Math Phys seminar

**Speaker: Hólmfríður Sigríðar Hannesdóttir, Harvard University**

**Title: ** Infrared Finite S-matrix elements and Cross Section

Room: A-050

Time: Tuesday 27th August, 2:00 pm

#### Abstract:

Quantum field theory (QFT) works remarkably well for making theoretical predictions in collider scattering experiments. One of the fundamental objects in these calculations, the scattering matrix (S-matrix), is inspired by a well defined operator in non-relativistic quantum mechanics, but is plagued with both ultraviolet (UV) and infrared (IR) divergences in QFT. The UV divergences are now understood through the program of renormalization, but IR divergences remain an active area of research. Three approaches have been explored to define IR finite quantities, which will all be discussed in this talk: i) The cross section method, ii) modification of the S-matrix, and iii) the dressed state formalism. The minimal set of processes required for a finite cross section will be examined, along with the need for forward scattering and disconnected diagrams. We will furthermore explore how the usual assumptions about evolution of the scattering states at asymptotic times are broken in a theory with massless particles, and how the universality of the interactions leading to IR divergences in such theories can be exploited to define finite S-matrix elements.

PhD midway evaluation

**Speaker: Juan Fernando Angel Ramelli, HÍ**

**Title: **Entanglement entropy of the quantum Lifshitz model

Room: A-050

Time: Tuesday 27th August, 09:00hrs

#### Abstract:

The quantum Lifshitz model (QLM) is a toy model used for studying scale invariant systems in which time and space scale on different footing. This type of behavior occurs, for example, in the context of critical phenomena. Entanglement entropy is a purely quantum measure of correlation between subsystems. It is a useful theoretical probe, as it encodes certain universal properties of critical models. I review the QLM as well as its generalization to higher dimensions, and show how to construct its excited states on compact manifolds. Through careful application of the replica method, I demonstrate that analytic results can be obtained for the entanglement entropy of both its ground state and its excited states.

Math Phys seminar

**Speaker: Mohab Abou Zeid, Imperial College London**

**Title: **T-duality in (2,1) superspace and SKT geometry

Room: VR-II, V-156

Time: Friday 23rd August, 14:00hrs

#### Abstract:

I will explain our recent derivation of the T-duality transformation rules for two-dimensional (2,1) supersymmetric sigma-models in (2,1) superspace. To this end I will first review the gauging of sigma-models in (2,1) superspace and present a new manifestly real and geometric expression for the gauged action. I will also discuss the obstructions to gauging (2,1) sigma-models and show that the obstructions to (2,1) T-duality are considerably weaker than the obstructions to gauging. Our complexified T-duality transformations are equivalent to the usual Buscher duality transformations (including an important refinement) together with diffeomorphisms. Time permitting, I will mention some possible applications of our results.

Math Phys seminar

**Speaker: Ro Jefferson, Albert Einstein Institute Potsdam**

**Title: **Black hole interiors and modular inclusions

Room: VR-II, V-156

Time: Thursday 22nd August, 14:00hrs

#### Abstract:

We show how the traversable wormhole induced by a double-trace deformation of the thermofield double state can be understood as a modular inclusion of the algebras of exterior operators. The effect of this deformation is the creation of a new region of spacetime deep in the bulk, corresponding to a non-trivial center between the left and right algebras. This set-up provides a precise framework for investigating how black hole interiors are encoded in the CFT. In particular, we use modular theory to demonstrate that state dependence is an inevitable feature of any attempt to represent operators behind the horizon. Building on this geometrical structure, we propose that modular inclusions may provide a more precise means of investigating the nascent relationship between entanglement and geometry in the context of the emergent spacetime paradigm.

Math Phys seminar

**Speaker: Jorge Laraña Aragon,University of Stockholm**

**Title: Quenched coupling, effective thermalization and evanescence in the free $O(N)$ vector model**

Room: VR-II, V-156

Time: Thursday 15th August, 14:00hrs

#### Abstract:

In this project we study two coupled free $O(N)$ vector models implementing the coupling via a quench of mixing. Since the theory is integrable pure thermalization does not occur but the quantum modes thermalize independently. Generalized thermal behavior is obtained at late times in the limit of weak initial mixing. We investigate this from both the effective density matrix and the spectral density function. When computing the latter, we observe that late-time correlation functions of composite operators in the post-quench free QFT surprisingly share interesting properties with correlators in strongly coupled systems. We discuss a possible holographic interpretation in terms of the so called evanescent modes.

Math Phys seminar

### Speaker: Christiana Pantelidou, Durham University

### Title: Incoherent hydrodynamics and density waves

Location: VR-II, V-258

Time: Monday June 24 at 2.00 pm

### Abstract:

I will consider thermal phases of holographic lattices at finite chemical potential in which a continuous internal bulk symmetry can be spontaneously broken. In the normal phase, translational symmetry is explicitly broken by the lattice and the long wavelength excitations are captured by incoherent hydrodynamics describing two perturbative diffusive modes. In the broken phase an additional diffusive hydrodynamic degree of freedom appears. In this talk I will use holography to write an effective theory describing the coupling of incoherent hydrodynamics to this goldstone mode.

Math Phys Seminar

### Speaker: Bo Sundborg, Stockholm University

### Title: Field theory compositeness, holographic gravity and higher spins

Location: Tg 227 (“smart room” Tæknigarður)

Time: Tuesday April 30 at 14.00 am

### Abstract:

I would like to discuss the idea that composite operators in field theory

are key ingredients in holograms of bulk gravitational physics. In practice,

this is well known: Composites are essential in both matrix (Maldacena) and vector (Klebanov-Polyakov) AdS/CFT. I argue that composite correlators in thermal and quenched field theory backgrounds illuminate the importance of compositeness further. They behave as boundary-boundary correlators in geometries with a strong gravitational source in the interior, whether the bulk is Einsteinian or higher spin-like.

Math Phys Seminar

### Speaker: Lukas Schneiderbauer, HÍ

### Title: Holographic Complexity of Two-Dimensional Black Holes

Location: VR-II 158

Time: Monday April 29 at 10.00 am

### Abstract:

Interest in holographic complexity arose when it was proposed as the dual quantity of the complexity of a quantum state in the context of AdS/CFT. However, it was realized that this quantity behaves reasonably even for geometries which are asymptotically flat and therefore do not fall under the AdS/CFT umbrella.

Motivated by this, we adapt and evaluate proposals for holographic complexity in black hole solutions of the CGHS model and variants thereof. These solutions describe 1+1 dimensional black hole geometries which are asymptotically flat.

The advantage of working with these models is that they allow for analytic treatment even in time-dependent backgrounds such as gravitational collapse or even black hole evaporation.

Math Phys/Phys seminar

### Speaker: Jens H. Bárðarson, KTH Stockholm

### Title: On the chiral anomaly in Weyl semimetals

Location: VR-II 158

Time: Tuesday December 4, at 13.30

### Abstract:

I will give an introduction to the physics of Weyl semimetals focussing on Fermi arcs and chiral anomaly. I will then discuss some transport properties of Weyl semimetals including strongly angular-dependent magnetotransport in the presence of long range disorder, and the difference between the longitudinal conductance in the presence of magnetic field and chiral pseudo-magnetic field. The latter points in opposite direction for opposite chiralities and can be induced, for example, by strain. This leads to a discussion of pseudo-landau levels and their connection with Fermi arcs and covariant and consistent chiral anomaly. I will also mention a new axial torsional contribution to the axial anomaly.

Math Phys seminar

### Speaker: Aruna Rajagopal, University of Iceland

### Title: Shock and Rarefaction Waves in Out of Equilibrium Lifshitz Fluids

Location: VR-II 155

Time: Thursday November 22 at 13.30

### Abstract:

Motivated by the recent developments and interest in the emergence of a non equilibrium steady state or NESS, for a relativistic fluid, and its possible holographic dual, we follow a similar framework to study the properties of such a state for a non-relativistic Lifshitz fluid with a general scaling exponent. This is carried out by solving the Riemann problem for the NESS which develops between two semi-infinite heat reservoirs that are brought into contact, and studying the shock and rarefaction waves which emerge as solutions to this problem.