Seminar
641 events

Seminar
Social behavior and social engineering in bacteria
August 1 (Thu) at 16:00  17:00, 2024
Ashleigh Griffin (Professor, Department of Biology, University of Oxford, UK)
This year is the 60th anniversary of WD Hamilton’s seminal paper in which he outlined his theory of inclusive fitness and showed how it could be used to understand altruism in the social insects. In this talk, I will describe efforts made to use his theory to understand social behavior in bacteria. And I’ll go on to explore the potential of using these insights to tackle problems of antibiotic resistance in infections.
Venue: Seminar Room #359 / via Zoom
Event Official Language: English

Seminar
Stringy Nonlocality: Operator Formalism and Implications
July 26 (Fri) at 14:00  15:30, 2024
WeiHsiang Shao (Ph.D. Student, Department of Physics, National Taiwan University, Taiwan)
Nonlocality is a fundamental property of string theory, where pointlike 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 The usual approach to canonical quantization is no longer applicable, and thus a nonperturbative treatment of nonlocal effects at the quantum level remains unclear.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

Seminar
MultiAgent 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 decisionmaking 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 multiagent reinforcement learning, a machine learning technique, to investigate the decisionmaking 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

Seminar
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 (LCNNs). 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 MaxPlanckInstitute in Heidelberg before returning to TU Wien in 2009. He completed his habilitation on "Yoctosecond dynamics of the quarkgluon 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

Seminar
Probing Majorana excitations in the Kitaev magnet αRuCl3 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 αRuCl3 exhibits several anomalous features that are consistent with expectations of the Kitaev quantum spin liquid (KQSL) under inplane magnetic field. Most remarkably, finite planar thermal Hall conductivity has been observed, whose magnitude is close to the halfinteger 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 highfield state of αRuCl3 is the difference in the field angle dependence of the excitation gap. Therefore, we distinguish these origins from combined lowtemperature measurements of highresolution specific heat and thermal Hall conductivity with rotating magnetic fields within the honeycomb plane. A distinct closure of the lowenergy bulk gap is observed for the fields in the RuRu 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 Majoranafermion origin of the thermal Hall effect in αRuCl3[2]. Furthermore, to understand the nature of the highfield 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 lowenergy excitations in the pristine and irradiated sample by highresolution specific heat measurements. We observed an additional ingap Tlinear term in C/T, whose coefficient shows distinct fieldsensitive 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 highquality crystals of αRuCl3[4].
Venue: via Zoom / Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English

Seminar
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, birthdeath 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

Seminar
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 radiationdominated 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 energymomentum tensor serves as background radiation. We computed thermal loop corrections for the gravitational wave power spectrum using the SchwingerKeldysh formalism. Interestingly, we found that the loop effect enhances the superhorizon 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

Seminar
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 nonperturvative calculations and realtime 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 realtime lightfront 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 lightcone 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

Seminar
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 YangMills theory, it is wellknown that the oneloop 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 socalled integrable state. We show that the state corresponding to the GukovWitten surface defect is integrable. We also calculate the treelevel onepoint function of the single trace operators and set up the perturbation calculation in this defect background for oneloop corrections.
Venue: Hybrid Format (3F #359 and Zoom), Main Research Building
Event Official Language: English

Seminar
Supernovae as Tracers of MassiveStar Evolution
July 17 (Wed) at 14:00  15:15, 2024
Daichi Hiramatsu (PostDoctoral fellow, Harvard University, USA)
Supernovae are the terminal explosions of massive stars with influences on every astrophysical scale. Advanced widefield and highcadence transient surveys routinely discover supernovae near the moment of explosion. Coupled with prompt and continuous followup 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 massloss 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 massloss histories. Finally, I will discuss our ongoing 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

Seminar
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 prespecified 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 KnillLaflamme 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 lowqubit and translation invariant examples.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English

Seminar Tomorrow
Mapping the Phase Space of toric CalabiYau 3folds using Explainable Machine Learning
July 16 (Tue) at 13:30  14:30, 2024
RakKyeong 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 4dimensional quiver gauge theories and their moduli space which is a toric CalabiYau 3fold  a cone over a SasakiEinstein 5manifold. 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 CalabiYau 3fold. Changes to the complex structure moduli of the mirror CalabiYau 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 4dimensional quiver gauge theory. This talk will discuss recent progress in understanding when such mutations occur from the point of view of CalabiYau 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

Seminar Tomorrow
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 nonsingular 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 blackbox nature makes them undesirable for scientific tasks. In this talk, I will show how we can make these blackbox AI models more interpretable and transparent and use them to discover physical laws, including conservation laws (AI Poincare), symmetries, phase transitions and symbolic relations (KolmogorovArnold Networks). Ziming is a physicist and a machine learning researcher. Ziming received BS in physics from Peking Univeristy in 2020, and is current a fourthyear 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

Seminar
Tensionless Strings in a KalbRamond 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 KalbRamond background turned on. In analogy with the tensile case, we find that the constant KalbRamond field has a nontrivial 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

Seminar
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

Seminar
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 radiationdominated 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 scalarinduced 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 scalarinduced secondary gravitational waves. Finally, I will conclude my talk by elaborating on the effect of quantum correction on the Hawking radiation for ultralight PBHs and its observational signature through dark matter and gravitational waves.
Venue: via Zoom
Event Official Language: English

Seminar
Recent progress of microscopic equation of state for hyperonmixed 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 threebody 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

Worldline Path Integrals for the Graviton and 1Loop Divergences in Quantum Gravity
June 28 (Fri) at 16:00  17:20, 2024
Fiorenzo Bastianelli (Professor, University of Bologna, Italy)
In this talk, I will discuss perturbative quantum gravity at the 1loop level by reviewing and systematizing old results on UV divergences and presenting new findings along with new methods for their calculation. The traditional approach to this problem employs the SchwingerDeWitt heat kernel method. We extend this approach by incorporating worldline path integrals to compute the perturbative expansion at small proper time. In addition, we explore a more principled approach that utilizes the BRST path integral quantization of the N=4 spinning particle, which describes the graviton in first quantization. Using these methods, we calculate the oneloop divergences in quantum gravity with a cosmological constant in arbitrary dimensions. When evaluated onshell, these calculations yield a set of gaugeinvariant coefficients that characterize pure quantum gravity with a cosmological constant. These coefficients may serve as benchmarks for comparing various approaches to quantum gravity.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English

Seminar
Challenging conventional wisdom in binary evolution
June 28 (Fri) at 14:00  15:15, 2024
Ryosuke Hirai (Special Postdoctoral Researcher, Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))
The majority of massive stars, stars with more than 8 times the mass of the Sun, are known to be born in binary or higherorder multiple systems. During the course of their evolution, the stars can interact in many different ways and cause interesting astrophysical phenomena such as eruptions and explosions or create objects like Xray binaries, gravitational wave sources, etc. Many studies have been conducted over the last few decades to tie our latest models to these observables in order to refine our understanding of massive binary evolution. However, in some cases "refining" a model is not enough and a paradigm shift is required to explain all the observables in a coherent way. In this talk, I will introduce some topics from my past work where I challenge conventional wisdom to resolve longstanding problems. The topics are as follows: 1. impact of supernova ejecta on companion star evolution, 2. wind accretion onto black holes, 3. commonenvelope evolution, 4. neutron star kicks. I will also discuss how these new views impact the overall landscape of binary evolution theory.
Venue: via Zoom
Event Official Language: English
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