

 at: 14:00 ICroom H503 abstract: In this talk we consider BPS states in 4D, N=2 gauge theory in the presence of defects. We give a semiclassical description of these `framed BPS states' in terms of kernels of Dirac operators on moduli spaces of singular monopoles. For both framed and ordinary BPS states we present a conjectural map between the data of the semiclassical construction and the data of the low-energy, quantum-exact Seiberg-Witten description. This map incorporates both perturbative and nonperturbative field theory corrections to the supersymmetric quantum mechanics of the monopole collective coordinates. We use it to translate recent developments in the study of N=2 theories, including wall-crossing formulae and the no-exotics theorem, into geometric statements about the Dirac kernels. The no-exotics theorem implies a broad generalization of Sen's conjecture concerning the existence of L^2 harmonic forms on monopole moduli space. This talk is based on work done in collaboration with Greg Moore and Dieter Van den Bleeken.

### 05.10.2016 (Wednesday)

Regular Seminar Cobi Sonnenschein (Tel Aviv University)

### 03.08.2016 (Wednesday)

#### (2,2) Abelian GLSM string vacua and mirror symmetry

Regular Seminar Ronen Plesser (Duke)

 at: 14:00 ICroom Blackett 1004 abstract: Mirror symmetry of Calabi—Yau spaces is best understood for families presented as complete intersections in toric varieties; these models have a description as the low-energy limit of Abelian gauged linear sigma models (GLSMs). We investigate the combinatorial conditions on GLSM data such that the generic member of the family determines a non-singular low energy theory. A sufficient condition is reflexivity; this has the pleasant feature that the mirror of a reflexive model is reflexive. This condition is certainly not necessary, in particular it is not preserved by extremal transitions. We propose a weaker condition that is preserved, but is also too strong. Along the way we describe how the Berglund—Hubsch mirror construction is related to the Batyrev—Boris combinatorial duality related to Abelian duality. We study the locus in parameter space along which the model becomes singular and the invariance of this under mirror symmetry, finding support for the observation of Hori and Vafa that mirror symmetry is most naturally stated in terms of local Calabi—Yau models.

### 19.07.2016 (Tuesday)

#### Super String Field Theory Revisited

Exceptional Seminar Ivo Sachs (LMU)

 at: 14:00 ICroom B1004 abstract: I first describe the remarkably simple algebraic structure underlying covariant string field theory. No knowledge of string field theory and only very basic concepts of string theory are required for this. I then use this structure to construct the superstring field theory action recursively and discuss its relation to the decomposition of (super) moduli spaces. I will focus mostly on open string theory but will comment on the generalisation to closed string.

### 15.06.2016 (Wednesday)

#### Extending the scope of holographic mutual information and chaotic behavior

Regular Seminar Nilanjan Sircar (Tel Aviv)

 at: 14:00 ICroom H503 abstract: We extend the use of holography to investigate the "Scrambling" (a.k.a "Chaos", "Butterfly Effect", "Thermalization") properties of various physical systems at finite temperature. Specifically, we consider: (i) non-conformal backgrounds of black Dp branes, (ii) asymptotically Lifshitz black holes, and (iii) black AdS solutions of Gauss-Bonnet gravity. We use the disruption of the mutual information as a probe of the chaotic feature of such systems. Our analysis shows that these theories share the same type of behavior as conformal theories as they undergo chaos, however, in the case of Gauss-Bonnet gravity, we find a stark difference in the evolution of the mutual information for negative Gauss-Bonnet coupling. This may signal an inconsistency of the latter.

### 01.06.2016 (Wednesday)

#### Non-abelian particle-vortex duality

Regular Seminar Jeff Murugan (University of Cape Town)

 at: 14:00 ICroom H503 abstract: Quantum field theories in (2+1)-dimensions exhibit a beautiful property known as particle-vortex duality. It relates, in a precise way, two different excitations on the plane, the familiar particle-like excitations that arise from quantisation of the field and vortices, solitonic-excitations defined by the winding of a local order parameter. Originally studied in the context of anyonic superconductivity and Neilsen-Olesen vortices, extensions of the duality have recently found application to, for example, topological quantum matter. I will review some of these developments and show how recent progress in understanding non-abelian T-duality can be used to define a non-abelian particle-vortex duality in (2+1)-dimensions.

### 11.05.2016 (Wednesday)

#### Semiholographic QFTs and applications

Regular Seminar Ayan Mukhopadhyay (TU Wien)

 at: 14:30 ICroom H503 abstract: I will argue that a semiholographic framework can lead us to unravel general aspects of many quantum many body systems with strongly interacting degrees of freedom. I will present the case for QCD and some other QFTs. In particular, applications to heavy ion physics and hadronic physics will be discussed. A derivation of the framework combining exact Wilsonian RG and holographic RG will be sketched.

### 17.03.2016 (Thursday)

#### Constraints from unitarity in Conformal Field Theories

Exceptional Seminar Andrei Parnachev (Trinity College Dublin)

 at: 13:30 ICroom H503 abstract: We discuss implications of unitarity in CFTs and derive a number of constraints, including positivity of energy flux constraint.

### 10.03.2016 (Thursday)

#### Six-dimensional field theories from string theory

Exceptional Seminar Alessandro Tomasiello (Milan)

 at: 13:00 ICroom H503 abstract: I will review some recent progress about superconformal field theories in six dimensions. A simple class has an effective description in terms of a chain of gauge fields, coupled by tensors and hypermultiplets. Their holographic duals are now known analytically, and some precision checks can be performed, involving Weyl anomalies. An extension of this class can be studied using F-theory. All these theories have some features in common with the elusive theory describing M5-branes, and indeed suggest new phenomena, such as M5 fractionation.

### 09.03.2016 (Wednesday)

#### Dirty black holes and transport in strange metals

Regular Seminar Andrew Lucas (Harvard)

 at: 15:00 ICroom H503 abstract: I will discuss what we have learned in the past year about transport in strongly interacting metallic phases by studying the linear response of planar black holes with broken translational symmetry. Firstly, I will discuss how holography tells us that the conductivity of weakly disordered metals is described by the Drude formula, confirming a "prediction" of the many-body memory matrix approach. Secondly, I will discuss the derivation of non-perturbative conductivity bounds, invoking a new version of the holographic membrane paradigm. These bounds rule out disorder-driven phase transitions (which would generically exist in traditional condensed matter models) in the boundary theory under rather mild assumptions about the existence and nature of solutions to Einstein's equations. Connections between these black-hole inspired theories of transport and novel experiments in graphene will also be discussed.

### 07.03.2016 (Monday)

#### Superconformal theories from M5 branes

Exceptional Seminar Simone Giacomelli (ULB - Bruxelles)

 at: 13:30 ICroom H503 abstract: In this talk I will discuss superconformal theories in four dimensions obtained wrapping M5 branes on a Riemann surface. I will first review N=2 class S theories introduced some years ago by Gaiotto and then describe a generalization to the N=1 case. For this class of theories it is possible to write down a spectral curve encoding the properties of the chiral ring and I propose a method to determine from the spectral curve the scaling dimension of chiral operators in the SCFT. This proposal reduces to the correct prescription in the special case of N=2 theories. I will provide several consistency checks and apply this method to study some new superconformal theories.

### 24.02.2016 (Wednesday)

#### Non-perturbative effective action of N=4 SYM in the Coulomb branch

Regular Seminar Massimo Bianchi (Rome (Tor Vergata))

 at: 14:00 ICroom H503 abstract: We derive the one-instanton effective action of N=4 super Yang-Mills theory in terms of the N=4 on-shell superfields. In the Coulomb branch, instantons correct both the MHV and next-to-next-MHV higher derivative terms D^4F^{2n+2} and F^{2n+4}. We confirm at the non-perturbative level the non-renormalization theorems for MHV F^{2n+2} terms that are expected to receive perturbative corrections only at n-loops. We compute also the one and two-loop corrections to the D^4F^4 term and show that its completion under SL(2,Z) duality is consistent with the one-instanton results.

### 17.02.2016 (Wednesday)

#### Charge-hyperscaling violating Lifshitz hydrodynamics from black-holes

Regular Seminar Yoshinori Matsuo (Crete)

 at: 14:00 ICroom H503 abstract: Non-equilibrium black hole horizons are considered in scaling theories with generic Lifshitz invariance and an unbroken U(1) symmetry. There is also charge-hyperscaling violation associated with a non-trivial conduction exponent. The boundary stress tensor is computed and renormalized and the associated hydrodynamic equations derived. Upon a non-trivial redefinition of boundary sources associated with the U(1) gauge field, the equations are mapped to the standard non-relativistic hydrodynamics equations coupled to a mass current and an external Newton potential in accordance with the general theory of [arXiv:1502.00228]. The shear viscosity to entropy ratio is the same as in the relativistic case.