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Donaldson-Thomas invariants, wall-crossing and categorifications

October 1 (Fri) 16:00 - 18:10, 2021

Yukinobu Toda (Professor, Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), The University of Tokyo)

It is an important subject to study algebraic curves inside algebraic varieties, both in classical algebraic geometry and also enumerative geometry inspired by string theory. The Donaldson-Thomas theory is one of curve counting theories on Calabi-Yau 3-folds, and has developed in these 20 years from several aspects of mathematics and mathematical physics. Among them, the wall-crossing in derived category turned out to be a key phenomena in proving deep structures of generating series of Donaldson-Thomas invariants. In the first one hour, I will review the classical aspect of counting curves inside algebraic varieties, and explain how it leads to modern enumerative geometry such as Gromov-Witten invariants, Donaldson-Thomas invariants. In the second one hour, I will explain wall-crossing phenomena in Donaldson-Thomas theory, and its categorification in the case of the resolved conifold. *Please contact Keita Mikami's mail address to get access to the Zoom meeting room.

Venue: via Zoom

Event Official Language: English

Evolutionary dynamics of seasonal influenza viruses

September 30 (Thu) 10:00 - 11:00, 2021

Takashi Okada (Senior Research Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

Seasonal influenza viruses undergo rapid evolution, which allows them to escape from human-immune-system responses and infect humans repeatedly. In this talk, I present some counter-intuitive properties observed in time-series data of viral sequence variation and then discuss how these strange properties can be explained by extending the standard framework of population genetics.

Venue: via Zoom

Event Official Language: English

Finding Gravitational Waves from the Early Universe

September 27 (Mon) 16:00 - 17:30, 2021

Eiichiro Komatsu (Director, Max Planck Institute for Astrophysics, Germany)

The Cosmic Microwave Background (CMB) gives a photographic image of the Universe when it was still an “infant”. We have been using it to test our ideas about the origin of the Universe. The CMB research told us a remarkable story: the structure we see in our Universe such as galaxies, stars, planets, and eventually ourselves originated from tiny quantum fluctuations in the period of the early Universe called cosmic inflation. While we have accumulated strong evidence for this picture, the extraordinary claim requires extraordinary evidence. The last prediction of inflation that is yet to be confirmed is the existence of primordial gravitational waves whose wavelength can be as big as billions of light years. To this end we have proposed to JAXA a new satellite mission called LiteBIRD, whose primary scientific goal is to find signatures of gravitational waves in the polarisation of the CMB. In this presentation we describe physics of gravitational waves from inflation, and the LiteBIRD proposal.

Venue: via Zoom

Event Official Language: English

S-matrix Unitarity toward UV Completion

September 13 (Mon) 13:30 - 15:00, 2021

Keisuke Izumi (Assistant Professor, Kobayashi-Maskawa Institute for the Origin of Particles and the Universe (KMI), Nagoya University)

Einstein gravity is not renormalizable and does not hold perturbative unitarity at high energy. This is the main reason why the construction of quantum gravity is difficult. A conjecture was proposed by Llewellyn Smith, "renormalizablility and tree-unitarity at high energy give the same conditions for theories". This conjecture would be important because it shows that, if a theory is constructed s.t. unitarity is satisfied, renormalizablility holds automatically, and vice versa. Unfortunately, a counterexample was pointed out. If a theory involves higher derivatives, there exists a theory which is renormalizable but does not satisfy tree-unitarity. A candidate of quantum gravity, the quadratic gravity (R_{\mu\nu}^2 gravity), is one of the examples. Therefore, Llewellyn Smith's conjecture would not be useful for the discussion of quantum gravity. Then, we introduce a new conjecture, "renormalizablility and S-matrix unitarity (or often called pseudo-unitarity) at high energy give the same conditions for theories". In this talk, Llewellyn Smith's conjecture and our contribution to it will be explained. Then, our new conjecture will be introduced. Finally, it will be shown that our conjecture works well even in theories with higher derivatives.

Venue: via Zoom

Event Official Language: English

The Polarised ring of the Supermassive Black Hole in M87: EHT observations and theoretical modeling

September 3 (Fri) 14:00 - 16:00, 2021

Yosuke Mizuno (T.D. Lee Fellow / Associate Professor, Tsung-Dao Lee Institute, Shanghai Jiao Tong University, China)

The Event Horizon Telescope has mapped the central compact radio source of the elliptical galaxy M87 at 1.3 mm with unprecedented angular resolution. These images show a prominent ring with a diameter of ~40 micro-arcsecond, consistent with the size and shape of the lensed photon orbit encircling the “shadow” of a supermassive black hole. Recently EHT has provided new images of the polarised emission around the central black hole in M87 on event-horizon scale. This polarised synchrotron emission probes the structure of magnetic fields and the plasma properties near the black hole. We found that the net azimuthal linear polarisation pattern may result from organised, poloidal magnetic fields in the emission region. In a quantitative comparison with a large simulated polarimetric image library, we found that magnetically arrested accretion disks are favoured to explain polarimetric EHT observations. In this talk, I also briefly discuss about a new modelling study of M87 jets in the collimation and acceleration region.

Venue: via Zoom

Event Official Language: English

Journal Club: A quantitative quasispecies theory-based model of virus escape mutation under immune selection

September 2 (Thu) 10:00 - 11:00, 2021

Yingying Xu (Special Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

I would like to introduce the paper "a quantitative quasispecies theory-based model of virus escape mutation under immune selection", written by Hyung-June Woo and Jaques Reifman [1]. Paper abstract: Viral infections involve a complex interplay of the immune response and escape mutation of the virus quasispecies inside a single host. Although fundamental aspects of such a balance of mutation and selection pressure have been established by the quasispecies theory decades ago, its implications have largely remained qualitative. Here, we present a quantitative approach to model the virus evolution under cytotoxic T-lymphocyte immune response. The virus quasispecies dynamics are explicitly represented by mutations in the combined sequence space of a set of epitopes within the viral genome. We stochastically simulated the growth of a viral population originating from a single wild-type founder virus and its recognition and clearance by the immune response, as well as the expansion of its genetic diversity. Applied to the immune escape of a simian immunodeficiency virus epitope, model predictions were quantitatively comparable to the experimental data. Within the model parameter space, we found two qualitatively different regimes of infectious disease pathogenesis, each representing alternative fates of the immune response: It can clear the infection in finite time or eventually be overwhelmed by viral growth and escape mutation. The latter regime exhibits the characteristic disease progression pattern of human immunodeficiency virus, while the former is bounded by maximum mutation rates that can be suppressed by the immune response. Our results demonstrate that, by explicitly representing epitope mutations and thus providing a genotype–phenotype map, the quasispecies theory can form the basis of a detailed sequence-specific model of real-world viral pathogens evolving under immune selection. *Please refer to the email to get access to the Zoom meeting room.

Venue: via Zoom

Event Official Language: English

RIKEN-Vancouver Joint Workshop on Quantum Computing

August 24 (Tue) - 25 (Wed) 2021

The main aim of this workshop is that the quantum people in RIKEN (iTHEMS and RQC) and Vancouver (Quantum BC) get together online to discuss scientific activities and explore future collaborations. Program: August 24, 2021 (8:30am - 1:00pm) Tokyo August 23, 2021 (4:30pm - 9:00pm) Vancouver Tetsuo Hatsuda (iTHEMS): Welcome + iTHEMS overview Yasunobu Nakamura (RQC): RQC overview Lukas Chrostowski (UBC): Quantum BC overview Shunji Matsuura (1QBit): Accurate state preparation on noisy quantum devices Olivia Di Matteo (UBC): Operational, gauge-free quantum tomography Yasunobu Nakamura (RQC): Towards superconducting quantum computing Jason Chang (iTHEMS): Improving Schroedinger equation implementations with gray code for adiabatic quantum computers Robert Raussendorf (UBC): Computationally universal phase of quantum matter Akira Furusawa (RQC): Large-scale quantum computing with quantum teleportation August 25, 2021 (8:30am - 1:30pm) Tokyo August 24, 2021 (4:30pm - 9:30am) Vancouver Etsuko Itou (iTHEMS): Digital quantum simulation for screening and confinement in gauge theory with a topological term Joe Salfi (UBC): Engineering long coherence times of spin-orbit qubits in silicon Seiji Yunoki (RQC): Quantum simulations for quantum many-body systems: Variational quantum algorithms and beyond Takumi Doi (iTHEMS): Hybrid quantum annealing via molecular dynamics Drew Potter (UBC): Simulating highly-entangled matter with quantum tensor networks Seigo Tarucha (RQC): High-fidelity quantum gates in silicon quantum computing Organizing Institutes: iTHEMS: RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program RQC: RIKEN Center for Quantum Computing Quantum BC Organizers: Tetsuo Hatsuda (iTHEMS) Yasunobu Nakamura (RQC) Shunji Matsuura (1QBit) Joseph Salfi (UBC) Erika Kawakami (RQC / RIKEN CPR) Neill Lambert (RIKEN CPR)

Venue: via Zoom

Event Official Language: English

The damped circadian oscillator in cyanobacterium: kaiA-less oscillator

August 19 (Thu) 10:00 - 11:00, 2021

Naohiro Kawamoto (Researcher, Research Center for Solar Energy Chemistry, Osaka University)

Circadian clocks are conserved in almost all organisms and provide fitness advantages to their owners through scheduling biological processes at appropriate time of diurnal cycles. Cyanobacteria possess circadian clock genes named kaiA, kaiB and kaiC. The phosphorylation cycle of KaiC, driven by KaiA and KaiB, is assumed to be a core oscillator in the cyanobacterial clock, and it has been believed that all of the three genes are essential for circadian oscillations since their finding in 1998. However, we found that the kaiA-disrupted strains exhibited a faint damped oscillation. Measuring the bioluminescence rhythms of mutants revealed that the damped oscillation is generated by transcriptional-translational feedback of kaiBC, but not by the phosphorylation cycle of KaiC. In this talk, in addition to the mechanism of the kaiA-less oscillator, I will propose how it can be beneficial for the total circadian system in the cyanobacterium by analyzing the simple model of the two coupled oscillators.

Venue: via Zoom

Event Official Language: English

Application of AdS/CFT to non-equilibrium phenomena in external electric fields

August 16 (Mon) 13:00 - 15:00, 2021

Shunichiro Kinoshita (Collaborative Researcher, Faculty of Science and Engineering, Chuo University)

The AdS/CFT correspondence is a useful tool for studying strongly-coupled gauge theories. According to this correspondence, the D3/D7 brane system in string theory is one of the simplest toy model dual to supersymmetric QCD-like gauge theory. In the dual field theory, the mesons, i.e., the quark-antiquark bound states are stable in vacuum when the quark is massive, while the dielectric breakdown occurs by pair production of quark-antiquark under strong electric fields. In this talk, I will review a series of our works of D3/D7 systems and show time-dependent, non-equilibrium phenomena driven by external electric fields such as suddenly increasing or rotating electric fields.

Venue: via Zoom

Event Official Language: English

Rational design of autonomous, peptide-based ion channels

August 5 (Thu) 10:00 - 11:00, 2021

Ai Niitsu (Special Postdoctoral Researcher, Theoretical Molecular Science Laboratory, RIKEN Cluster for Pioneering Research (CPR))

Designing artificial ion channel proteins has been a major challenge since rational design of membrane proteins is still in its infancy. To address this challenge, we aim to understand the most fundamental interaction in membrane proteins, helix-helix packing, using artificial peptides. Here, we rationally design, synthesise and characterise transmembrane peptides which self-assemble into stable channels. In this talk, I will present our computational de novo peptide design, structure modelling and molecular dynamics simulations, followed by biophysical experiments indicating structure and function of the designed channels. These works shed light on a sequence-to-structure/stoichiometry of membrane alpha-helices, which will aid more accurate membrane protein designs in future. *Please refer to the email to get access to the Zoom meeting room.

Venue: via Zoom

Event Official Language: English

iTHEMS Science Outreach Workshop 2021

July 11 (Sun) - 12 (Mon) 2021

RIKEN iTHEMS, an interdisciplinary Theoretical and Mathematical Sciences Program (RIKEN iTHEMS), was established in November 2016. RIKEN iTHEMS aims to use interdisciplinary methods with a focus on mathematics to elucidate the universe, matter and life, and to solve fundamental problems in society. Building on the achievements of the past exchanges through the Journalist in Residence Program, we will hold an outreach workshop to explore the relevance of the research of the RIKEN iTHEMS to society and to discuss how to build interactive relationships between journalists and researchers in the future. Like last year, this year's workshop will be held as a ZOOM workshop as the seminar house is closed. Contact: Takashi Tsuboi (iTHEMS Deputy Director)

Venue: via Zoom

Event Official Language: Japanese

Fallback Accretion in Binary Neutron Star Mergers

July 9 (Fri) 16:00 - 17:30, 2021

Wataru Ishizaki (Postdoctoral Fellow, Yukawa Institute for Theoretical Physics, Kyoto University)

The gravitational wave event GW170817 with a kilonova shows that a merger of two neutron stars ejects matter with radioactivity including r-process nucleosynthesis. A part of the ejecta inevitably falls back to the central object, possibly powering long-lasting activities of a short gamma-ray burst (sGRB), such as extended and plateau emissions. We investigate the fallback accretion with the r-process heating by performing one-dimensional hydrodynamic simulations and developing a semi-analytical model. We show that the usual fallback rate dM/dt \propto t^{-5/3} is halted by the heating. The characteristic halting timescale is $\sim 10^4$--$10^8$ sec for the GW170817-like r-process heating, which is long enough to continue the long-lasting emission of sGRBs. Furthermore, we propose a new interpretation of the recently reported re-brightening in the annual-scale X-ray light curve of GW170817. We model the fallback of the merger ejecta and construct a simple light curve model from the accreting ejecta. We find that the X-ray flux excess can be well explained by the fallback of the post-merger ejecta such as the disk wind from the accretion disk of the merger remnant rather than by the fallback of the dynamical ejecta. The duration of the constant luminosity phase conveys the initial fallback timescale t_0 in the past. Future observations in the next decades will probe the timescale of t_0 \sim 10--10^4 sec, around the time of extended emission in short gamma-ray bursts.

Venue: via Zoom

Event Official Language: English

Quantitative Population Dynamics in Interdisciplinary Biology

July 8 (Thu) 10:30 - 12:00, 2021

Shingo Iwami (Professor, Graduate School of Science, Nagoya University)

Through the course of life, from the moment of birth till death, an organism will achieve various states of equilibrium or ‘homeostasis’ which will inevitably encounter perturbations. The processes of cell growth, differentiation, infection, mutation, evolution and adaptation work together as a coordinated ‘system’, described by mathematical models for population dynamics, to maintain a healthy state. Any disruptions to this system leads to disease including infection, allergy, cancer, and aging. We are conducting interdisciplinary research to elucidate “Quantitative Population Dynamics” through the course of life with original mathematical theory and computational simulation, which are both our CORE approach. Our mathematical model-based approach has quantitatively improved a current gold-standard approach essentially relying on the statistical analysis of “snapshot data” during dynamic interaction processes in life sciences research. In this talk, I will explain how our interdisciplinary approach extends our understanding for complicated clinical data and apply real world problem with an example of the Novel Coronavirus Disease, COVID-19.

Venue: via Zoom

Event Official Language: English

Non-Unitary TQFTs from 3d N=4 Rank-0 SCFTs

July 5 (Mon) 13:00 - 14:30, 2021

Myungbo Shim (Kyung Hee University, Republic of Korea)

We propose a novel procedure of assigning a pair of non-unitary topological quantum field theories (TQFTs), TFT_\pm[T_0], to a (2+1)D interacting N=4 superconformal field theory (SCFT) T_0 of rank 0, i.e. having no Coulomb and Higgs branches. The topological theories arise from particular degenerate limits of the SCFT. Modular data of the non-unitary TQFTs are extracted from the supersymmetric partition functions in the degenerate limits. As a non-trivial dictionary, we propose that F = max{ -log |S^{(+)}_{0\alpha}| } = max{ -log |S^{(-)}_{0\alpha}| }, where F is the round three-sphere free energy of T_0 and S^{(\pm)}_{0\alpha} is the first column in the modular S-matrix of TFT_\pm. From the dictionary, we derive the lower bound on F, F > -log(\sqrt{(5-\sqrt{5})/10}) \simeq 0.642965, which holds for any rank 0 SCFT. The bound is saturated by the minimal N=4 SCFT proposed by Gang-Yamazaki, whose associated topological theories are both the Lee-Yang TQFT. We explicitly work out the (rank 0 SCFT)/(non-unitary TQFTs) correspondence for infinitely many examples. Before going to the technical part, we provide some background materials including some peculiar features in 3d gauge theories, some supersymmetries, anyons, and some modular data of MTC in this talk.

Venue: via Zoom

Event Official Language: English

An introduction to modular functions, conformal field theories, and moonshine phenomena

July 2 (Fri) 16:00 - 18:10, 2021

Mizuki Oikawa (Junior Research Associate, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS) / Student Trainee, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS) / Ph.D. Student, Graduate School of Mathematical Sciences, The University of Tokyo)

Moonshine phenomena are certain mysterious connections between modular functions and finite groups. The first example is the celebrated monstrous moonshine, which connects the J-invariant and the Monster group. Surprisingly, this relationship can be well understood in terms of chiral conformal field theory. In this talk, I would like to explain what is chiral conformal field theory and how it gives moonshine phenomena. In the first part of the talk, the notion of modular function will be introduced and the precise statement of the monstrous moonshine will be given. Then the monstrous moonshine will be explained in terms of vertex operator algebra, a mathematical model of chiral conformal field theory. In the second part of the talk, we focus on the question: what is chiral conformal field theory mathematically? In addition to vertex operator algebras, other mathematical models of chiral conformal field theory, namely conformal nets and Segal conformal field theories, will be introduced. Recent progress on the relationship among these three models, including the Carpi--Kawahigashi--Longo--Weiner correspondence and the geometric realization of conformal nets will also be reviewed.

Venue: via Zoom

Event Official Language: English

How nucleus size affects chromatin motion? - Experimental measurements and a polymer physics theory

July 1 (Thu) 10:00 - 11:00, 2021

Takahiro Sakaue (Associate Professor, Department of Physics and Mathematics, College of Science and Engineering, Aoyama Gakuin University)

Chromatin moves dynamically inside the cell nucleus, and its motion is often correlated with gene functions such as DNA recombination and transcription. A recent study has shown that during early embryogenesis of the nematode, Caenorhabditis elegans, the chromatin motion markedly decreases with the cell stage. However, the underlying mechanism for this transition has yet to be elucidated. Here we systematically investigate the impact of nuclear size to demonstrate that it is indeed a decisive factor in chromatin mobility. We show that a simple theoretical description, which takes into account the length and time scales of chromatin polymer solution, can quantitatively describe the relationship between the nucleus size and the chromatin motion in vivo. Our results emphasize a regulatory role of nuclear size in restricting chromatin motion, and a generic polymer physics model plays a guiding role in capturing this essential feature. *Please refer to the email to get access to the Zoom meeting room.

Venue: via Zoom

Event Official Language: English

Toward QCD-based description of dense baryonic matter

June 29 (Tue) 13:00 - 14:30, 2021

Yuki Fujimoto (Postdoctoral Scholars, Department of Physics, University of Washington, USA)

The equation of state (EoS) of dense baryonic/quark matter is the crucial ingredient for understanding neutron stars. I briefly review the current state of the high-density matter EoS based on the QCD perspectives. In this talk, I particularly focus on the perturbative QCD (pQCD) EoS, which was previously thought to be useless at realistic density because it is plagued by the large uncertainty. I introduce our recent analysis of the EoS calculated within the pQCD framework with the resummation [Fujimoto & Fukushima, 2011.10891]. I discuss our scheme for the Hard Dense Loop resummation, which turns out to reduce the uncertainty compared with the conventional pQCD estimate without resummation. Our result apparently extends the applicability of the QCD-based EoS down to densities realized inside neutron stars and infers a smooth matching with the baryonic EoS.

Venue: via Zoom

Event Official Language: English

Period variability can provide valuable information in oscillatory systems

June 24 (Thu) 10:00 - 11:00, 2021

Fumito Mori (Assistant Professor, Department of Human Science, Faculty of Design, Kyushu University)

Biological clocks generate temporally precise oscillations although they are subjected to various types of noise. In other words, oscillations with only a small variability in the period are observed under action of noise. In this talk, I focus on period variability in coupled phase oscillators and complex oscillatory dynamical systems, and present the following topics:(i) A phase oscillator subjected to noise can become to generate more precise oscillations not only when it is synchronized with periodic signal but also when it is mutually synchronized with another phase oscillator with stronger noise. (ii) In complex oscillatory dynamical systems, period variability is sensitive to the choice of an output variable and output checkpoint; it can be reduced by an appropriate selection of them. (iii) Noise intensity and coupling strength in synchronized phase oscillators can be inferred from data about period variability. This talk is based on joint researches with Dr. Hiroshi Kori (Tokyo Univ) and Dr. Alexander Mikhailov (Kanazawa Univ). *Please refer to the email to get access to the Zoom meeting room.

Venue: via Zoom

Event Official Language: English

Black Hole Information Paradox and Wormholes

June 21 (Mon) 13:00 - 14:30, 2021

Kanato Goto (Special Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

In this talk, I will explain about the recent progress in the black hole information paradox that I am involved with. The information paradox arises when a black hole evaporates by emitting Hawking radiation due to the quantum effects. Time dependence of the entropy of Hawking radiation is diagnosis of information loss caused by the black hole evaporation. If information is not lost, the entropy of Hawking radiation should obey the so-called Page curve. In recent research developments, it was found that “the quantum extremal islands” reproduce the unitary Page curve in an evaporating black hole. I will argue about how the quantum extremal islands are derived from the computation of the entropy of Hawking radiation using the gravitational path-integral.

Venue: via Zoom

Event Official Language: English

Stable eigenvalues of compact anti-de Sitter 3-manifolds

June 18 (Fri) 16:00 - 18:10, 2021

Kazuki Kannaka (Special Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

Geometric objects that have been investigated in detail so far, such as closed Riemann surfaces, are sometimes locally homogeneous. Loosely speaking, their infinitesimal behavior is the same at each point. In this talk, I would like to explain the idea of investigating such objects using the Lie group theory.In the first part of the talk, I will recall the notions of Lie group actions and their quotient spaces with examples, and then explain the definitions of locally homogeneous spaces and their deformations (Teichmüller spaces). In the second part of the talk, I will consider anti-de Sitter manifolds as a special case, i.e., Lorentzian manifolds of negative constant curvature. As in the Riemannian case, a differential operator called the Laplacian (or the Klein-Gordon operator) is defined on Lorentzian manifolds. Unlike the Riemannian case, it is no longer an elliptic differential operator but a hyperbolic differential operator. In its spectral analysis, new phenomena different from those in the Riemannian case have been discovered in recent years, following pioneering works by Toshiyuki Kobayashi and Fanny Kassel. I would like to explain stable eigenvalues of the hyperbolic Laplacian of anti-de Sitter 3-manifolds with recent progress.

Venue: via Zoom

Event Official Language: English