SEMINAR

This group was newly established in April 2024. We hold seminars irregularly.

FY2025

17. Date: July 14th, 2025, 15:30 PM
    Place: C104
    Presenter: Dr. M. Eto (Yamagata University)
    Title: Topological solitons in field theories: How ubiquitous are they in nature?
    Abstract: Show abstract
    

    My talk consists of two parts: The first half is devoted to a basic introduction to topological solitons in field theories, including order parameter spaces, homotopy, topological stability and so on. In the second part I introduce three concrete examples from my recent studies. 1) Knot solitons in particle physics and cosmology, 2) Dislocations (topological defects known in atomic crystals and nematic liquid crystal) and crystallization of chiral soliton lattice in finite density QCD, 3) Skeletal structure of marine planktons and SUSY topological solitons. By bringing together these seemingly unrelated topics in one talk, I hope to demonstrate how ubiquitous topological solitons are in nature.

    Language: Japanese
    Other: This seminar is held with theoretical physics group. This seminar will be held in-person without streaming online.
18. Date: June 23rd, 2025, 15:30 PM
    Place: C204
    Presenter: Dr. K. Mizukoshi (RCNS, Tohoku University)
    Title: 南極長期周回気球を用いた暗黒物質探索実験GAPS
    Abstract: Show abstract
    

    暗黒物質は宇宙物理学・素粒子物理学の重要な未解決問題のひとつである. ハドロン生成物質による間接探索は背景事象フリーな探索が期待される一方で, フラックス量が小さく, 反重陽子や反ヘリウム-3は未検出である. これらの粒子が1事象でも検出されれば, 暗黒物質の正体に大きく迫ることが可能になる. 飛来する反重陽子や反ヘリウム-3が大気との相互作用で消滅してしまう上, 地球の磁場によって遮蔽されてしまう. 南極長期周回気球を用いるGAPS実験では, 大気の影響を低減し, 磁場によるエネルギーカットオフが小さい極域上空での観測を行うことで, 低エネルギー領域での粒子探索を実施し, 暗黒物質の正体解明をめざす. 本セミナーでは, 暗黒物質探索の現状, GAPS実験の詳細について説明し, 実験の進捗状況, 今後の展望について議論する.

    Language: Japanese
    Other: This seminar is held with theoretical physics group. This seminar will be held in-person without streaming online.
19. Date: May 19th, 2025, 14:45 PM
    Place: A238
    Presenter: Lukas Treuer (KEK, Sokendai)
    Title: Higgs Boson Production at μ^+ μ^+ Colliders
    Abstract: Show abstract
    

    Before discussing our work, I will begin the talk with a broader introduction to make it accessible to people from various fields. Motivated by recent advancements in antimuon cooling, we study Higgs boson production at μ^+ μ^+ colliders at high energy. Since both initial-state particles are positively charged, there is no W boson fusion at the leading order, as it requires a W^+ W^- pair. However, we find that the cross section of the higher-order, γ- and Z-mediated W boson fusion process is large at high center-of-mass energies √s, growing as (log s)^3. This is in contrast to the (log s) behavior of the leading-order W boson fusion. Thus, even though it is a higher-order process, the rate of Higgs boson production for 10 TeV energies at μ^+ μ^+ colliders with polarized beams can be as high as about half of the one at μ^+ μ^- colliders, assuming the same integrated luminosity. To calculate the cross section of this process accurately, we carefully treat the collinear emission of the photon in the intermediate state. The thereby obtained large cross section furthermore shows the significance of Higgs production with an extra W boson in the final state also at μ^+ μ^- and e^+ e^- colliders. Reference: Higgs boson production at μ^+ μ^+ colliders -- hep-ph/2408.01068

    Language: English
    Other: This seminar is held with theoretical physics group. This seminar will be held in-person without streaming online.
20. Date: April 21st, 2025, 15:30 PM
    Place: A425
    Presenter: Dr. T. Nakamori (Department of Physics, Ymagata University)
    Title: Science communitaion activities on cosmic-rays and astronomy
    Abstract: Show abstract
    

    It goes without saying that science communication between experts and the general public is of vital importance. While scientific explanations and the transfer of knowledge?often referred to as “research outreach” in a narrower sense?constitute one aspect of science communication, they do not represent its entirety. In this seminar, I will introduce two projects in which I have been involved. The first project focuses on increasing public understanding of cosmic rays. We have developed a low-price, compact detector called OSECHI, which is used in cosmic-ray measurement workshops and made available to schools for inquiry-based learning. I will discuss the background of these activities, share examples, and consider future prospects, as well as provide a demonstration of the device. The second project involves running an NPO dedicated to science outreach in astronomy and space. This organization manages a university observatory by and for local citizens, and we have established a cycle of capacity building among our volunteers and staff to sustain its operations. If time permits, I will also introduce an undergraduate class on science communication that I coordinate.

    Language: Japanese
    Other: This seminar is held with theoretical physics group. This seminar will be held in-person without streaming online.

FY2024

1. Date: March 24th, 2025, 16:30 PM
   Place: A228
   Presenter: Dr. M. Tachibana (Department of Physics, Saga University)
   Title: Neutron Stars and Quark-Hadron Continuity
   Abstract: Show abstract
   

   Neutron star is a stellar remnant after supernova explosion. Since its density is so high, exotic matter states, such as superfluid neutrons, Bose-Einstein condensate of mesons, superconducting quarks (and even dark matter), are expected to be present inside of it. In this talk, I will focus on a concept "quark-hadron continuity", originally proposed by Schafer and Wilczek and construct a concrete model to realize it. Then I will apply the idea into physics of neutron stars, such as vortex formation and mass-radius relation.

   Language: Japanese
   Other: This seminar is held with theoretical physics group. This seminar will be held in-person without streaming online.
2. Date: March 24th, 2025, 14:30 PM
   Place: A228
   Presenter: Dr. K. Funakubo (Department of Physics, Saga University)
   Title: Renormalization group improvement for thermally resummed effective potential
   Abstract: Show abstract
   

   To improve the perturbative calculations of the effective potential used in the study of phase transitions in the early universe, the resummation of the thermal masses is often employed. In conventional perturbation theory, the improvement using the renormalization group have benn well known, but the application of the renormalization group in the presence of resummation had not been well understood. We developed a new renormalization group method by interpreting resummation as a form of finite renormalization and demonstrated improvements in perturbation theory using a simple scalar theory. In this talk, I will present the construction of a renormalization scale-invariant effective potential and introduce a method to improve perturbation theory by utilizing scale invariance.

   Language: Japanese
   Other: This seminar is held with theoretical physics group. This seminar will be held in-person without streaming online.
3. Date: March 17th, 2025, 16:30 PM
   Place: A228
   Presenter: Stefan Lederer (Tokyo University of Science)
   Title: Excited bound states and their role in dark matter production
   Abstract: Show abstract
   

   I will discuss the impact of highly excited bound states on the evolution of number densities of new physics particles, focusing on dark matter, in the early Universe. In case of non-Abelian gauge interactions, which source dipole interactions between fundamental particles, highly excited states can prevent the particles from freezing, supporting a continuous depletion in the regime consistent with perturbativity and unitarity. Unitarity violation does in fact arise systematically, that is even for arbitrary small interaction strengths, once sufficiently highly excited states become relevant at low velocities. Novel analytic expressions for bound state formation, which we found recently, allow to accurately compute the freeze-out dynamics down to very low temperatures. I will highlight the importance of bound states to dark SU(N) sectors. For a more concrete dark matter model, I will focus on a colored and charged t-channel mediator model in the regime of superWIMP production. Here, excited states render the mediator depletion efficient all the way until its decay, introducing a dependence of the dark matter density on the mediator lifetime as a novel feature.

   Language: English
   Other: This seminar is held with theoretical physics group. This seminar will be held in-person without streaming online.
4. Date: March 13th, 2025, 16:30 PM
   Place: A228
   Presenter: S. Taniguchi (Osaka University Particle Physics Theory group)
   Title: An extended Higgs model for neutrino mass, dark matter, and baryon asymmetry
   Abstract: Show abstract
   

   Aoki-Kanemura-Seto model(AKS model) is a new physics model that can explain neutrino mass, dark matter and baryon asymmetry of the universe, but the baryon number has not yet been evaluated in the original model. The CP-violating phase in the Higgs potential is important to explain the observed baryon asymmetry in the electroweak baryogenesis scenario. However, such a phase is severely constrained by the measurements of the electron electric dipole moment (EDM). In this talk, we show that the electron EDM can be suppressed by interference between the phase in the Higgs sector and the dark sector. We discuss the possibility of explaining three phenomena.

   Language: Japanese
   Other: This seminar is held with theoretical physics group. This seminar will be held in-person without streaming online.
5. Date: February 17th, 2025, 10:30 AM
   Place: A228
   Presenter: Dr. K. Hashino (National Institute of Technology, Fukushima College)
   Title: Probing BSM effects for realizing first-order phase transition
   Abstract: Show abstract
   

   In the standard model (SM), the electroweak phase transition (EWPT) is crossover. When new physics effects beyond the SM modify the Higgs potential, the EWPT can become a first-order phase transition. Such a phase transition in the early Universe could produce gravitational waves (GWs) and primordial black holes (PBHs). Since the GW spectra and PBH abundance depend on the shape of Higgs potential, we can use their observations to probe beyond the SM effects for first-order phase transition. In this talk, we consider the Higgs potential which can realize the first-order phase transition, and discuss the testability of new physics effects via the GW and PBH observations.

   Language: Japanese
   Other: This seminar is held with theoretical physics group. This seminar will be held in-person without streaming online.
6. Date: February 13th, 2025, 16:30 PM
   Place: A228
   Presenter: Dr. S. Mishima (Saitama Medical University)
   Title: Current status of B anomalies
   Abstract: Show abstract
   

   Semi-leptonic B-meson decays have attracted significant attention over the past decade due to persistent discrepancies between experimental data and Standard Model predictions, commonly referred to as "B anomalies." In this talk, I will first review the current status of these decays and discuss possible new physics scenarios that could account for the anomalies. In the second part of the talk, I will focus on a sum rule that approximately holds for b → cτν semi-leptonic decays. The sum rule was empirically found a few years ago, and we have recently demonstrated that an exact version of the sum rule can be derived in the heavy quark limit. I will present our findings and discuss how the sum rule is modified in more realistic scenarios beyond the heavy quark limit.

   Language: Japanese
   Other: This seminar is held with theoretical physics group. This seminar will be held in-person without streaming online.
7. Date: January 28th, 2025, 10:30 AM
   Place: A228
   Presenter: N. Osamura (Nagoya University Theoretical Elementary Particle Physics Laboratory)
   Title: Impact of the Electroweak Weinberg Operator on the Electric Dipole Moment of Electron
   Abstract: Show abstract
   

   Recent progress in the electric dipole moment (EDM) measurements of the electron using the paramagnetic atom or molecule is remarkable. In this paper, we calculate a contribution to the electron EDM at three-loop level, introducing the CP-violating Yukawa couplings of new SU(2)$_L$ multiplets. At two-loop level, the Yukawa interactions generate a CP-violating dimension-six operator, composed of three SU(2)$_L$ field strengths, called the electroweak-Weinberg operator. Another one-loop diagram with this operator inserted induces the electron EDM. We derive the matching condition and find that even if new SU(2)$_L$ particles have masses around the TeV scale, the electron EDM may be larger than the Standard Model (SM) contribution to the paramagnetic atom or molecule EDMs. We also discuss the relation between the Barr-Zee diagram contribution at two-loop level and three-loop one, assuming that the SM Higgs has new Yukawa interactions with the SU(2)$_L$ multiplets.

   Language: Japanese
   Other: This seminar is held with theoretical physics group. This seminar will be held in-person without streaming online.
8. Date: January 27th, 2025, 16:30 PM
   Place: A228
   Presenter: N. Osamura (Nagoya University Theoretical Elementary Particle Physics Laboratory)
   Title: Novel loop-diagrammatic approach to QCD theta parameter
   Abstract: Show abstract
   

   When the QCD axion is absent in full theory, the strong $CP$ problem has to be explained by an additional mechanism, {\it e.g.}, the left-right and extended CP symmetry. Even though tree-level QCD $\bar\theta$ parameter is restricted by the mechanism, radiative corrections to $\bar\theta$ are mostly generated, which leads to a dangerous neutron electric dipole moment (EDM). The ordinary method for calculating the radiative $\bar\theta$ utilizes an equation $\bar \theta= -\text{arg}\, \text{det}\, m_q^{\rm loop}$ based on the chiral rotations of complex quark masses. In this paper, we point out that when full theory includes extra heavy quarks, the ordinary method is unsettled for the extra quark contributions and does not contain its full radiative corrections. We formulate a novel method to calculate the radiative corrections to $\bar\theta$ through a direct loop-diagrammatic approach, which should be more robust than the ordinary one. Then, we scrutinize a radiatively generated QCD $\theta$ parameter at the two-loop level based on both full analytical loop functions with the Fock-Schwinger gauge method and the effective field theory approach, using simplified models. We observe that the radiatively generated $\theta$ parameters at the low energy scale precisely match between them.

   Language: Japanese
   Other: This seminar is held with theoretical physics group. This seminar will be held in-person without streaming online.
9. Date: November 25th, 2024, 16:30 PM
   Place: A228
   Presenter: T. Shimasaki (The university of Tokyo, Kavli IPMU Belle II experiment group)
   Title: Belle II実験におけるステライルニュートリノ探索
   Abstract: Show abstract
   

   素粒子標準理論においては、ニュートリノは左巻き成分のみを持ち質量をもたない粒子とされているが、ニュートリノ振動実験はニュートリノが小さいながらも質量をもつことを強く示唆している。 右巻きで有限な質量をもつマヨラナ型の（ステライル）ニュートリノが存在すれば、小さいニュートリノ質量や宇宙初期のバリオジェネシスを説明し、さらに暗黒物質の候補にもなる[1]。 電子陽電子型衝突実験BelleまたはBelle IIではB中間子の崩壊過程を精密に調べることでGeV領域のステライルニュートリノ探索が可能である。先行研究Belleでは、2 - 3.5 GeV/c^2の質量領域においてステライルニュートリノとSMニュートリノとの混合定数Uに対して|U|^2~10^-4程度の制限を付けた[2]。本研究では、Belleとは異なる探索手法を用いてより小さな混合定数の領域での探索を目指す。 本トークでは、加速器実験としてのBelle IIの特徴と解析手法の基本的な事柄を述べた後、Belle IIにおけるステライルニュートリノ探索の戦略と現在の進捗について報告する。 参考文献： [1] Phys. Lett. B 620 (2005) [2] Phys. Rev. D. 87 071102 (2013)

   Language: Japanese
   Other: This seminar is held with theoretical physics group. This seminar will be held in-person without streaming online.
10. Date: November 25th, 2024, 14:30 PM
    Place: A228
    Presenter: Dr. T. Asaka (Niigata university Theoretical Particle Physics Group)
    Title: Lepton number violation in the low-scale seesaw mechanism
    Abstract: Show abstract
    

    We investigate the origin of neutrino masses and its implications to the lepton number violation. Especially, we consider the seesaw mechanism by introducing right-handed neutrinos which mass scale is close to the weak scale. The contributions of such right-handed neutrinos to the lepton number violating processes, such as the neutrinoless double beta decay and the inverse neutrinoless double beta decay, are explained. In particular, we discuss the impact of the radiative corrections to these processes. 参考文献： T. Asaka, H. Ishida and K. Tanaka, ``Neutrinoless double beta decays tell nature of right-handed neutrinos,'' JHEP 07, 062 (2023) doi:10.1007/JHEP07(2023)062 [arXiv:2101.12498 [hep-ph]]. T. Asaka and T. Tsuyuki, ``Seesaw mechanism at electron-electron colliders,'' Phys. Rev. D 92, no.9, 094012 (2015) doi:10.1103/PhysRevD.92.094012 [arXiv:1508.04937 [hep-ph]].

    Language: Japanese
    Other: This seminar is held with theoretical physics group. This seminar will be held in-person without streaming online.
11. Date: October 10th, 2024, 16:30 PM
    Place: A228
    Presenter: Dr. T. Kuwahara (Peking University)
    Title: 複合非対称ダークマターの現象論
    Abstract: Show abstract
    

    複合非対称ダークマター（ADM）は、宇宙のバリオン密度と暗黒物質密度の一致を自然に説明する枠組みである。 ダークセクターはダークマターやそれに付随する新粒子の粒子群のことを指し、標準模型の粒子群とは独立した相互作用を持っている。 この枠組みでは、ダークセクターにおける強い相互作用の帰結として現れる、ダークセクターのバリオン的粒子がダークマターとして振る舞う。 標準模型とダークセクターをつなげるポータル相互作用を二種類導入する。一つ目は、宇宙初期に生成された粒子-反粒子の非対称性を共有するポータル相互作用。 もう一つは、宇宙後期におけるエントロピーの移動を担う低エネルギーポータル相互作用として、我々の光子と混合するダークフォトンを導入する。 これらのポータル相互作用は、複合ADMシナリオを実験・観測的に探る上で非常に重要な役割を果たす。 ダークハドロンは加速器実験においてダークフォトンを介して生成され、ダークフォトンを通じた標準模型粒子への崩壊シグナルを残す。 特に特定の質量スペクトルにおいて、FASERやFACETのような前方検出器実験やDarkQuestのようなビームダンプ実験で、長寿命粒子探索を通じて複合ADM模型を探ることができる。 これらの実験では、ダークハドロンの崩壊シグナルから、10^-3から10^-5のオーダーの運動的混合が探索可能であると期待される。 このパラメータ領域は、ダークフォトンの短寿命崩壊シグナルでも探索できる領域と一致します。 粒子-反粒子の非対称性を共有するポータル相互作用は、宇宙後期でのダークマターの崩壊を引き起こす。 ダークマターは反ニュートリノとダークセクターのメソン的粒子（またはダークフォトン）に崩壊する。 その後のダークメソンやダークフォトンのカスケード崩壊を通じて、電子・陽電子・ガンマ線などを放出する。 これらの宇宙線の観測と無矛盾であることから、複合ADMシナリオにおけるダークマターの寿命に制限がかけられる。 AMS-02による宇宙線陽電子測定からの制限が2 GeV以上のダークマター質量で最も厳しく、寿命は10^26秒より長くなければならない。 これらの結果は、粒子-反粒子の非対称性を共有するポータル相互作用のカットオフスケールが約10^8?10^9 GeVより大きくならなければならないことを示唆する。 参考文献: JHEP 06 (2023) 208 (arXiv: 2303.03736) by TK, S.Yuan , JHEP 03 (2022) 176 (arXiv: 2112.01202) by A.Kamada, TK, work in progress by S.Das, A. Kamada, TK, K. Murase, D. Song
    

    Language: Japanese
    Other: This seminar is held with theoretical physics group. This seminar will be held in-person without streaming online.
12. Date: October 3rd, 2024, 16:30 PM
    Place: A228
    Presenter: Dr. M. Fujiwara (Technical University of Munich)
    Title: Dark matter self-interaction: the fate of the spike and annihilation-boosted dark matter in the Milky Way galaxy
    Abstract: Show abstract
    

    One of the characteristic predictions of dark matter (DM) self-interaction is the number changing processes that convert n initial DM particles into m final ones (n to m process). The inner region of the Milky Way galaxy may host an extremely high density of DM, called DM spike, and can be an ideal environment to explore these n to m processes. We systematically study the consequences of n to m processes in the local galaxy, including all possible DM processes originating from the same interactions, such as annihilation and self-scattering. We find that for n > 2, the spike is depleted significantly by the annihilation and isothermal core formation via self-scattering, erasing the potential of the galactic center as a DM rich region. On the other hand, semi-annihilation processes of form 2 to 1 may maintain the spike power law and induce boosted DM signatures at direct detection and neutrino experiments. We can discriminate this boosting mechanism from others, thanks to its distinct recoil spectra with a sharp cutoff at the mass limit. We probe DM mass down to 2 MeV at CRESST (20 MeV at XENONnT) in evident complementarity with the signature from the Sun, where the evaporation mass (> 5 GeV) limits the sensitivity. Collaboration with: Boris Betancourt-Kamenetskaia, Alejandro Ibarra, and Takashi Toma (work in progress)

    Language: Japanese
    Other: This seminar is held with theoretical physics group. This seminar will be held in-person without streaming online.
13. Date: June 13th, 2024, 16:30 PM
    Place: A228
    Presenter: Dr. K. Sakurai (Tohoku University, Particle Theory and Cosmology Group)
    Title: LFV decays of the Higgs boson in variant DFSZ axion models
    Abstract: Show abstract
    

    アクシオンは強いCP問題を解決するPeccei-Quinn機構の枠組みで予言される擬南部ゴールドストンボソンである。 微弱な結合を持ち安定である為、軽い暗黒物質の候補として考えられている。 加えて、アクシオンの存在は宇宙発展に影響を及ぼし、とりわけ星の進化の様々な段階で観測されている冷却異常を説明する新たな冷却機構を与える。 このような星の冷却異常を上手く説明するアクシオン模型（Variant DFSZ axion模型）はフレーバーを破る湯川結合を要求する。 我々はVariant DFSZ axion模型において予言される125GeVヒッグスボソンのレプトンフレーバーを破る(LFV)崩壊の大きさについて詳細に解析を行った。 本セミナーでは、このアクシオン模型が2023年に報告されたALTASのヒッグスボソンLFV崩壊におけるエクセスを説明出来ることを示す。 また、このような拡張ヒッグスセクターを伴うアクシオン模型において、アクシオンとヒッグスボソンがどのように関連しているかについて議論する。

    Language: Japanese
    Other: This seminar is held with theoretical physics group. This seminar will be held in-person without streaming online.