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
Searching for Non-Gravitational Signals of Dark Matter: The Particle Nature of Dark Matter
The simplest dark matter models are those in which dark matter candidates annihilate via s-wave interactions during the thermal equilibrium process in the early universe. However, particle collider experiments and direct detection experiments have yet to yield decisive results. For dark matter candidates with masses ranging from MeV to 10 TeV, satisfying the conditions of early universe thermal equilibrium requires annihilation processes with velocity dependencies. In this talk, I will explore how indirect detection methods and cosmological observations can be used to search for such non-s-wave annihilating dark matter candidates.
18.11.2025
Marc Schiffer
Radboud University
Towards systematic error estimates in asymptotically safe gravity-matter systems
Asymptotically safe quantum gravity might provide a unified description of the fundamental dynamics of quantum gravity and matter. The realization of asymptotic safety - i.e., scale symmetry at high energies - constrains the possible interactions and dynamics of a system. In this talk, I will first motivate the scenario of asymptotic safety for gravity and highlight the matter sector as a powerful test ground for the scenario. I will explain mechanisms how asymptotic safety could UV-complete individual sectors of the Standard Model and review the status of these mechanisms. I will then highlight recent results for the Yukawa and abelian gauge sector, where we find strong evidence that an asymptotically safe UV completion is possible. Furthermore, I will introduce two methods to estimate the robustness and the systematic uncertainties of the scaling exponents of the abelian gauge and Yukawa couplings.
Slides
2.12.2025
Daniele Perri
University of Warsaw
Gravothermal collapsing into primordial black holes
Very little is known about the cosmological history from after the end of inflation until the Big Bang nucleosynthesis, which spans more than $10^{39}$ orders of magnitude in time scales. Various well-motivated models predict that the universe could have undergone a period of matter domination in this early epoch. In this seminar, we show that if during this unknown period there was a long period of matter domination by a massive scalar field, and if the particle causing the matter domination has moderate self-interactions, the matter particles can undergo gravothermal collapse to form exotic states as primordial black holes (PBHs), boson stars, and cannibal stars. Thus, our study highlights that structure formation in the early universe can have a rich phenomenology. We found that for some choice of parameters, our model can predict an amount of PBHs surviving until today comparable to dark matter. For an optimistic estimate of PBH abundance, we also find that PBHs with masses less than 10^10 g can reheat the universe before BBN. From the bounds on the PBH abundance, we also constrain the models of massive scalar fields in a large range of parameters.
Slides
27.01.2026
Deanna Hooper
University of Helsinki
Reconstructing early universe physics with future LISA data
A gravitational wave background from a first order phase transition at the electroweak scale may be observable with future detectors such as LISA. While the Standard Model does not predict a first order phase transition, these occur in many extended scenarios. Therefore, detecting a stochastic gravitational wave background could point to new physics, while a null detection could constrain or even exclude many scenarios beyond the Standard Model. However, finding a signal in LISA data coming from an early universe phase transition is not straightforward, due to the expected instrument noises and astrophysical backgrounds, such as a the one produced by a population of white dwarf binaries. In this talk I will give an overview of first order phase transitions and how we numerically study them. I will then present our recent advances in recovering injected phase transition signals in realistic simulated LISA data, where we exploit the annual modulation of the astrophysical sources to improve the signal separation. I will also discuss how we could map such a gravitational wave signal onto the underlying phase transition parameters by using parameterised templates as an approximation to a more complete physical model, which greatly speeds up the process.