NPPSG Seminar

Scaling regimes of the 1D Kuramoto-Sivashinsky equation

November 5 (Thu) 17:00 - 18:30, 2026

Liubov Gosteva (Ph.D. Student, LPMMC, Université Grenoble Alpes, France)

The Kuramoto-Sivashinsky (KS) equation describes various phenomena exhibiting chaos and instabilities, including flames, reaction-diffusion processes, and thin liquid film flows. It resembles the Kardar-Parisi-Zhang (KPZ) equation, with the key difference being the negative "viscosity" in the KS equation. The KS equation is known to belong to the KPZ universality class: at large scales, its statistical properties are governed by the KPZ fixed point, characterized by the dynamical exponent $z=3/2$ and a known PDF of fluctuations. In this talk, I will analyze the stochastic KS equation using the functional renormalization group (FRG). In this framework, attractive fixed points of the FRG equation determine large-scale universal behavior, while unstable fixed points govern small-scale universal behavior. I will show that the two-point correlation function exhibits, besides the KPZ scaling at large scale, another universal scaling regime at small scale, with $z=1$. This scaling is very robust as it is intrinsic to the KS dynamics. It arises from the vanishing of the effective viscosity when evolving from its microscopic negative KS value, to its macroscopic effective positive KPZ value. I will also present results from direct numerical simulations, which confirm these findings.

Venue: via Zoom

Event Official Language: English

Bubble wall dynamics from non-equilibrium quantum field theory

October 21 (Wed) 13:30 - 15:00, 2026

Matthias Carosi (Postdoctoral Researcher, International Centre for Theoretical Physics Asia-Pacific (ICTP-AP), University of Chinese Academy of Sciences, China)

First-order phase transitions in the early universe are a unique probe of physics beyond the Standard Model, with potential implications for electroweak baryogenesis, the formation of primordial black holes, magnetogenesis and the production of dark matter. Upcoming gravitational wave detectors may capture the imprint of such transitions, but to extract theoretical insights from these signals requires a precise understanding of the underlying dynamics. In this talk, I show how the language of non-equilibrium quantum field theory, combined with the two-particle-irreducible effective action, provides a natural framework for describing the dynamics of a bubble after nucleation. After a brief introduction to the closed-time-path formalism, I derive the dynamical equations governing the bubble and the plasma and identify all sources of friction for the bubble expansion. This framework unifies the pre-existing approaches within a single, consistent description. In the ultrarelativistic regime, I demonstrate how to compute the friction induced by the pair production of heavy scalar particles.

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

Event Official Language: English

Four Fermi Theory in Four Dimensions is Renormalisable

August 3 (Mon) 15:00 - 16:30, 2026

Charlie Cresswell-Hogg (Post-Doctoral fellow, Department of Physics, Sussex University, UK / Post-Doctoral fellow, Dortmund University, Germany)

We demonstrate the renormalisability of quantum field theories in four dimensions with elementary self-interacting Dirac fermions and to leading order in the limit of many fermion flavours Nf. Starting from the underlying divergence structure and using Gross-Neveu-type interactions as a template, we explain why extended four-fermion theories including higher-derivative interactions are well-defined, renormalisable, and predictive with only a few free parameters. We also provide the exact large-Nf leading beta functions of couplings and discuss quantum scaling dimensions, universality, 1/Nf corrections, and extensions to other types of four fermion interactions. Implications for effective theory and model building are indicated.

Venue: via Zoom

Event Official Language: English

Center-vortex condensation and monopole condensation in 4d gapped phases

July 27 (Mon) 14:00 - 15:30, 2026

Yui Hayashi (JSPS Postdoctoral Research Fellow, Yukawa Institute for Theoretical Physics, Kyoto University)

Two well-known scenarios for quark confinement are center-vortex proliferation and monopole condensation. We consider gauge-invariant criteria for center-vortex condensation and monopole condensation in terms of Z(N) 1-form symmetry. The condensation of a soliton can be characterized by the non-suppression of the partition function with a proper twisted boundary condition, and we utilize this idea for these criteria. With these definitions, we show that gapped phases with the center-vortex condensation necessarily exhibit the monopole condensation.

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

Event Official Language: English

Entanglement suppression for ΩΩ scattering

July 17 (Fri) 15:00 - 16:30, 2026

Katsuyoshi Sone (Ph.D. Student, Graduate School of Science, Tokyo Metropolitan University)

The S-matrix describing the scattering process can be expressed in terms of projection operators onto the allowed spin–flavor channels and the corresponding phase shifts. Using the entanglement entropy in the spin space of the two-particle state, one can define the entanglement power, which quantifies the ability of the S-matrix to generate entanglement in the system. By investigating the conditions under which the entanglement power of the S-matrix is minimized, namely, the conditions for entanglement suppression, one can derive relations among the phase shifts in different spin–flavor channels. Furthermore, by comparing these relations with the interaction Lagrangian, one can identify the underlying symmetries [1,2]. In this work, we apply the entanglement suppression framework to two-baryon scattering involving spin-3/2 baryons in the flavor decuplet [3]. Lattice QCD calculations have shown that the spin-0 ΩΩ system exhibits scattering close to the unitary limit. Combining this result with the relation between the phase shifts obtained from entanglement suppression, we discuss the scattering behavior of the spin-2 ΩΩchannel.

Venue: #445-447, 4F, Main Research Building (Main Venue) / via Zoom

Event Official Language: English

Symmetry origin of the quantum-classical transition, hydrodynamics, and decodability.

June 26 (Fri) 14:00 - 16:00, 2026

Cenke Xu (Professor, University of California, Santa Barbara, USA)

We discuss the following question: when a quantum system evolves into classical one, is there a sharp transition? We will show that the “strong-to-weak” spontaneous symmetry breaking (SW-SSB) provides a sharp onset of classical physics. We present the theoretical framework and summarize recent experimental progress toward observing SW-SSB. We will also discuss the consequence of the SW-SSB, including the emergence of hydrodynamics, and also its information aspect, such as the transition of decodability and distinguishability. Much of the theoretical analysis maps to a problem of defect in the Euclidean spacetime.

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

Event Official Language: English

't Hooft anomaly matching and symmetry enforced gaplessness

June 1 (Mon) 13:00 - 14:00, 2026

Kantaro Ohmori (Senior Research Scientist, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

I will talk about the 't Hooft anomaly matching and its enforcement of gaplessness. I will also briefly touch on my recent work with Takamasa Ando on this topic.

Venue: via Zoom / Seminar Room #359

Event Official Language: English

Basics of chiral lattice fermion

May 18 (Mon) 13:00 - 14:00, 2026

Arata Yamamoto (Senior Research Scientist, Quantum Mathematical Science Team, Division of Applied Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

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

Event Official Language: English

Cooking up holographic black holes

April 27 (Mon) 13:30 - 15:00, 2026

Daichi Takeda (Special Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

I have recently been investigating holography for open systems and have developed a method to compute correlation functions of a CFT governed by the Lindblad equation from its gravitational dual. In an open system, the state of the subsystem of interest cannot remain pure, and one naively expects its entropy to grow over time. It is then natural to expect that this thermalization process is accompanied, on the gravity side, by black hole formation. In this talk, after giving an overview of holography for open systems, I will present a numerical nonperturbative analysis of the dynamics of JT gravity coupled to a scalar field, and show that black holes indeed form in this setup.

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

Event Official Language: English