Week 06.11.2021 – 14.11.2021


Lonti: CPT symmetry in Quantum Field Theory

Regular Seminar Andreas Fring (City)

10:30 Other
room Online

Lonti Autumn 2021 Series: Lecture 3. Live Tutorial. Please register at https://lonti.weebly.com/registration.html to receive joining instructions for this live session which will be held via Zoom. CPT-symmetry is a fundamental symmetry of nature that is respected by all physical processes. It consists of a consecutive reversal of time (T), reflection of space at an arbitrary point (Parity P) and a charge conjugation that replaces particles by antiparticles. I briefly discuss the role P T -symmetry plays in quantum mechanics and how it may be utilised for a consistent formulation of non-Hermitian theories. For a relativistic quantum field theory the CPT-theorem provides the general framework for the validity of this symmetry to occur, by stating that the CPT-symmetry is equivalent to a strong reflection and a simultaneous Hermitian conjugation. In this lecture I will prove the theory in the framework of a Lagrangian quantum field theory for spin 0, 1 and spin 1/2 Dirac fields, by first identifying the separate transformation and a subsequent combination. Subsequently these transformations are used to identify the behaviour of various interaction terms under their action. I conclude by commenting on the experimental observations of CP-symmetry violation in the neutral K-meson decay.

Lonti: An Introduction to Observables in Gauge Theories

Regular Seminar Nadav Drukker (KCL)

10:00 Other
room Youtube

Lonti Autumn 2021 Series: Lecture 4. Release of Recorded Lecture. Available at https://youtu.be/JLbuSnt2OyA. Gauge theories are ubiquitous in theoretical physics, not to mention that the standard model is one. It is therefore of utmost importance to know what the observables of these theories are, quantities that can be calculated and measured. I start with a long discussion based on the most familiar gauge theory, Maxwell's electromagnetism, where a lot of computations can be done explicitly. I then take the lessons from that to non-abelian gauge theories. The observables covered are local, Wilson loops, and briefly 't Hooft loops and surface operators.

Berry Phases and Complexity as Probes of Bulk Geometry

Exceptional Seminar Claire Zukowski (University of Amsterdam)

15:45 KCL
room Norfolk Building G.01

I will describe two new quantum information theoretic probes of bulk geometry that access information inaccessible to spacelike geodesics. The first arises from considering a parallel transport process of modular Hamiltonians on the boundary under a change of state. I will show that the Berry curvature for this process computes the entanglement wedge symplectic form associated to a family of Euclidean cosmic brane solutions. Next, I will derive the circuit complexity for conformal field theory in arbitrary dimensions. I will show that circuits are dual to timelike geodesics in the bulk, and that the complexity metric admits a simple bulk geometric description in terms of distances between geodesics. In either case, these quantities are governed by the geometry of coadjoint orbits, which are special symplectic manifolds arising from group theory. The state-changing modular Berry transport process naturally describes the geometry of new, Virasoro-like coadjoint orbits that extend beyond the current classification. The complexity metric describes the geometry of a particular coadjoint orbit of the conformal group in arbitrary dimensions.


Black holes, information and wormholes

Regular Seminar Geoff Penington (UC Berkeley and IAS)

15:00 IC
room Online

Hawking famously argued, based on semiclassical calculations, that the radiation from evaporating black holes is contains no information about the matter that fell in. This would be inconsistent with the unitarity of quantum mechanics. In this talk, I will show that, in more careful ‘replica trick’ calculations, the gravitational path integral becomes dominated at late times by saddles containing spacetime wormholes. These wormholes cause the entropy to decrease after the Page time, consistent with unitarity, and allow information to escape from the interior of the black hole.


Phase transitions for deformations of JT supergravity and matrix models

Regular Seminar G Joaquin Turiaci (IAS)

13:45 KCL
room online

We analyze black holes in deformations of Jackiw-Teitelboim (JT) supergravity by adding a gas of defects, equivalent to changing the dilaton potential. For some range of deformations, the black hole density of states extracted from the gravitational path integral becomes negative, yielding an ill-defined sum over topologies. To solve this problem, we use an equivalent matrix model description and show the negative spectrum is resolved via a phase transition analogous to the Gross-Witten transition. The matrix model contains a rich and novel phase structure that we explore in detail, using both perturbative and non-perturbative techniques.

Defect Central Charges

Regular Seminar Adam Chalabi (Southampton)

14:00 IC
room 503

Conformal defects can be characterised by their contributions to the Weyl anomaly. The coefficients of these terms, often called defect central charges, depend on the particular defect insertion in a given conformal field theory. I will review what is currently known about defect central charges across dimensions, and present novel results. I will discuss many examples where they can be computed exactly without requiring any approximations or limits. These include defects in free theories, and recently developed tools for defects in superconformal field theories.


Gauged sigma models from four-dimensional Chern-Simons

Journal Club Jake Stedman (King's College London)

15:45 Other
room Zoom, instructions in abstract

Several years ago, a new gauge theory called four-dimensional Chern-Simons was introduced by Costello in an attempt to explain the integrability of various two-dimensional models using techniques in gauge theory. My work focuses on the use of four-dimensional Chern-Simons to explain the integrability of two-dimensional sigma models. I will begin by reviewing the construction of the Wess-Zumino-Witten (WZW) model as the boundary theory of three-dimensional Chern-Simons theory as was introduced by Moore and Seiberg. This will allow me to introduce the analogous construction of Costello and Yamazaki, in which two-dimensional sigma models appear as theories on defects in four-dimensional Chern-Simons. This naturally leads to a discussion of my work in which I construct a large class of gauged sigma models by coupling together two four-dimensional Chern-Simons theories. I will argue that the structure of four-dimensional Chern-Simons suggests that these models are integrable and finish by constructing the gauged WZW model and conformal Toda theories. This talk is based on: https://arxiv.org/abs/2109.08101. ---- Part of the London Integrability Journal Club. Please register at integrability-london.weebly.com if you are a new participant. The link will be emailed on Tuesday.

Amplitudes, Loops, and Gravity

Regular Seminar Mao Zeng (Edinburgh)

14:00 QMW
room zoom

[for zoom link please email s.nagy@qmul.ac.uk] Next-generation gravitational wave detectors demand highly precise predictions for waveforms. We present advances in binary inspiral dynamics by taking classical limits of scattering amplitudes in perturbative quantum gravity. The amplitudes are calculated efficiently using modern methods for scattering amplitudes and loop integration techniques developed for colliders. Classical physics can be extracted by several complementary approaches, including effective field theory, eikonal exponentiation, and extrapolation of quantum observables defined by the S-matrix. For both conservative and radiative dynamics, we obtain new terms in the post-Minksowskian expansion which represent first advances in decades.