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Nuclear Energy-Density Functional Approach to Bridging Neutron-Rich Nuclei and Neutron Stars

February 5 (Mon) 13:30 - 15:00, 2024

Kenichi Yoshida (Associate Professor, Research Center for Nuclear Physics, Osaka University)

Understanding the properties of neutron-rich nuclei has been a central subject in low-energy nuclear physics. The great interest lies not only in the pursuit of a variety of structures and the elucidation of the mechanisms of their occurrence but also in obtaining insights into the structure of the inner crust of neutron stars. With advances in neutron-star observation techniques, the structure of neutron stars has been becoming better understood. The data accumulated from these observations unveil properties of neutron-rich matter that are otherwise inaccessible through terrestrial experiments. In this talk, I will introduce an attempt to construct a nuclear energy-density functional (EDF) inspired by the observations and then demonstrate its applicability to nuclear structure problems, including mass and deformation. One intriguing aspect of neutron stars is the emergence of superfluidity, especially the occurrence of spin-triplet pairing. I will discuss the unconventional pairing in nuclei within the nuclear EDF framework and give perspectives on the study of the phase diagram of the superfluidity in neutron stars. This seminar is co-hosted by UKAKUREN.

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

Event Official Language: English

Can social issues be solved by mathematical science!? - Mathematical Research in Corporations

February 3 (Sat) 13:00 - 17:00, 2024

Continuing from the previous fiscal year, we will be hosting a symposium exploring the potential of solving societal issues through mathematical science research. In the past two years, the themes were "Attempts and Challenges" and "Connecting Corporate Issues and Mathematical Sciences." This year, we will focus on concrete examples under the title "Mathematical Research in Corporations" with the aim of introducing activities utilizing mathematics in the corporate sector. Currently, we are inviting speakers with diverse backgrounds, including researchers actively engaged in mathematical activities within corporations or those who have transitioned from corporate mathematical research to academic settings. We are particularly interested in learning about the experiences of individuals who have used mathematics in corporate settings. Additionally, there has been a growing trend in connecting activities of graduate students in mathematics with corporate endeavors. By sharing such initiatives with participants, we plan to conduct a panel session for exchanging opinions on the role of mathematical science in addressing societal issues through corporate collaboration in the future. We sincerely look forward to the active participation of corporate researchers and mathematicians who have an interest in these activities.

Venue: Hybrid Format (Noyori Conference Hall, Nagoya University and Zoom)

Event Official Language: Japanese

Competition across scales in biology

January 31 (Wed) 11:00 - 12:00, 2024

Sidhartha Goyal (Associate Professor, Department of Physics, University of Toronto, Canada)

Many biological phenomena emerge from interaction and competition between its parts. I will share some examples across biological scales where data-driven theory can reveal new rules of biological competition. At the molecular scale competition between mitochondrial genomes within budding yeast depends on genome architecture; dynamics of adaptive immunity in microbes reveal different modalities of competition and coexistence of bacteria and its phages; in mammals cellular reprogramming may be driven by elite clones, and tumor response to drugs is driven by "epigenetic" switching. Going beyond, I will present some ideas on understanding dynamical systems that govern cell fate dynamics and if competition may play a role in it. Short bio: Sidhartha Goyal got his PhD in Physics at Princeton in 2009 and then moved to Kavli Institute for Theoretical Physics, Santa Barbara for a postdoc. He got his first degree in Electrical Engineering from IIT Bombay. He is now an Associate Professor in the Physics Department at University of Toronto interested in collective phenomena in biology across scales.

Venue: via Zoom

Event Official Language: English

Knot Theory in Doubly Periodic Tangles and Applications

January 19 (Fri) 15:00 - 16:30, 2024

Sonia Mahmoudi (Assistant Professor, Mathematical Science Group, Advanced Institute for Materials Research (AIMR), Tohoku University)

Doubly periodic entangled structures offer an interesting framework for modeling and investigating diverse materials and physical phenomena, from micro to large scales. Specifically, a doubly periodic tangle (DP tangle) is characterized as an embedding of an infinite number of curves in the thickened plane, derived as the lift of a link in the thickened torus to the universal cover. DP tangles play a crucial role in scientific research, particularly in fields such as materials science, molecular chemistry, and biology. Despite their widespread applications, a universally accepted mathematical description of DP tangles is currently lacking. One of the key challenges arises from the infinite possibilities in choosing a periodic cell (referred to as a motif) for a DP tangle, taking into account various periodic boundary conditions. In this presentation, we conduct a comprehensive examination of the concept of topological equivalence of DP tangles, offering insights into potential classifications and applications in the process.

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

Event Official Language: English

Gravitational lensing on superposed curved spacetime

January 18 (Thu) 13:30 - 14:15, 2024

Youka Kaku (Ph.D. Student, Graduate School of Science, Nagoya University)

In 2017, Bose et al. proposed a tabletop experiment to observe the gravitational effect induced by a spatially superposed mass source, particularly gravity-induced entanglement. This experiment is expected to be the first step in exploring the quantum nature of gravity. Also, there are ongoing efforts to extend their proposal to the relativistic region to observe the unique quantum nature of gravity. In this talk, I will investigate gravitational lensing in a weak gravitational field induced by a spatially superposed mass source. I will show the Einstein ring image of a quantum scalar field propagated on a superposed curved spacetime and compare it with the image of the semi-classical gravity case. This work is currently in progress and is a collaboration with Yasusada Nambu.

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

Event Official Language: English

Bayesian mechanics of classical, neural, and quantum systems

January 17 (Wed) 16:30 - 17:45, 2024

Takuya Isomura (Unit Leader, Brain Intelligence Theory Unit, RIKEN Center for Brain Science (CBS))

(This is a joint seminar with iTHEMS Biology group.) Bayesian mechanics is a framework that addresses dynamical systems that can be conceptualised as Bayesian inference. However, the elucidation of requisite generative models is required for empirical applications to realistic self-organising systems. This talk introduces that the Hamiltonian of generic dynamical systems constitutes a class of generative models, thus rendering their Helmholtz energy naturally equivalent to variational free energy under the identified generative model. The self-organisation that minimises the Helmholtz energy entails matching the system's Hamiltonian with that of the environment, leading to an ensuing emergence of their generalised synchrony. In short, these self-organising systems can be read as performing variational Bayesian inference of the interacting environment. These properties have been demonstrated with coupled oscillators, simulated and living neural networks, and quantum computers. This notion offers foundational characterisations and predictions regarding asymptotic properties of self-organising systems exchanging with the environment, providing insights into potential mechanisms underlying emergence of intelligence.

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

Event Official Language: English

Methods for neural decoding using machine learning, deep learning, and quantum-inspired algorithms

January 17 (Wed) 15:00 - 16:15, 2024

Kei Majima (Researcher, National Institutes for Quantum Science and Technology (QST))

Note: The format of this event has changed from hybrid to Zoom only. However, you will still be able to watch it on the screen in Room #359 of the Main Research Building. (This is a joint seminar with iTHEMS Biology group.) Recent advances in machine learning have enabled the extraction of intrinsic information from neural activities, a field known as neural decoding. In this presentation, I will introduce several machine learning methods recently developed for neural decoding analysis: 1) a method for visualizing subjective images in the human mind based on brain activity [1], 2) a supervised algorithm designed for predicting discrete ordinal variables [2], and 3) a fast classical algorithm algorithm inspired by quantum computation for approximating principal component analysis (PCA) and canonical correlation analysis (CCA), potentially allowing for the analysis of vast-dimensional neural data [3]. Following these presentations, I am eager to engage in discussions with participants at the RIKEN Quantum Seminar regarding potential collaborations.

Venue: via Zoom

Event Official Language: English

Dust-driven instabilities in protoplanetary disks: toward understanding formation of planetesimals

January 17 (Wed) 10:30 - 11:30, 2024

Ryosuke Tominaga (Special Postdoctoral Researcher, Star and Planet Formation Laboratory, RIKEN Cluster for Pioneering Research (CPR))

Planet formation starts from collisional growth of sub-micron-sized dust grains in a gas disk called a protoplanetary disk. They are expected to grow toward km-sized objects called planetesimals. The resulting planetesimals further coalesce by gravity and form planets. However, there are some barriers preventing planetesimal formation, which includes fast radial drift and collisional fragmentation of dust grains. To circumvent the barriers and to explain planetesimal formation, previous studies have proposed hydrodynamic instabilities of dusty-gas disks. The instabilities can cause dust clumping, and planetesimals form if the resulting clumps collapse self-gravitationally. We have been investigating the linear/nonlinear development of these dust-gas instabilities. We also found a new instability driven by collisional growth of dust, which can bridge a potential gap between the first dust growth and the later planetesimal formation via the previous instabilities. In this talk, I will introduce our work on the dust-driven instabilities and their impact on planetesimal formation.

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

Event Official Language: English

Probing structure of neutron stars through X-ray bursters

January 12 (Fri) 14:00 - 15:15, 2024

Akira Dohi (Special Postdoctoral Researcher, Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))

Type-I X-ray bursts are rapidly brightening phenomena triggered by the nuclear burning of light elements near the surface of accreting neutron stars. Most of the X-ray bursters show irregular behavior of light curves. However, some X-ray bursters are somehow quite regular, i.e., constant recurrence time and constant shaper of light curves, and are often called Clocked bursters, which are powerful sites to probe uncertainties of many model parameters such as accretion rate, the composition of accreted matter, reaction rates, neutron star structure, and temperature. In this study, we focus on the uncertainties of the equation of states, which determines the latter two properties. Based on our numerical models covering whole areas of neutron stars, we will present their impact on X-ray burst light curves. Furthermore, we will discuss the possibility of constraining the equation of states from Clocked bursters such as GS 1826-24 and 1RXS J180408.9-342058.

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

Event Official Language: English

Seoul National University student group visit

January 10 (Wed) 14:00 - 20:00, 2024

Catherine Beauchemin (Deputy Program Director, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
Akinori Tanaka (Senior Research Scientist, RIKEN Center for Advanced Intelligence Project (AIP))
Misako Tatsuuma (Research Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
Ryo Namba (Senior Research Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
Dongwook Ghim (Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
Steffen Backes (Senior Research Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

A group of 22 undergraduate students in the GLEAP programme at Seoul National University will visit iTHEMS to hear short talks by our members, exchange one-on-one, and visit our facilities. I would like to encourage all available iTHEMS members to take part in this event which will be held in different spaces throughout the day, all at the RIKEN Wako campus Main Research Building: 15:50-16:10 at iTHEMS Common Room (#246-248) Coffee break with iTHEMS members and SNU visitors 16:10-17:50 on 4th floor, room #435-437 Short talks by iTHEMS members 18:05-18:30 in 3rd floor common space Short intro talks by SNU visitors 18:30-20:00 in 3rd floor common space Free informal discussion between SNU visitors and iTHEMS members over some light food [Note some slight changes in the times previously announced]

Venue: 3rd floor public space, Main Research Building / #435-437, Main Research Building / Common Room #246-248

Event Official Language: English

Oscillatory data analysis using the extended Hilbert transform method

December 26 (Tue) 16:00 - 17:00, 2023

Akari Matsuki (Postdoctoral Researcher, Department of Advanced Transdisciplinary Sciences, Hokkaido University)

Oscillatory phenomena are observed in various biological systems, such as spinal nervous systems and circadian rhythms. These macroscopic oscillatory phenomena appear as a result of synchronization of microscopic oscillators, such as pacemaker cells. The first step in the analysis of synchronization is to reconstruct the "phase" from the observed signal. The Hilbert transform method is one of the popular methods for phase reconstruction, but it is known that it can only accurately reconstruct the phase from a limited class of signals such as narrowband signals. In this study, we show that the Hilbert transform method has a low-pass filter-like effect on the phase modulation and propose an "extended Hilbert transform method" that can be applied to a wider class of signals. In this talk, I will introduce the extended Hilbert transform method, and its application to phase shift detection and coupling network inference.

Venue: via Zoom

Event Official Language: English

Inflationary Cosmology with a scalar-curvature mixing term $\frac{1}{2} \xi R \phi^2$

December 20 (Wed) 16:00 - 17:30, 2023

Payel Sarkar (Visiting Researcher, Kyoto University)

We use the PLANCK 2018 and the WMAP data to constraint inflation models driven by a scalar field $\phi$ in the presence of the non-minimal scalar-curvature mixing term $\frac{1}{2}\xi R \phi^2$. We consider four distinct scalar field potentials $\phi^p e^{-\lambda\phi},~(1 - \phi^{p})e^{-\lambda\phi},~(1-\lambda\phi)^p$ and $\frac{\alpha\phi^2}{1+\alpha\phi^2}$ to study inflation in the non-minimal gravity theory. We calculate the potential slow-roll parameters, predict the scalar spectral index $n_s$, tensor-to-scalar ratio $r$, leading and higher order non-Gaussianity parameters ($f_{NL},~\tau_{NL}$ and $g_{NL}$) and the amplitude of the scalar spectrum $A_s$ in the parameter ($\lambda, p, \alpha$) space of the potentials corresponding to different values of the non-minimal coupling parameter $\xi$. We have compared our results with the ones existing in the literature, and this indicates the present status of non-minimal inflation after the release of the PLANCK 2018 data.

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

Event Official Language: English

Application of mathematical models to the COVID-19 cohort study

December 19 (Tue) 16:00 - 17:00, 2023

Takara Nishiyama (Ph.D. Student, Graduate School of Science, Nagoya University)

The COVID-19 pandemic, which began in 2019, has caused widespread morbidity and mortality across the globe. In response, a multitude of studies focusing on SARS-CoV-2 have been undertaken. Among these, cohort studies have been particularly significant. These studies, as a key observational research method, play a crucial role in exploring the links between various factors and the onset of diseases, offering valuable insights for disease control. Mathematical model, applied within these studies, provide essential quantitative data. In my talk, I will introduce how mathematical models are instrumental in cohort studies, drawing on two of my own COVID-19 cohort studies as examples.

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

Event Official Language: English

Plasticity in the endogenous rhythms and the adaptation to the tidal environment in a freshwater snail

December 14 (Thu) 16:00 - 17:00, 2023

Takumi Yokomizo (JSPS Research Fellow PD, Graduate School of Science, Chiba University)

Organisms have diverse biological clocks synchronized with environmental cycles　depending on their habitats. The change in endogenous rhythms could contribute to range expansion in a novel rhythmic environment. For example, the Anticipation of tidal changes has driven the evolution of circatidal rhythms in some marine species. I am interested in the genetic and non-genetic changes in the biological rhythms and adaptation to tidal environments in the freshwater snail, Semisulcospira reiniana. Chronobiological analyses of behavior and gene expression revealed that snails had habitat-specific endogenous rhythms: individuals in a nontidal population showed the circadian rhythm while those in a tidal population showed the circadian and circatidal rhythms. The entrainment to the simulated tidal cycles increased the strength of circatidal activity only in snails in a tidal population. Although the circatidal rhythms in the transcriptome were clearer in individuals entrained to tidal cycles, the number of circatidal rhythmic transcripts was greater in a tidal population than in a nontidal population. These results suggest biological rhythms in the snails plastically change at the molecular level, but the strength of circatidal rhythm is different between populations. Finally, transcriptome-wide population genetic analysis revealed that these two populations can be clearly distinguished genetically, though the genetic distance was very small. Thus, genetic differentiation in biological rhythms could result from the evolution of a small number of genes. These findings suggest that adaptive plasticity and genetic changes in the biological rhythms play an important role in coping with tidal environments.

Venue: via Zoom

Event Official Language: English

Tropical geometry and period integrals

December 13 (Wed) 14:00 - 16:30, 2023

Yuto Yamamoto (Special Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

Tropical geometry is a field of mathematics that naturally emerges when considering the limits of spaces with respect to some parameters. One of the motivations to study tropical geometry is to describe the behaviors of the spaces under the limit. In this math seminar, starting with a brief introduction to tropical geometry, we discuss its application to computation of period integrals, which are one of the most fundamental quantities of complex manifolds. The goal is to compute asymtptotics of period integrals for complex hypersurfaces in toric varieties using tropical geometry, and observe that the Riemann zeta values (or the gamma classes) appear in the result of the computation. The first half of the talk will be a brief introduction to tropical geometry for non-experts including those who are working outside mathematics, and everyone will be welcome.

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

Event Official Language: English

Exploring material strengths of dust aggregates in planet formation by numerical simulations

December 8 (Fri) 14:00 - 15:15, 2023

Misako Tatsuuma (Research Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

The planet formation process is the growth from sub-micrometer-sized cosmic dust grains to thousand-kilometer-sized planets. This growth process has broadly two phases: the growth from dust grains to kilometer-sized planetesimals, mainly driven by intermolecular forces like van der Waals forces and hydrogen bonds, and the subsequent growth from planetesimals to planets, governed by gravitational forces. However, the planetesimal formation process encounters various challenges, including fragmentation and bouncing resulting from collisions among dust aggregates. To gain insights into the planetesimal formation process and how to avoid these obstacles, I have been focused on measuring and formulating the material strengths of dust aggregates using grain simulations. In this talk, I will introduce my works on the material strengths of dust aggregates and their applications to kilometer-sized bodies in the solar system, such as comets and asteroids.

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

Event Official Language: English

Gravitational Lensing in Black Hole Spacetimes of the Plebanski-Demianski Class

December 6 (Wed) 16:00 - 17:30, 2023

Torben Christian Frost (Postdoctoral Researcher, Kavli Institute for Astronomy and Astrophysics, Peking University, China)

Einstein's field equations allow various different black hole solutions. Among these solutions, the most famous are most likely the Schwarzschild and the Kerr spacetimes, which are both special cases of the so-called Plebanski-Demianski spacetime. Besides the Schwarzschild and Kerr spacetimes, the Plebanski-Demianski spacetime also includes other solutions as special cases, among them the C-metric and the NUT metric. They describe a linearly accelerating black hole and a black hole with gravitomagnetic charge, respectively. The question is now how we can determine if an astrophysical black hole can be described by one of these spacetimes. We will address this question using gravitational lensing for the three spacetimes with the most salient lensing features, namely the C-metric, the NUT metric, and the Kerr metric. For this purpose, we will first outline how to solve the equations of motion analytically using elementary and Jacobi's elliptic functions as well as Legendre's elliptic integrals. Then we will fix an observer in the domain of outer communication and relate the constants of motion of the lightlike geodesics to latitude-longitude coordinates on the observer's celestial sphere. We will use the analytic solutions to write down the lens equations, calculate the redshift, and the travel time. Finally, we will discuss and compare the results and comment on how we can use them to place constraints on the spin parameter, the acceleration parameter, and the gravitomagnetic charge of a black hole.

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

Event Official Language: English

Translating between evolutionary game theory and theoretical ecology

December 5 (Tue) 16:30 - 17:30, 2023

Arne Traulsen (Director, Department for Theoretical Biology, Max Planck Institute for Evolutionary Biology, Germany)

Both theoretical ecology and evolutionary game theory describe the dynamics of interacting populations. More than 40 years ago, Hofbauer and Sigmund established a mathematical equivalence between the Lotka-Volterra equations and the replicator dynamics from evolutionary game theory. However, this equivalence has not been exploited by empiricists so far. One of the issues is dimensionality: An ecological interaction of two species corresponds to an evolutionary game between three types. Only when we focus on a special case with identical growth rates, it is possible to translate without this trick, leading to a more direct equivalence between the frameworks. Consequently, one has to be particularly careful how to classify interactions and how to assess dynamical outcomes. For example, a ‘Prisoner's Dilemma’ interaction where the `cooperators' have a higher intrinsic growth rate than `defectors' can result in stable coexistence of the two types and may ultimately not represent a social dilemma at all.

Venue: via Zoom

Event Official Language: English

Rotating discs on the Kerr black hole background

December 5 (Tue) 15:00 - 16:30, 2023

David Kofroň (Postdoctoral Researcher, Institute of Theoretical Physics, Charles University, Czechia)

Analytical solution of a rotating black hole surrounded by accretion disc in full GR is, so far, unknown. The Ernst equation is nonlinear. In this talk, we will provide a framework in which the solutions of linearised Ernst equations can be obtained from the linear perturbations of Kerr black hole treated in the formalism of the Debye potentials. In this way, we recover all the metric perturbations in term of a single complex scalar function (which solves the Laplace equation).

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

Event Official Language: English

Gravity of Accretion Discs and Black Holes

December 5 (Tue) 14:00 - 15:00, 2023

Petr Kotlařík (Ph.D. Student, Institute of Theoretical Physics, Charles University, Czechia)

The typical black hole solutions describe only isolated black holes. However, in astrophysics, such a condition is never strictly satisfied. As matter accretes onto the black hole, disc structures are often formed. In this talk, I will summarize our recent attempts to find the gravitational field of such a nonisolated black hole. We start from the simplest case of static and axially symmetric metric. Although it is a somewhat "rough" approximation in the astrophysical context, this idealization may already help us to understand some interesting implications of the disc's gravity. Moreover, with such a simplification, we can obtain exact analytical "superpositions" of the Schwarzchild black hole and a disc. When some rotation is present, dragging effects complicate the situation dramatically. Then, one typically has to resort to numerical relativity or some approximate methods, e.g., perturbations. In the talk, I also address the stationary case and demonstrate what we can do on the level of the direct metric perturbation.

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

Event Official Language: English