春に素粒子若手が新しいことを考える会

Thursday Apr 23, 2026, 1:00 PM → 6:00 PM Asia/Tokyo

Yukawa Hall Y306 (YITP)

素粒子大学院生を中心に、興味を持っていることを自由に語ってもらう会。そして老人を教育する会。

以前の若手会:
https://indico.yukawa.kyoto-u.ac.jp/event/74/ (2025師走会)
https://indico.yukawa.kyoto-u.ac.jp/event/69/ (2025皐月会)
https://indico.yukawa.kyoto-u.ac.jp/event/67/ (2025卯月会）

世話人：
浅井健人（YITP）
井黒就平（名古屋大/KEK）
北野龍一郎（YITP）
高山史宏（YITP）
濱田佑（大阪大）
 

1:00 PM → 1:40 PM Revisiting axion string in DFSZ axion model

I think I found something interesting. I will talk about a new aspect of axion strings in DFSZ model and give a very rough idea that there is an exotic electroweak instanton in the model. I will also discuss phenomenological/cosmological consequence.

Speaker: Yu Hamada (University of Osaka)

1:40 PM → 2:20 PM break

2:20 PM → 3:00 PM Maximal parameter space of sterile neutrino dark matter with lepton asymmetries

We delineate the maximal parameter space of sterile neutrino dark matter in the presence of lepton flavor asymmetries. We focus on large flavor asymmetries with vanishing total lepton asymmetry, which are washed out by neutrino oscillations at MeV temperatures and hence are consistent with BBN and CMB constraints. We derive a semi-classical Boltzmann equation for sterile neutrinos applicable in this regime and validate it against quantum kinetic equations. For sterile neutrino masses up to 60 keV, the viable range of mixing angles extends by up to two orders of magnitude, with broad prospects for tests in forthcoming X-ray, CMB, and structure formation observations. We will also discuss some related topics: the origin of lepton flavor asymmetries and baryon asymmetry, and the recently reported helium anomaly in the Universe.

Speaker: Kensuke Akita (University of Tokyo)

3:00 PM → 3:40 PM break

3:40 PM → 4:20 PM Do cosmic string segments emit gravitational waves?

Cosmic strings are predicted in various extensions of the Standard Model, including grand unified theories. Depending on the symmetry-breaking pattern, they can be either topologically stable or metastable. Intriguingly, metastable strings have been proposed as a possible origin of the gravitational wave (GW) background observed by recent pulsar timing array experiments. When metastable strings decay, they fragment into segments with monopoles and antimonopoles attached at their endpoints. The monopole and antimonopole are strongly pulled by the string tension. Violent oscillations of these segments have been considered as a potential GW source, in addition to contributions from string loops. We show that, in realistic situations, the monopoles frequently collide with thermal fluctuations on the string segments, which act as a resistance and prevent the oscillation. As a result, we find that the contribution from string segments to the GW background is negligible.

Speaker: Akifumi Chitose (ICRR, the University of Tokyo)

4:20 PM → 5:00 PM Break

5:00 PM → 5:40 PM Electroweak Scattering Through the FD Gauge

We study the production a neutrino pair in association with a $Zh$ final state at future lepton colliders, whose cross section exceeds that of Zh production in the multi-TeV energy region.
We classify the amplitudes into three main groups based on their Feynman diagram topologies: vector boson scattering, $\ell^-W^+$ scattering, and $W^-\ell^+$ scattering, and investigate the interference patterns among them.
We show that the subtle gauge cancellations among amplitudes at high energies, which appear in the unitary gauge, are absent in the recently proposed Feynman-diagram (FD) gauge. As a result, physical distributions can be interpreted in terms of contributions from each subgroup.

Speaker: Hiroyuki Furusato (Iwate university)

5:40 PM → 6:00 PM discussion