SEMESTER 2025/26 - WINTER
30.09.2025
Junichiro Kawamura
Waseda University
Finite modular flavor symmetry and pseudo Nambu-Goldstone boson
In this talk, I will discuss how finite modular symmetries can explain the observed flavor hierarchies of quarks and leptons, and how a pseudo Nambu–Goldstone (NG) boson may arise in such frameworks. One of the longstanding questions in the Standard Model (SM) is why the three generations of quarks and leptons exhibit distinct hierarchical structures. A finite modular symmetry, which is a quotient group of SL(2,Z), provides an elegant framework to address this problem through residual symmetries. After presenting an explicit example that successfully reproduces the observed flavor patterns, I will show that, under certain conditions, a pseudo NG mode can also appear in modular flavor models. As an illustration, I will focus on a simple model inspired by the KSVZ axion framework, which also addresses the strong CP problem. These results suggest a possible connection between the flavor structure and a light scalar field that could potentially serve as a dark matter candidate.
14.10.2025
Priyanka Lamba
University of Bologna
Exploring Quantum Observables and their sensitivity to different interactions in polarised lepton collisions
The study of quantum observables at high-energy colliders provides a powerful probe of the quantum structure underlying particle interactions. In this talk, I will begin by outlining the general framework for defining and measuring such observables. I will then discuss the impact of beam polarisation on quantum properties emerging in fermionic inelastic scattering processes, considering different possible mediators such as scalar, vector, and tensor operators. Focusing on top–antitop pair production at future lepton colliders, I will present analytic results for the spin density matrix and examine a range of quantum information–theoretic quantities, including entanglement, spin correlations, Bell inequality violations, and measures of purity and magic. The overarching goal is to assess how beam polarization can be exploited to probe the quantum character of the system and enhance sensitivity to potential new physics effects.
03.11.2025
Kensuke Akita
University of Tokyo
Maximal parameter space of sterile neutrino dark matter with lepton asymmetries
Large lepton flavor asymmetries with zero total lepton asymmetry could be generated in the Early Universe. They are loosely constrained by current observations, being washed out at MeV temperatures by neutrino oscillations. We show that such lepton flavor asymmetries open up a new parameter space for sterile neutrino dark matter, consistent with all observational bounds. To this end, we construct the semi-classical Boltzmann equation for sterile neutrinos applicable in the case of arbitrarily large lepton asymmetries, and confirm its validity by quantum kinetic equations. This way, we derive the maximal parameter space for sterile neutrino dark matter with lepton asymmetries. The allowed range of sterile neutrinos' squared couplings extends by up to two orders of magnitude across a 5-60 keV mass range, and may be testable by X-ray, structure formation, and upcoming CMB observations. We will also discuss the origin of lepton flavor asymmetries: leptoflavorgenesis, and the recently reported helium-4 anomaly in the Universe associated with it.
04.11.2025
Yue-Lin Sming Tsai
Purple Mountain Observatory, Chinese Academy of Sciences
18.11.2025
Marc Schiffer
Radboud University
2.12.2025
Daniele Perri
Warsaw University