Given the current travel and gathering lockdown situation due to the coronavirus, a few of us from Osaka University, KIAS at Seoul, GIST at Gwangju and National Taiwan University at Taipei are starting a biweekly webinar series.

The webinar's address is announced in our mailing list, to which you can register via this Google form.

To add the seminars to the calendar of your choice (Google, Apple, Microsoft...), please go to the bottom of this page on ResearchSeminars.org.

Organizers: Heng-Yu Chen, Koji Hashimoto, Norihiro Iizuka, Keun-Young Kim, Sungjay Lee; Webmaster: Yuji Tachikawa

Justin Kaidi (SCGP),

"Adventures in Non-Supersymmetric String Theory"

Dec 4, 11am Taiwan time, 12pm Japan/Korea time

It has long been known that there exist strings with supersymmetry on the world sheet, but not in spacetime. These include the well-known Type 0 strings, as well as a series of seven heterotic strings, all of which are obtained by imposing unconventional GSO projections. Besides these classic examples though, relatively little is known about the full space of non-SUSY theories. One of the reasons why non-SUSY strings have remained understudied is the fact that nearly all of them have closed string tachyons, and hence do not admit ten-dimensional flat space as a stable vacuum. The goal of this talk is two-fold. First, using recent advances in condensed matter theory, we will reinterpret GSO projections in terms of topological phases of matter, thereby providing a framework for the classification of non-SUSY strings. Having done so, we will show that for all non-SUSY theories in which a tachyon exists, it can be condensed to give a stable lower-dimensional vacuum. In many cases, these stable vacua will be two-dimensional string theories already known in the literature.

Yang-Hui He (Oxford),

"Universes as Bigdata: Superstrings, Calabi-Yau Manifolds and Machine-Learning" [slides]

Sep 18, **4pm** Taiwan time, **5pm** Japan/Korea time **(Note the unusual time!)**

We review how historically the problem of string phenomenology lead theoretical physics first to algebraic/diffenretial geometry, and then to computational geometry, and now to data science and AI. With the concrete playground of the Calabi-Yau landscape, accumulated by the collaboration of physicists, mathematicians and computer scientists over the last 4 decades, we show how the latest techniques in machine-learning can help explore problems of physical and mathematical interest.

Yang Zhou (Fudan Univ),

"Reflected entropy for an evaporating black hole" [slides]

Sep 4, 11am Taiwan time, 12pm Japan/Korea time

Black hole information paradox is a problem over 40 years. Recent progress shows hint towards a new understanding of the black hole interior as part of the Hawking radiation, which they called island. In particular the island formula for the Von Neumann entropy of Hawking radiation is consistent with unitarity. In this talk I will discuss how to extract more information of the island by computing the correlation in Hawking radiation. In particular we study reflected entropy (as a correlation measure of mixed states) and propose a quantum extremal surface for reflected entropy. Based on that, we find a formula for reflected entropy with island cross section as its area term in gravitational system.

Richard A. Davison (Herriot-Watt Univ),

"Relations between transport and chaos in holographic theories" [slides]

Aug 21, **4pm** Taiwan time, **5pm** Japan/Korea time **(Note the unusual time!)**

I will describe recent work illustrating general relations between the transport properties and chaotic properties of quantum field theories with holographic duals. I will firstly show how a simple analysis of near-horizon dynamics yields exact constraints on the spectrum of collective excitations. I will then describe how this can be exploited to identify a universal feature in the spectrum, and its implications for the collective transport properties of strongly interacting field theories with gravity duals.

Julian Sonner (Univ of Geneva),

"Causal symmetry breaking: late time physics of holographic quantum chaos" [slides]

Aug 7, **4pm** Taiwan time, **5pm** Japan/Korea time **(Note the unusual time!)**

Quantum chaotic systems are often defined via the assertion that their spectral statistics coincides with, or is well approximated by, random matrix theory. In this talk I will explain how the universal content of random matrix theory emerges as the consequence of a simple symmetry-breaking principle and its associated Goldstone modes. Within AdS/CFT this gives a universal framework to express correlations between "different universes", even for individual theories rather than ensembles. I will illustrate the bulk picture of this EFT using the example minimal string theory. I will end by discussing the construction of the EFT of quantum chaos also in higher dimensional field theories, as applicable for example for higher-dimensional AdS/CFT dual pairs.

Xiangyu Cao (UC Berkeley),

"Scrambling vs chaos" [slides]

July 24, 11am Taiwan time, 12pm Japan/Korea time

How to characterize information scrambling in dynamical systems is a question of interest to physicists across energy scales. Inspired by classical chaos, which addresses this question quite successfully in classical physics, out of order time correlators (OTOC) have been considered a promising approach for quantum systems. In this talk, I will first discuss the relation between scrambling (as defined by exponential growth of OTOC) and chaos in the semiclassical regime, and show that they can differ qualitatively (joint work with Tianrui Xu andThomas Scaffidi). In a second part, I will discuss scrambling in a family of generalized Sachdev-Ye-Kitaev models, which are arguably more realistic, and which allow us to clarify the behavior of the Lyapunov exponent in non-maxically chaotic Fermi and non-Fermi liquids (joint work with Jaewon Kim and Ehud Altman).

David Tong (University of Cambridge),

"Boundary Conditions for Chiral Fermions" [slides]

July 10, **4pm** Taiwan time, **5pm** Japan/Korea time **(Note the unusual time!)**

I will describe boundary conditions for fermions in d=1+1 dimensions, using the framework of boundary conformal field theory. I will explain how boundary states are classified by a mod 2 index, related to their SPT phase, describe how one can follow RG flows from one boundary state to another, and give a novel perspective on the Z_8 classification of d=2+1 SPT phases.

João Penedones (École Polytechnique Fédérale de Lausanne),

"Nonperturbative Mellin Amplitudes" [slides]

June 26, **4pm** Taiwan time, **5pm** Japan/Korea time **(Note the unusual time!)**

I will argue that nonperturbative CFT correlation functions admit a Mellin amplitude representation. I will discuss the main properties of Mellin amplitudes: subtractions, analyticity, unitarity, Polyakov conditions and polynomial boundedness at infinity. These can be used to write dispersion relations for Mellin amplitudes. Combining crossing, dispersion relations and Polyakov conditions I will write down a set of (extremal) functionals that act on the OPE data.

Miranda Cheng (Univ. of Amsterdam/National Taiwan University),

"Quantum Modularity from 3-Manifolds" [slides]

June 12, 11am Taiwan time, 12pm Japan/Korea time

Quantum modular forms are functions on rational numbers that have rather mysterious weak modular properties. Mock modular forms and false theta functions are examples of holomorphic functions on the upper-half plane which lead to quantum modular forms. Inspired by the 3d-3d correspondence in string theory, a new topological invariants named homological blocks for (in particular plumbed) three-manifolds have been proposed a few years ago. My talk aims to explain the recent observations on the quantum modular properties of the homological blocks, as well as the relation to logarithmic vertex algebras. The talk will be based on a series of work in collaboration with Sungbong Chun, Boris Feigin, Francesca Ferrari, Sergei Gukov, Sarah Harrison, and Gabriele Sgroi.

Chang-Tse Hsieh (Kavli IPMU & ISSP, Univ. of Tokyo),

"Anomaly of the Electromagnetic Duality of Maxwell Theory" [slides]

May 29, 11am Taiwan time, 12pm Japan/Korea time

Every physicist knows that the classical electromagnetism is described by Maxwell's equations and that it is invariant under the electromagnetic duality S: (E, B) → (B, −E). However, the properties of the electromagnetic duality in the quantum theory might not be as well known to physicists and are in fact not very well understood in the literature. Recently, we uncovered a feature of Maxwell theory in the situation where going around nontrivial paths in the spacetime involves the action of the duality transformation, namely that it has a quantum anomaly. We found that the anomaly of this system in a particular formulation is 56 times that of a Weyl fermion. Our result reproduces, as a special case, the known anomaly of the all-fermion electrodynamics — a version of the Maxwell theory where particles of odd (electric or magnetic) charge are fermions — discovered in the last few years.

Kantaro Ohmori (Simons Center for Geometry and Physics),

"1+1d adjoint QCD and non-invertible topological lines" [slides]

May 15, 11am Taiwan time, 12pm Japan/Korea time

In 1990s, it is claimed that the QCD with massless adjoint quark in 1+1-dimensions is confined, although naively the fermions cannot screen the Wilson line in the fundamental representation. In this talk, we will see that the confinement can be explained from the topological line operators live in the theory. In particular, non-invertible topological lines, which do not correspond to a symmetry, play a crucial role. Based on a ongoing work with Zohar Komargodski, Konstantinos Roumpedakis and Sahand Seifnashri.

Tadashi Takayanagi (Yukawa Institute, Kyoto Univ.),

"Holographic Pseudo Entropy" [slides]

May 1, 11am Taiwan time, 12pm Japan/Korea time

In this talk, we will study a generalization of holographic entanglement entropy. This quantity is defined by a mininal area surface in an asymptotically AdS space with Euclidean time-dependence. We will give its CFT counterpart, which we call pseudo entropy. We will also provide quantum information theoretic interpretations of this quantity in several specific examples of qubit systems, which help us to understand our results of pseudo entropy in CFTs and gravity duals.

Daniel Baumann (Univ. of Amsterdam),

"The Cosmological Bootstrap" [slides]

Apr 17, 11am Taiwan time, 12pm Japan/Korea time

The past decade has seen an explosion of progress in our understanding
of scattering amplitudes in gauge theory and gravity. New bootstrap
methods have revealed hidden symmetries and new mathematical
structures that are completely invisible in the standard approach of
Lagrangians and Feynman diagrams. Inspired by these developments, the
bootstrap philosophy has recently been applied to cosmology.
In this talk, I will describe our work on the bootstrapping of
cosmological correlations.

The talk will have two parts: In the first part, I will describe the
conceptual foundations of the "cosmological bootstrap” as developed
together with Arkani-Hamed, Lee and Pimentel in [arXiv:1811.00024].
In the second part, I will describe the extension of these ideas to
massless particles with spin, where locality provides important new
constraints. This is work to appear with Duaso Pueyo, Joyce, Lee and
Pimentel.