Stringy Nonlocality: Operator Formalism and Implications

July 26 (Fri) at 14:00 - 15:30, 2024

Wei-Hsiang Shao (Ph.D. Student, Department of Physics, National Taiwan University, Taiwan)

Nonlocality is a fundamental property of string theory, where point-like particles are replaced by extended strings. This feature is especially evident in string field theories, where field components interact through form factors containing spacetime derivatives of infinite order. The usual approach to canonical quantization is no longer applicable, and thus a non-perturbative treatment of nonlocal effects at the quantum level remains unclear. In this seminar, I will discuss a recent attempt to construct an operator formalism for stringy nonlocal field theories, and explore the potential implications for black hole radiation and primordial fluctuations in the early universe.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Multi-Agent Reinforcement Learning for Exploring Collective Behavior

July 25 (Thu) at 16:00 - 17:00, 2024

Kazushi Tsutsui (Assistant Professor, Graduate School of Arts and Sciences, The University of Tokyo)

Humans and other organisms develop collective behaviors through interactions with diverse environments and various species. These behaviors are significant topics across multiple research fields, including evolutionary biology, behavioral ecology, and animal sociology. Unraveling the decision-making mechanisms of individuals in groups within cooperative and competitive contexts has captured the attention of many researchers but remains a complex challenge. This seminar will present research cases that employ multi-agent reinforcement learning, a machine learning technique, to investigate the decision-making processes underlying collective behavior. Through this approach, we aim to provide deeper insights into the dynamics and mechanisms that drive group behaviors in various biological systems.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Symmetries and Generalization for Machine Learning on a Lattice

July 23 (Tue) at 15:00 - 16:30, 2024

Andreas Ipp (Senior Scientist, Institute for Theoretical Physics, TU Wien, Austria)

Symmetries such as translations and rotations are crucial in physics and machine learning. The global symmetry of translations leads to convolutional neural networks (CNNs), while the much larger space of local gauge symmetry has driven us to develop lattice gauge equivariant convolutional neural networks (L-CNNs). This talk will discuss how the challenges of simulating the earliest stage of heavy ion collisions led us to use machine learning and how these innovations could improve lattice simulations in the future. Andreas Ipp is a Senior Scientist at the Institute for Theoretical Physics at TU Wien. He received his PhD in 2003 and held postdoctoral positions at ECT* in Trento and the Max-Planck-Institute in Heidelberg before returning to TU Wien in 2009. He completed his habilitation on "Yoctosecond dynamics of the quark-gluon plasma" in 2014. His current research focuses on symmetries in machine learning for applications in lattice gauge theory and heavy ion collisions.

Venue: Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

Probing Majorana excitations in the Kitaev magnet α-RuCl3 through bulk heat capacity measurements

July 22 (Mon) at 10:30 - 11:45, 2024

Kumpei Imamura (Ph.D. Student / JSPS Research Fellow DC, Department of Advanced Materials Science, The University of Tokyo)

Recently, the layered honeycomb material α-RuCl3 exhibits several anomalous features that are consistent with expectations of the Kitaev quantum spin liquid (KQSL) under in-plane magnetic field. Most remarkably, finite planar thermal Hall conductivity has been observed, whose magnitude is close to the half-integer quantization value expected for the chiral edge currents of Majorana fermions[1]. However, it has been reported that the thermal Hall conductivity shows strong sample dependence. Also, there are attempts to offer a different explanation by the bosonic edge excitations due to topological magnons or phonon. A key to distinguishing between fermionic and bosonic origins of unusual features in the high-field state of α-RuCl3 is the difference in the field angle dependence of the excitation gap. Therefore, we distinguish these origins from combined low-temperature measurements of high-resolution specific heat and thermal Hall conductivity with rotating magnetic fields within the honeycomb plane. A distinct closure of the low-energy bulk gap is observed for the fields in the Ru-Ru bond direction, and the gap opens rapidly when the field is tilted. Notably, this change occurs concomitantly with the sign reversal of the Hall effect. General discussions of topological bands show that this is the hallmark of an angle rotation–induced topological transition of fermions, providing conclusive evidence for the Majorana-fermion origin of the thermal Hall effect in α-RuCl3[2]. Furthermore, to understand the nature of the high-field state, it is crucial to elucidate the effects of disorder, which inevitably exists in real materials. We artificially introduce point defects by electron irradiation and compare the low-energy excitations in the pristine and irradiated sample by high-resolution specific heat measurements. We observed an additional in-gap T-linear term in C/T, whose coefficient shows distinct field-sensitive behaviors suggestive of Majorana physics in the KSL. This can be interpreted by the weak localization of Majorana fermions, which is induced by the disorder[3]. Moreover, recently, we succeed in synthesizing very high-quality crystals of α-RuCl3[4].

Venue: via Zoom / Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

iTHEMS NOW & NEXT 2024

July 19 (Fri) at 9:30 - 18:00, 2024

9:30-10:00 Keynote by Lucy Mcneill 10:00-10:30 Keynote by Jose Said Gutierrez Ortega 10:35-11:05 Keynote by Puttarak Jai-akson 11:05-11:35 Keynote by Kannaka Kazuki 11:35-12:30 Lunch Time Session 12:30-13:30 Working Group / Study Group Report 
13:30-15:30 Flash Talk & Poster Presentation Part 1 
15:30-17:30 Flash Talk & Poster Presentation Part2 17:30 Concluding Remarks by the Director 18:00 reception

Venue: RIKEN Wako Campus, Head Quarter Build., 2F Large Conference Room (Main Venue) / via Zoom

Event Official Language: English

Optimal control of stochastic reaction networks

July 18 (Thu) at 16:00 - 17:00, 2024

Shuhei Horiguchi (JSPS Postdoctoral Research Fellow, Nano Life Science Institute, Kanazawa University)

Optimal control problems for the population of interacting particles arise in various fields, including pandemic management, species conservation, cancer therapy, and chemical engineering. When the population size is small, the time evolution of the particle numbers is inherently noisy and modeled by stochastic reaction networks, a class of jump processes on the space of particle number distributions. However, compared to deterministic and other stochastic models, optimal control problems for stochastic reaction networks have not been extensively studied. In this talk, I will review a formulation of stochastic reaction networks and present a new class of optimal control problems that are efficiently solvable and widely applicable. The optimal solution can be efficiently obtained using the Kullback–Leibler divergence as a control cost. We apply this framework to the control of interacting random walkers, birth-death processes, and stochastic SIR models. Both numerical and analytical solutions will be presented, highlighting the practical applications and theoretical significance of this approach.

Venue: via Zoom

Event Official Language: English

Thermal radiation exchange in primordial gravitational waves

July 18 (Thu) at 13:30 - 15:00, 2024

Atsuhisa Ota (Postdoctoral Fellow, Institute for Advanced Study, Hong Kong University of Science and Technology, China)

The radiation-dominated universe is a key component of standard Big Bang cosmology. Radiation comprises numerous quantum elementary particles, and its macroscopic behavior is described by taking the quantum thermal average of its constituents. The dynamics of gravitational waves are considered in this smooth fluid. While interactions between individual particles and gravitational waves are often neglected in this context, it raises the question of whether such a hydrodynamical approximation is reasonable. To address this question, we explored the quantum mechanical aspects of gravitational waves in a universe dominated by a massless scalar field, whose averaged energy-momentum tensor serves as background radiation. We computed thermal loop corrections for the gravitational wave power spectrum using the Schwinger-Keldysh formalism. Interestingly, we found that the loop effect enhances the super-horizon primordial gravitational wave spectrum, indicating that the inflationary spectrum is not conserved, contrary to conventional wisdom. These findings have significant implications for our understanding of the early universe. In this talk, I will begin with the basics of cosmology and explain the significance of these results and their relevant observational consequences.

Venue: Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

Quantum Simulation in High Energy Nuclear Physics

July 18 (Thu) at 10:00 - 11:30, 2024

Xingyu Guo (Lecturer, Institute of Quantum Matter, South China Normal University, China)

Quantum simulation is a novel method of simulation physical systems with quantum computers. Compared to conventional methods, quantum algorithms have various advantages in doing non-perturvative calculations and real-time evolutions, which makes it very promising to apply them in high energy nuclear physics. We propose a systematic quantum algorithm, which integrates both the hadronic state preparation and the evaluation of real-time light-front correlators. This algorithm can be applied to the calculation of a wide range of quantities in high energy nuclear physics. As a demonstration, we calculate the parton distribution functions, the light-cone distribution amplitudes and scattering amplitudes in the 1+1 dimensional NJL model. The results are qualitatively consistent with QCD calculations.

Venue: Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

Surface defect in N=4 SYM and integrability

July 17 (Wed) at 16:00 - 17:00, 2024

Hiroki Kawai (Ph.D. Student, University of California, Santa Barbara, USA)

In the N=4 super Yang-Mills theory, it is well-known that the one-loop anomalous dimension operator for the single trace operators is equivalent to an integrable spin chain. Recent works have extended the application of integrability to scenarios involving a BPS boundary or defects such as 't Hooft line. One can describe the correlators of the single trace operators as an overlap between the Bethe state and the corresponding defect state. This overlap can be exactly calculated if the defect state is a so-called integrable state. We show that the state corresponding to the Gukov-Witten surface defect is integrable. We also calculate the tree-level one-point function of the single trace operators and set up the perturbation calculation in this defect background for one-loop corrections.

Venue: Hybrid Format (3F #359 and Zoom), Main Research Building

Event Official Language: English

Supernovae as Tracers of Massive-Star Evolution

July 17 (Wed) at 14:00 - 15:15, 2024

Daichi Hiramatsu (Post-Doctoral fellow, Harvard University, USA)

Supernovae are the terminal explosions of massive stars with influences on every astrophysical scale. Advanced wide-field and high-cadence transient surveys routinely discover supernovae near the moment of explosion. Coupled with prompt and continuous follow-up facilities, these observations have revealed unprecedented features of dense circumstellar medium in various spatial scales as traced by the expanding supernova ejecta. Such circumstellar medium is thought to originate from mass-loss activities in the final years to decades of stellar evolution; however, their inferred densities exceed the expectations from standard theory by many orders of magnitude. In this talk, I will first introduce standard stellar evolution and supernova explosion mechanisms, and then describe novel observational probes and modeling techniques of supernovae interacting with circumstellar medium to reconstruct their explosion properties and progenitor mass-loss histories. Finally, I will discuss our on-going largest sample study of interacting supernovae and emerging pictures of dramatic dying breaths of massive stars.

Venue: Hybrid Format (3F #359 and Zoom), Main Research Building

Event Official Language: English

Quantum Error Transmutation

July 17 (Wed) at 10:30 - 11:30, 2024

Daniel Zhang (Postdoctoral Fellow, University of Oxford, UK)

We introduce a generalisation of quantum error correction, relaxing the requirement that a code should identify and correct a set of physical errors on the Hilbert space of a quantum computer exactly, instead allowing recovery up to a pre-specified admissible set of errors on the code space. We call these quantum error transmuting codes. They are of particular interest for the simulation of noisy quantum systems, and for use in algorithms inherently robust to errors of a particular character. Necessary and sufficient algebraic conditions on the set of physical and admissible errors for error transmutation are derived, generalising the Knill-Laflamme quantum error correction conditions. We demonstrate how some existing codes, including fermionic encodings, have error transmuting properties to interesting classes of admissible errors. Additionally, we report on the existence of some new codes, including low-qubit and translation invariant examples.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Differential Topology Seminar: Rigidity and Flexibility of Isometric Embeddings

July 16 (Tue) at 15:00 - 16:30, 2024

Dominik Inauen (Academic Staff, University of Leipzig, Germany)

The problem of embedding abstract Riemannian manifolds isometrically (i.e. preserving the lengths) into Euclidean space stems from the conceptually fundamental question of whether abstract Riemannian manifolds and submanifolds of Euclidean space are the same. As it turns out, such embeddings have a drastically different behaviour at low regularity (i.e. C¹) than at high regularity (i.e. C²). For example, by the famous Nash--Kuiper theorem it is possible to find C1 isometric embeddings of the standard 2-sphere into arbitrarily small balls in R³, and yet, in the C² category there is (up to translation and rotation) just one isometric embedding, namely the standard inclusion. Analoguous to the Onsager conjecture in fluid dynamics, one might ask if there is a sharp regularity threshold in the Holder scale which distinguishes these flexible and rigid behaviours. In my talk I will review some known results and argue why the Holder exponent 1/2 can be seen as a critical exponent in the problem.

Venue: ＃609, Department of Mathematics, Faculty of Science Bldg. No. 6, , Kyoto University

Event Official Language: English

Mapping the Phase Space of toric Calabi-Yau 3-folds using Explainable Machine Learning

July 16 (Tue) at 13:30 - 14:30, 2024

Rak-Kyeong Seong (Assistant Professor, Department of Mathematical Sciences, Ulsan National Institute of Science and Technology (UNIST), Republic of Korea)

This talk will give a brief introduction on how bipartite graphs on a torus represent 4-dimensional quiver gauge theories and their moduli space which is a toric Calabi-Yau 3-fold - a cone over a Sasaki-Einstein 5-manifold. Under mirror symmetry, the bipartite graph can be identified with the tropical projection of the mirror curve obtained from the Newton polytope associated to the toric Calabi-Yau 3-fold. Changes to the complex structure moduli of the mirror Calabi-Yau determine the overall shape of the bipartite graph on the torus. For certain choices of complex structure moduli, the bipartite graph undergoes a graph mutation which is identified with Seiberg duality of the associated 4-dimensional quiver gauge theory. This talk will discuss recent progress in understanding when such mutations occur from the point of view of Calabi-Yau mirror symmetry with the help of new computational techniques such as machine learning.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Topological recursion and twisted Higgs bundles

July 16 (Tue) at 10:30 - 12:00, 2024

Christopher Mahadeo (Research Assistant Professor, Department of Mathematics, The University of Illinois at Chicago (UIC), USA)

Prior works relating meromorphic Higgs bundles to topological recursion have considered non-singular models that allow the recursion to be carried out on a smooth Riemann surface. I will discuss some recent work where we define a "twisted topological recursion" on the spectral curve of a twisted Higgs bundle, and show that the g=0 components of the recursion compute the Taylor expansion of the period matrix of the spectral curve, mirroring a result of for ordinary Higgs bundles and topological recursion. I will also discuss some current work relating topological recursion to a new viewpoint of quantization of Higgs bundles.

Venue: Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

Discovering Physical Laws with Artificial Intelligence

July 12 (Fri) at 10:00 - 11:30, 2024

Liu Ziming (Ph.D. Student, Department of Physics, Massachusetts Institute of Technology, USA)

Deep neural networks have been extremely successful in language and vision tasks. However, their black-box nature makes them undesirable for scientific tasks. In this talk, I will show how we can make these black-box AI models more interpretable and transparent and use them to discover physical laws, including conservation laws (AI Poincare), symmetries, phase transitions and symbolic relations (Kolmogorov-Arnold Networks). Ziming is a physicist and a machine learning researcher. Ziming received BS in physics from Peking Univeristy in 2020, and is current a fourth-year PhD student at MIT and IAIFI, advised by Max Tegmark. His research interests lie generally in the intersection of artificial intelligence (AI) and physics (science in general).

Venue: via Zoom

Event Official Language: English

Tensionless Strings in a Kalb-Ramond Background

July 10 (Wed) at 16:00 - 17:00, 2024

Ritankar Chatterjee (Ph.D. Student, Indian Institute of Technology Kanpur, India)

We investigate tensionless (or null) bosonic string theory with a constant Kalb-Ramond background turned on. In analogy with the tensile case, we find that the constant Kalb-Ramond field has a non-trivial effect on the spectrum only when the theory is compactified on an S^1 ⊗d background with d ≥ 2. We discuss the effect of this constant background field on the tensionless spectrum constructed on three known consistent null string vacua. We elucidate further on the intriguing fate of duality symmetries in these classes of string theories when the background field is turned on. Based on: https://arxiv.org/abs/2404.01385

Venue: Hybrid Format (3F #359 and Zoom), Main Research Building

Event Official Language: English

The role of demographic stochasticity in the evolution of spite and altruism

July 9 (Tue) at 16:00 - 17:00, 2024

Troy Day (Professor, Head of Department, Department of Mathematics and Statistics, Queen's University, Canada)

The evolution of spiteful and altruistic behaviour remains a fascinating and somewhat puzzling phenomenon. In recent years there has been interest in examining how stochasticity arising from a finite population size might affect the evolution of these traits. Some results suggest that such stochasticity can reverse the direction of selection and promote the evolution of traits like altruism and spitefulness that are selected against in very large (deterministic) populations. However, other results seem to call this finding into question. In this talk I will consider a simple but quite general model of spite and of altruistic behaviour and examine how demographic stochasticity affects the evolution of these traits. I will show that stochasticity can indeed affect the direction of evolution but not in the way that previous studies have suggested. The results also help to clarify the broader issue of how and why stochasticity can sometimes reverse the direction of evolution.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

The 26th MACS Colloquium

July 8 (Mon) at 14:45 - 18:00, 2024

Satoshi Taguchi (Professor, Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University)
Michitaka Notaguchi (Professor, Division of Biological Sciences, Graduate School of Science, Kyoto University)

14:45-15:00 Teatime discussion 15:00-16:00 Talk by Prof. Satoshi Taguchi (Professor, Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University) 16:15-17:15 Talk by Prof. Yoshihiro Morishita (Professor, Division of Biological Sciences, Graduate School of Science, Kyoto University) 17:15-18:00 Discussion

Venue: Science Seminar House (Map 9)

Event Official Language: Japanese

Dynamics of the very early universe: towards decoding its signature through primordial black hole abundance, dark matter, and gravitational waves.

July 5 (Fri) at 14:00 - 15:15, 2024

Riajul Haque (Postdoctoral Researcher, Department of Physics, Indian Institute of Technology, India)

I will start my talk with a brief overview of the standard reheating scenario. Then, I will discuss reheating through the evaporation of primordial black holes (PBHs) if one assumes PBHs are formed during the phase of reheating. Depending on their initial mass, abundance, and inflaton coupling with the radiation, I discuss two physically distinct possibilities of reheating the universe. In one possibility, the thermal bath is solely obtained from the decay of PBHs, while inflaton plays the role of the dominant energy component in the entire process. In the other possibility, PBHs dominate the total energy budget of the universe during evolution, and then their subsequent evaporation leads to a radiation-dominated universe. Furthermore, I will discuss the impact of both monochromatic and extended PBH mass functions and estimate the detailed parameter ranges for which those distinct reheating histories are realized. The evaporation of PBHs is also responsible for the production of DM. I will show its parameters in the background of reheating obtained from two chief systems in the early universe: the inflaton and the primordial black holes (PBHs). Then, I will move my discussion towards stable PBHs and discuss the effects of the parameters describing the epoch of reheating on the abundance of PBHs and the fraction of cold dark matter that can be composed of PBHs. If PBHs are produced due to the enhancement of the primordial scalar power spectrum on small scales, such primordial spectra also inevitably lead to strong amplification of the scalar-induced secondary gravitational waves (GWs) at higher frequencies. I will show how the recent detection of the stochastic gravitational wave background (SGWB) by the pulsar timing arrays (PTAs) has opened up the possibility of directly probing the very early universe through the scalar-induced secondary gravitational waves. Finally, I will conclude my talk by elaborating on the effect of quantum correction on the Hawking radiation for ultra-light PBHs and its observational signature through dark matter and gravitational waves.

Venue: via Zoom

Event Official Language: English

Recent progress of microscopic equation of state for hyperon-mixed nuclear matter

July 4 (Thu) at 14:00 - 15:00, 2024

Togashi Hajime (Specially Appointed Assistant Professor, Research Center for Nuclear Physics, Osaka University)

The presence of hyperons in the neutron star interior have been investigated by many researchers using both phenomenological and microscopic approaches for the equation of state (EOS) of neutron star matter with hyperons. However, hyperon fractions in nuclear matter are still far from being understood, since there are relatively large uncertainties in hyperon interactions due to the small amount of the experimental data. Furthermore, recently observed masses of massive pulsars impose severe constraints on the hyperon EOS. In this seminar, I will review the recent results of microscopic nuclear EOS including hyperons and its applications to astrophysical compact objects to discuss the possible signatures of the presence of hyperons in compact star interiors. In particular, I will discuss the effect of three-body forces including hyperons on the structure and particle composition of (proto) neutron stars.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English