https://www-conf.kek.jp/kincha/
ハイパーカミオカンデ計画は、超大型水チェレンコフ検出器「ハイパーカミオカンデ」と、J-PARCからの大強度・高品質なニュートリノビーム、さらに高性能の前置・中間検出器を組み合わせることで、素粒子・宇宙の研究を推進するプロジェクトである。東京大学とKEKをホスト機関とし、現在、国際共同研究グループによる建設・準備が佳境を迎えている。
本講演では、計画の概要と進捗状況、期待される科学的成果と国際的な優位性、将来の展望について紹介し、世話人から与えられたタイトルの問いに答えられるよう努力する。

The Weak Gravity Conjecture (WGC) plays an important role in the swampland program. While the WGC has been intensively studied and refined since its inception, most studies have been focused on formulating and testing the conjecture in Minkowski space. In this talk, I will discuss our proposal of a version of the Weak Gravity Conjecture that applies to AdS space. Adhering to one of the original motivations for the WGC by demanding extremal black holes to decay, the dynamics of black hole charged emission is analyzed, leading to an AdS WGC that is more stringent than the Minkowski version. The new AdS WGC bound can also be obtained from a black hole-particle repulsiveness condition and the near horizon Breitenlohner-Freedman (BF) instability of extremal black holes. I will discuss extensions and implications of the AdS WGC bound, including the convex hull version and consistency with supersymmetry, and comment on further steps to embed the AdS WGC into string theory settings.

https://www.rcnp.osaka-u.ac.jp/~suenaga/Hadron_Cafe/

Gravitational chiral anomaly connects the topological charge of spacetime and the chirality of fermions. It has been known that the chirality is carried by the particles (or the excited states) and also by vacuum. However, in the study of gravitational leptogenesis, for example, lepton asymmetry associated with the chiral gravitational waves sourced during inflation is conventionally evaluated only by integrating the anomaly equation. In this evaluation, no distinction between the excite states and vacuum contribution has been made. In this talk, I apply an analogy between U(1) electromagnetism and the weak gravity to the spacetime that resembles the one considered in the gravitational leptogenesis scenario. By assuming the emergence of Landau level-like dispersion relation in our setup, I suggest that level-crossing does not seem to be efficient while the charge accumulation in the vacuum likely takes place. Phenomenological implication is also discussed.

Ultralight bosonic (ULB) fields with mass m << 1 eV often arise in theories beyond the Standard Model (SM). If such fields exist, violent astrophysical events that result in emission of gravitational wave, photon, or neutrino signals could also produce bursts of high-density relativistic ULB fields. Detection of such ULB fields in terrestrial or space-based laboratories correlated with other signals from transient astrophysical events opens a novel avenue for multimessenger astronomy. This additionally provides a route for directly detected for fields that are otherwise very challenging to detect. I will discuss that quantum sensors are particularly well-suited to observe emitted scalar and pseudoscalar axion-like ULB fields coupled to SM, and demonstrate that multimessenger astronomy with ULB fields is possible even when accounting for matter screening effects.

Sterile neutrino with masses of the keV scale is a fascinating candidate for dark matter (DM). They can be produced via neutrino oscillations involving the Standard model neutrinos (“active” neutrinos), which are in thermal equilibrium in the early universe. Especially, in the presence of significant neutrino-antineutrino asymmetry, the production rate of the sterile neutrinos is resonantly enhanced and we can successfully account for all dark matter consistent with observational constraints.
In this seminar, we first present a comprehensive numerical analysis of the resonant production scenario and explore the consistency with current observations.Secondly, we examine a leptogenesis scenario that can naturally generate the neutrino-antineutrino asymmetry consistent with the resonant production of sterile neutrino DM. We demonstrate that this can be realized within the framework of Affleck Dine leptogenesis, based on the minimal supersymmetric Standard Model (MSSM). In our setup, spherical clumps of the slepton field—known as Q-balls—form and eventually decay into lepton asymmetric Standard model plasma.
Furthermore, in the above setup, Q-balls dominate the cosmic energy density and finally decay rapidly into radiation, triggering sudden reheating. This sudden reheating causes subhorizon modes of the gravitational potential to oscillate with large amplitudes, and then a significant amount of the gravitational waves (GWs) are produced by curvature perturbations at second order. Based on this idea, we discuss the testability of the scenario with the future GW observations.This presentation is mainly based on arXiv:2402.11902, but includes some updates on the analysis.

The QCD axion is currently the focus of intense experimental and theoretical research. A key feature of canonical QCD axion models is the fixed mass-coupling relation, which motivates experimental efforts to explore the so-called QCD axion band. But what if no axion is found within this range? Could a genuine QCD axion lie outside this band—and if so, at what theoretical cost? In this seminar, we will show that such a displacement can naturally arise if the axion propagates in extra dimensions, leading to distinctive patterns that motivates new regions of parameter space. We will discuss the theoretical framework, the associated experimental signatures, and the limitations of these constructions.

In the path-integral formalism, significant progress has been made in understanding chiral symmetry on the lattice using overlap fermions, which are constructed from Dirac operators satisfying the Ginsparg–Wilson relation. A corresponding formulation and understanding in the Hamiltonian formalism are also desirable yet remains under active investigation. Recently, A. Chatterjee, S. D. Pace, and S.-H. Shao proposed a novel construction of both vector and axial charge operators in a $1+1$ D staggered fermion system, which not only commute with the Hamiltonian but also possess quantized eigenvalues. The axial charge operator acts locally and generates a $\mathrm{U}(1)_A$ symmetry that can be gauged. These features provide a promising framework for the precise definition of chiral fermions.

In this work, we focus on the fact that the Hamiltonian of the $1+1$D staggered fermion system can be smoothly deformed into that of Wilson fermions. We reinterpret the structure of the axial charge operator proposed above using Wilson fermions. The eigenstates of the axial charge are expressed as linear combinations of positive-energy creation and negative-energy annihilation operators. Consequently, the corresponding Hamiltonian includes terms that violate particle number conservation. Interestingly, by applying the insights of Chatterjee et al., it can be shown that even such Hamiltonians admit a conserved charge operator associated with the vector $\mathrm{U}(1)$ symmetry in the continuum limit. Since this operator does not commute with the axial charge, the construction is consistent with the Nielsen–Ninomiya theorem.

The resulting $1+1$D Hamiltonian formulation is expected to be useful in constructing chiral gauge theories based on the symmetric mass generation (SMG) mechanism. SMG refers to a mechanism by which gapless systems can be gapped without fermion bilinears, purely through appropriate interactions, while preserving symmetries. To demonstrate this, we examine the feasibility of realizing SMG while maintaining the $\mathrm{U}(1)_A$ gauge symmetry generated by the axial charge operator $Q_A$, using the $1^4(-1)^4$ and 3-4-5-0 models as examples.

After a brief review of our current models of the early universe I will argue that the usual effective field theory techniques
which are currently employed inevitably break down, and we need a non-perturbative starting point. Matrix models such as the BFSS and IKKT models provide a promising starting point, and I will indicate a way to obtain an emergent spacetime and early universe cosmology from these models.

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. Refrence: [arXiv: 2308.01336, 2411.08737]