Seminar
764 events
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Seminar
Gamma-ray emission from the Sagittarius Dwarf Spheroidal galaxy due to millisecond pulsars
October 28 (Fri) at 17:00 - 18:00, 2022
Oscar Macias (Faculty of Science, University of Amsterdam, Netherlands)
The Fermi Bubbles are giant, gamma-ray emitting lobes emanating from the nucleus of the Milky Way discovered in ~1-100 GeV data collected by the Fermi Gamma-Ray Space Telescope. Previous work has revealed substructure within the Fermi Bubbles that has been interpreted as a signature of collimated outflows from the Galaxy's super-massive black hole. In this talk, I will show that much of the gamma-ray emission associated to the brightest region of substructure -- the so-called cocoon -- is likely due to the Sagittarius dwarf spheroidal (Sgr dSph) galaxy. This large Milky Way satellite is viewed through the Fermi Bubbles from the position of the Solar System. As a tidally and ram-pressure stripped remnant, the Sgr dSph has no on-going star formation, but I will demonstrate that the dwarf's millisecond pulsar (MSP) population can plausibly supply the observed gamma-ray signal. This finding plausibly suggests that MSPs produce significant gamma-ray emission amongst old stellar populations, potentially confounding indirect dark matter searches in regions such as the Galactic Centre, the Andromeda galaxy, and other massive Milky Way dwarf spheroidals.
Venue: via Zoom
Event Official Language: English
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Seminar
Measuring diversity: species similarity
October 28 (Fri) at 16:00 - 17:00, 2022
Tom Leinster (Professor, University of Edinburgh, UK)
Traditional measures of the diversity of an ecological community depend only on how abundant the species are, not the similarities or differences between them. To better reflect biological reality, species similarity should be incorporated. Mathematically, this corresponds to moving from probability distributions on sets to probability distributions on metric spaces. I will explain how to do this and how it can change ecological judgements. Finally, I will describe a surprising theorem on maximum diversity (joint with Meckes and Roff), which reveals close connections between maximum diversity and invariants of geometric measure.
Venue: via Zoom
Event Official Language: English
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Seminar
Migration dynamics and model of cells crawling on a matrix with cell-scale stiffness heterogeneity
October 27 (Thu) at 16:00 - 17:00, 2022
Hiroyuki Ebata (Assistant Professor, Graduate School of Science, Kyushu University)
In living tissues where cells migrate, spatial distribution of mechanical properties, especially matrix stiffness, are generally heterogenous with cell-scales ranging from 10 to 1000 μm. Since the cell migration in our body plays critical role in morphogenesis, wound healing, and cancer metastasis, it is essential to understand the migratory dynamics on the matrix with cell-scale stiffness heterogeneity. However, while cellular responses to homogeneous matrix have been extensively explored, studies of the cell motility with stiffness heterogeneity have been limited to the directional movement (durotaxis) driven by a simple stiffness gradient. Thus, we need to elucidate how cell migration is determined through the interaction among cell-scale stiffness heterogeneity and cellular responses such as dynamics of the cell-matrix adhesion site, the intracellular prestress, and cell shape. In this talk, we introduce our experiments on cell motility, shaping, adhesion, and traction forces at long time scales using microelastically patterned hydrogels that enable us to systematically control the cell-scale heterogeneity of the matrix-stiffness. Using microelastically patterned hydrogels, we showed that the cell exhibited a general mode of durotaxis depending on the shape and size of the stiff domains, which was coincided with the extraordinarily large fluctuation of the traction force. We proposed a cell migration model based on equations of a deformable self-propelled particle adopting an amoeboid swimmer-like velocity-shape relationship. By considering the cellular response to stiffness gradients, the model can reproduce general durotaxis driven by cell-scale heterogeneity of the matrix-stiffness.
Venue: via Zoom
Event Official Language: English
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Seminar
Measuring diversity: the axiomatic approach
October 21 (Fri) at 16:00 - 17:00, 2022
Tom Leinster (Professor, University of Edinburgh, UK)
Ecologists have been debating the best way to measure diversity for more than 50 years. The concept of diversity is relevant not only in ecology, but also in other fields such as genetics and economics, as well as being closely related to entropy. The question of how best to quantify diversity has surprising mathematical depth. I will argue that the best approach is axiomatic: to enable us to reason logically about diversity, the measures we use must satisfy certain mathematical conditions, and those conditions dramatically limit the choice of measures. This point will be illustrated with a theorem: using a simple model of ecosystems, the only diversity measures that behave logically are the Hill numbers, which are very closely related to the Rényi entropies of information theory.
Venue: via Zoom
Event Official Language: English
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Seminar
How can interspecific pollen transfer affect coexistence and evolution of sex ratio of two closely related plant species?
October 20 (Thu) at 16:00 - 17:00, 2022
Keiichi Morita (Ph.D. Student, School of Advanced Sciences Department of Evolutionary Studies of Biosystems, The Graduate University for Advanced Studies (SOKENDAI))
In co-flowering species with shared pollinators, interspecific pollen transfer (IPT) may occur, in which pollen grains are transferred between different species. Yet, it is unclear how the costs of IPT in reduced pollen grains (i.e., costs for males) and seed set (i.e., costs for females) can affect coexistence and evolution of sex ratio. We investigated the conditions for which two closely related plants can coexist and of evolution in sex ratio, by constructing a theoretical model that incorporates the dynamics of population, pollen export, pollen reception on an ovule, and seed production in two closely related plants with resource competition and IPT. The model analysis revealed that coexistence is likely with infrequent IPT, α, and weak interspecific resource competition, β, and high production rates of pollen, Am, and ovules, Af. Also, we found a trade-off where too low value of either Am or Af makes both species go extinct. Furthermore, even when α and β were small enough, too low Am and Af made extinction of both species likely, because of the Allee effect due to resource competition and interspecific pollen competition for a small number of ovules. Adaptive dynamics, analysis of evolutionary dynamics showed that sex ratio evolve to 1:1 as the optimum allocation of resource to produce pollen grains and ovules.
Venue: via Zoom
Event Official Language: English
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Seminar
Quantum nucleation of topological solitons
October 20 (Thu) at 13:30 - 15:00, 2022
Minoru Eto (Professor, Faculty of Science, Yamagata University)
The chiral soliton lattice is an array of topological solitons realized as ground states of QCD at finite density under strong magnetic fields or rapid rotation, and chiral magnets with an easy-plane anisotropy. In such cases, topological solitons have negative energy due to topological terms originating from the chiral magnetic or vortical effect and the Dzyaloshinskii-Moriya interaction, respectively. We study quantum nucleation of topological solitons in the vacuum through quantum tunneling in 2+1 and 3+1 dimensions, by using a complex ϕ4 (or the axion) model with a topological term proportional to an external field, which is a simplification of low-energy theories of the above systems. In 2+1 dimensions, a pair of a vortex and an anti-vortex is connected by a linear soliton, while in 3+1 dimensions, a vortex is string-like, a soliton is wall-like, and a disk of a soliton wall is bounded by a string loop. Since the tension of solitons can be effectively negative due to the topological term, such a composite configuration of a finite size is created by quantum tunneling and subsequently grows rapidly. We estimate the nucleation probability analytically in the thin-defect approximation and fully calculate it numerically using the relaxation (gradient flow) method. The nucleation probability is maximized when the direction of the soliton is perpendicular to the external field.
Venue: via Zoom
Event Official Language: English
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Seminar
An Introduction to Rough Geometry (with a view to Euclidean Gravity)
October 14 (Fri) at 14:00 - 16:30, 2022
Christy Koji Kelly (Special Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
The mathematical formulation of Einstein gravity typically utilises differentiable manifolds as models of smooth spacetimes. In many scenarios, however, it is desirable to have coarser models of spacetime and a correspondingly rough theory of geometry applicable to these coarser spacetime structures. In 2D Euclidean quantum gravity, for instance, the use of Regge calculus allows one to treat triangulations as regularisations of smooth spacetimes. There has been much recent progress in the mathematical (rigorous) understanding of this theory which we briefly review. We also introduce a rich alternative framework for the study coarse Euclidean geometry in the form of metric geometry augmented by optimal transport theory. In particular we introduce several optimal transport theoretic curvatures and demonstrate that these recover the familiar smooth notions under suitable limits.
Venue: Hybrid Format (Common Room 246-248 and Zoom)
Event Official Language: English
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Seminar
Introduction to cepstrum analysis and its applications
October 13 (Thu) at 16:00 - 17:00, 2022
Shingo Gibo (Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
Recent advances in experimental technique enable us to obtain many time-series data of biological oscillatory systems. These time-series are of various shapes, which means that the envelop of spectrum are also various. Cepstrum analysis is well-known method for evaluating the spectrum shapes in acoustic engineering. In this talk, I will introduce cepstrum analysis and its applications for analyzing biological oscillations.
Venue: via Zoom
Event Official Language: English
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Seminar
Ultra-Light Axion Dark Matter: Bose-Einstein condensates and superfluids in the sky
October 11 (Tue) at 13:30 - 15:00, 2022
Elisa G.M. Ferreira (Project Assistant Professor, Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), The University of Tokyo)
The nature of dark matter (DM) is one of the biggest mysteries in cosmology. There are many different models to explain the nature of this elusive component. In this talk I will present a class of dark matter models: ultra-light dark matter (ULDM) or ultra-light axions (ULA). I will show the different models present in the literature and classify them according to the way they behave on small scales. One of the most interesting features of this class of DM models is that it might condense in the interior of the halos of galaxies forming a Bose-Einstein condensate (BEC) or superfluid. This interesting quantum phenomena on macroscopic scales, and the wave nature of ULDM leads to different and interesting astrophysical consequences that can be probed on small scales. I will quickly review first the fuzzy dark matter model, one of the most well studied ULA models, where I will present its description, predictions and current bounds. Then I will introduce the DM superfluid model, where, upon condensation in the interior of galaxies, DM dynamics represents that of MOdified Newtonian Dynamics (MOND) on galactic scales. This behaviour can address some of the curiosities of the behaviour of DM on small scales. I plan to show the theoretical description of this model and its interesting phenomenology.
Venue: Hybrid Format (Common Room 246-248 and Zoom)
Event Official Language: English
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Seminar
Isometric tensor networks in two dimension
October 11 (Tue) at 10:30 - 12:00, 2022
Yantao Wu (Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
In this talk, I would like to explain the ansatz of isometric tensor network states (isoTNS) as candidate wavefunctions in two-dimensional condensed matter systems. I will explain how the isometric structure in 2D helps generalize many 1D tensor network algorithms, like the density matrix renormalization group and the time-evolution block decimation methods, to two dimensions. Both bosons and fermions; ground states and dynamics will be discussed. I will also explain why it is a friendly trial wavefunction in the context of variational Monte Carlo, where the sampling correlation time vanishes. I will also explain its relation to quantum error correction and how it provides an interesting playground of quantum information. If time permits, I would like to discuss some open questions about its representability of topological phases.
Venue: Common Room #246-248 (Main Venue) / via Zoom
Event Official Language: English
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Seminar
Magnetic fields at extragalactic scales: origin from the early universe?
October 7 (Fri) at 14:00 - 15:00, 2022
Ryo Namba (Senior Research Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
Blazar observations have provided tantalizing evidence for the presence of magnetic fields in the extragalactic regions, where astrophysical processes may not be an efficient source for their generation. While a natural speculation is to associate the production of such large-scale magnetic fields to inflationary physics, it has been known that magnetogenesis solely from inflation is quite challenging. In this talk I will discuss some mechanisms, successful/unsuccessful, for production of magnetic fields in the primordial universe, as well as the constraints from theoretical consistencies and observational data.
Venue: via Zoom
Event Official Language: English
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Seminar
Extremely low-coverage whole genome sequencing (XLC-WGS) as a cost-effective tool for pharmacogenomic profiling: Advantages and Challenges
October 6 (Thu) at 10:00 - 11:00, 2022
Lazaro-Guevara Jose Miguel (Postdoctoral Researcher, Department of Botany, The University of British Columbia, Canada)
Despite more than 10 years have passed since the first FDA labeling for pharmacogenomics (PGx), factors like the continuous updates on PGx related SNVs and the high cost for PGx testing have challenged its clinical implementation. However, next generation sequencing technologies like extremely low coverage whole genome sequencing (XLC-WGS) could overcome these difficulties. Our purpose is to explore the potential use of XLC-WGS as a cost-effective way for generate reliable PGx profiles that can be applied in preemptive and clinical scenarios. We sequenced 195 patients enrolled to the Utah Diabetes and Diabetic Complications Study using XLC-WGS, for further PGx Profiles generation. Additionally, we sequenced a subset of 190 individuals using Illumina CoreExome-24 v1.3 microarray and 50 individuals using Deep Coverage Whole Genome Sequencing (DC-WGS) for cross-platform comparisons. We built the Pharmacogenomic profiles extracting the genetic information from XLC-WGS in accordance to the extended manifest of the commercially available PGx microarray PharmacoScan. Once the PGx profiles were generated we perform a preemptive analysis using the Clinical Pharmacogenetics Implementation Consortium (CPIC®) Gene-Drugs interaction with “A” level. The cross-platform comparison revealed that genetic concordance between XLC-WGS, DC-WGS and Microarray platforms ranged from 98.25% to 99.7%. The preemptive PGx profiles identified patients at risk of potential adverse effects for intake of commonly prescribed medications, including 4 homozygote carriers of rs4149056 (in SLCO1B1) which has previously been associated with statin-related myopathy. As well as 25 homozygote carriers of rs9923231 (in VKORC1) that increased risk of Hemorrhage when starting Warfarin intake without dose adjustment/reduction.
Venue: via Zoom
Event Official Language: English
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Seminar
Transport coefficients of a Bose gas in one dimension
October 5 (Wed) at 13:30 - 15:00, 2022
Yusuke Nishida (Associate Professor, Department of Physics, Tokyo Institute of Technology)
I will present two of our recent studies on transport coefficients of a Bose gas in one dimension. The first part is on the thermal conductivity [1], which is typically divergent for quantum integrable systems in one dimension. However, it is found to be finite and dominated by an effective three-body interaction that inevitably arises by confining bosons into a tight matter waveguide. The second part is on the bulk viscosity [2], which is computed perturbatively in the high-temperature, weak-coupling, and strong-coupling limits. In particular, the strong-coupling limit is accessible thanks to the Bose-Fermi duality, which is shown for the dynamic bulk viscosity provided by the contact-contact response function.
Venue: via Zoom
Event Official Language: English
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Seminar
Axion electrodynamics in neutron stars
September 27 (Tue) at 14:00 - 15:00, 2022
Filippo Anzuini (Postdoc Fellow, Department of Astronomy, Monash University, Australia)
Axions are pseudo-Goldstone bosons that provide a solution to the strong CP problem, and are prominent candidates for dark matter. In neutron stars, it has been shown recently that the potential of the QCD axion acquires finite density corrections that shift the axion field expectation value, which can be large compared to the vanishing expectation value in vacuo. Such a shift leaves an imprint on typical neutron star observables such as the redshifted thermal luminosity, which can be used to constrain the axion parameter space. In this talk we focus on the coupling of axions with photons, which modifies Maxwell’s equations and alters the neutron star magnetic field. By performing state-of-the-art magneto-thermal simulations, we calculate the axion-induced perturbations to the neutron star’ magnetic field, and show that they grow on relatively short time-scales. At the same time, intense electric currents form, leading to enhanced ohmic dissipation, which increases the stars’ observable thermal luminosity. The activation of such mechanisms depends on the axion decay constant and the axion mass, two long-sought parameters at the center of several experimental and theoretical investigations. Both parameters can be constrained by comparing our simulations to observations of thermally-emitting neutron stars. The latter do not exhibit uncontrolled growth of the magnetic field that causes enhanced ohmic dissipation, allowing us to place bounds on axion parameters. Our results open a new astrophysical avenue to constrain axions, extending significantly the parameter range that can be probed with direct axion searches.
Venue: Hybrid Format (Common Room 246-248 and Zoom)
Event Official Language: English
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Seminar
Discrete Stochastic Model for the Co-infection Dynamics with Defective Interfering Particles
September 26 (Mon) at 16:00 - 17:00, 2022
Jizhou Li (Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
Defective interfering particle (DIP) in the context of influenza A virus is a virion with a significantly shortened RNA segment substituting one of its eight full-length parent RNA segments, such that it is preferentially amplified during replications. Therefore, a cell co-infected by standard viruses (STVs) and DIPs will produce mainly DIPs, suppressing the STV yield and displaying nontrivial co-infection dynamics. An important approach to quantifying the co-infection dynamics is mathematical modeling with ordinary differential equations (ODEs), which treat relevant quantities (such as numbers of target cells, STVs, and DIPs) as continuous numbers evolving with prescribed physical laws. However, the ODE models are mean-field in nature that is only valid for scenarios with large numbers of STVs and DIPs. For small-number scenarios, the infection outcomes can be dominated by random fluctuations and stochasticity, which cannot be captured by ODE models. In this week’s biology seminar, we introduce a new Discrete Stochastic Model (DSM) aimed to rectify the shortcomings of the ODEs by treating the co-infection dynamics as stochastic processes. As we will show, the new DSM is consistent with the ODE model in the large number regime. In the low number regime, the DSM yields bi-modal distributions for the infection outcomes (extinct vs established infections) that are otherwise unattainable by ODE models.
Venue: via Zoom
Event Official Language: English
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Seminar
Hyperons in neutron stars: fast cooling, Joule heating and hyperon superfluidity
September 26 (Mon) at 14:00 - 15:00, 2022
Filippo Anzuini (Postdoc Fellow, Department of Astronomy, Monash University, Australia)
Neutron stars challenge current models of highly dense matter. Despite be- ing the targets of numerous observational campaigns (e.g. gravitational-wave searches and X-ray observations), their equation of state is still unknown. One of the most exciting possibilities is that “unconventional” particles such as hy- perons may appear in neutron star cores. Hyperons have a major impact on the observed thermal luminosity, because they accelerate the cooling rate via direct Urca processes, which copiously increase the neutrino emission from the core. Such mechanism is often considered to be a key signature of hyperon concentrations at high densities. Hyperon superfluidity plays a major role as well, because it can suppress the neutrino emissivity exponentially. The hope is that a comparison of the theoretical cooling curves against the available data of thermally-emitting neutron star can hint towards the existence of hyperons and their superfluidity. There is one ingredient, however, that is often neglected in neutron star cooling models: internal heating. The magnetic field of neutron stars decays due to the dissipation of the electric currents circulating in the crust, generating substantial Joule heating in the shallower layers. The ther- mal power generated by this process can counterbalance hyperon fast cooling, making it difficult to infer the presence of hyperons from the available thermal luminosity data, and complicating the link between measured thermal emission and internal composition. We show that this is the case for magnetars, because their crustal temperature is almost independent of hyperon direct Urca cooling in the core, regardless of whether hyperons are superfluid or not. Likewise, ther- mal luminosity data of moderately magnetized neutron stars are not suitable to extract information about the internal composition, as long as hyperons are superfluid.
Venue: Hybrid Format (Common Room 246-248 and Zoom)
Event Official Language: English
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Seminar
Genetic Drift and Gnatural Selection
September 22 (Thu) at 16:00 - 17:00, 2022
Thomas Hitchcock (Special Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
Understanding how the various evolutionary forces of mutation, selection, and drift collectively shape the genetic composition of populations is a key goal of population genetics research. One classic method of study has been to compare different inheritance systems, and particularly popular has been the within genome comparison of autosomes and sex chromosomes. However, inferences from such comparisons can be limited by the fact that multiple factors differ between sex chromosomes and autosomes (e.g. ploidy and transmission genetics). Here, we study a group of black winged fungus gnats with a peculiar type of reproduction “digenic PGE” in which X and autosomes are inherited equally from females and males, but the X remains expressed in a haploid state in males compared to a diploid state in females. I first explain what is known about their inheritance system, and then show how we can extend classic theory to the various inheritance systems that coexists within the fungus gnats.
Venue: via Zoom
Event Official Language: English
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Seminar
MeV gamma-ray all sky simulation
September 16 (Fri) at 14:00 - 15:00, 2022
Naomi Tsuji (Assistant Professor, Faculty of Science, Kanagawa University)
The MeV gamma-ray domain is the only unexplored window among recent multiwavelength observations in astrophysics, often referred to as the "MeV gap". To fulfill this gap, there are several ongoing and planned projects of MeV gamma-ray telescopes. The measurement of MeV gamma rays (both continuum and line emission) would give us new insight into many topics in astrophysics, such as relativistic jets, particle acceleration, and origin of matter. In advance of the future MeV gamma-ray missions, we have been working on prediction of the MeV gamma-ray sky, which is helpful to determine what kinds of sources can be detectable with the future telescopes. In order to explore the MeV gamma-ray sources, we performed a catalog cross-matching between the hard X-ray (Swift/BAT) and GeV gamma-ray (Fermi/LAT) catalogs, resulting in 145 firmly cross-matched sources. Combined with the Galactic diffuse emission, which is calculated by GALPROP to reconcile the cosmic-ray and gamma-ray spectra with observations by AMS-02, Voyager, and Fermi-LAT, the all-sky maps in the MeV gamma-ray band can be produced. This is also used to investigate a long-standing problem in the MeV gamma-ray astrophysics: the origin of the diffuse emission from the inner Galaxy, measured by COMPTEL. I will report the analysis and results in detail, and introduce future missions of the MeV gamma-ray detectors.
Venue: via Zoom
Event Official Language: English
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Seminar
Phantom Bethe excitations and spin helices in integrable spin chains
September 15 (Thu) at 17:00 - 18:15, 2022
Vladislav Popkov (University Wuppertal, Germany)
We demonstrate the existence of a special chiral “phantom” mode with some analogy to a Goldstone mode in the anisotropic quantum XXZ Heisenberg spin chain. The phantom excitations contribute zero energy to the eigenstate, but a finite fixed quantum of momentum. The mode exists not due to symmetry principles, but results from nontrivial scattering properties of magnons with momentum k given by the anisotropy via cos (k) = Jz/Jx. The mode originates from special string-type solutions of the Bethe ansatz equations with unbounded rapidities, the phantom Bethe roots. All such Bethe states are chiral (the simplest representative being factorized state with helicoidal magnetization profile) and exist in both periodic and open XXZ spin chain under fine-tuning. I show how phantom Bethe states can be generated dissipatively, by setting a polarization gradient via coupling the ends of the open spin chain to suitable dissipative baths. Spin helix eigenstates were observed and used in recent cold atom experiments, and led to further surprising findings.
Venue: via Zoom
Event Official Language: English
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Seminar
Hessian Geometric Structure of Equilibrium and Nonequilibrium Chemical Reaction Newtworks
September 8 (Thu) at 16:00 - 17:00, 2022
Tetsuya Kobayashi (Associate Professor, Institute of Industrial Science, The University of Tokyo)
Cells are the basic units of all living things, and their functions are realized by circuits and networks of chemical reactions. Thus, understanding the mechanism how various cellular functions are implemented by chemical reaction networks (CRN) is the central challenge in biophysics and quantitive biology. Among various aspects of CRN, its thermodynamic property is particularly important because most of biological functions are energy-consuming nonequilibrium phenomena. However, even though the equilibrium chemical thermodynamics and kinetics of chemical reactions were founded more than one century ago, the nonequilibrium theory of CRN is still immature. One reason is the nonlinearity in the constitutive equation between chemical force and flux, which prevents us from associating the tangent and cotangent spaces of the dynamics by the usual inner product structure. In this work, we show that the nonlinear relation between chemical force and flux can be captured by Legendre transformation and the geometric aspects of CRN dynamics can be characterized by Hessian geometry. Hessian geometry is the geometry generated by Legendre dual pairs of convex functions and is the basis of dually flat structure of information geometry and also equilibrium thermodynamics. Thus, we have dually flat structures in CRN dynamics, one on the state-potential space where equilibrium and energetic aspect is formulated (1,2), and the other on the force-flux space where nonequilibrium and kinetics aspect is characterized(3). Two of them are consistently connected by topological property of the underlying hypergraph structure of CRN. We discuss potential applications of this structure not only for CRN but also for other phenomena and problems(4,5).
Venue: via Zoom
Event Official Language: English
764 events