State-of-the-art atomic clocks achieved fractional uncertainties below 10^-18. In these clocks, effects of conventional external fields, such as electric and magnetic fields, are suppressed for the high accuracy. Under this condition, the system can potentially be sensitive to tiny energy shifts caused by hypothetical fields weakly coupled to ordinary matter or by effects mediated by massive particles. Based on this idea, various searches for new particles and fields are performed using precision spectroscopy.
In this seminar, starting from the overview of precision spectroscopy of atoms and its applications to fundamental physics searches, I will describe some major recent progress in the field. Particularly, I focus on the topics related to the new clock transition at 431 nm in ytterbium that has high sensitivity to variation of the fine structure constant and fifth forces between a neutron and an electron.

We ask the question of how angular momentum is conserved in a number of related processes, from elastic scattering of a circularly polarized photon by an atom, where the scattered photon has a different spin direction than the original photon; to scattering of a fully relativistic spin-1/2 particle by a central potential; to inverse beta decay in which an electron is emitted following the capture of a neutrino on a nucleus, where the final spin is in a different direction than that of the neutrino – an apparent change of angular momentum.

The seeming non-conservation of angular momentum arises, in fact, in the quantum measurement process in which the measuring apparatus does not have an initially well-defined angular momentum, but is localized in direction in the outside world. We generalize the discussion to massive neutrinos and electrons, and examine nuclear beta decay and electron-positron annihilation processes through the same lens, enabling physical insights into angular and helicity distributions in these reactions.

Primordial black holes (PBHs), hypothesized to have largely been produced in the early Universe, span a broad mass range and offer rich cosmological and astrophysical implications. Among them, there has been a recent growing interest in the evaporating PBHs, looking for the cosmological consequences of their past existence and observable signatures. In this talk, I will provide a brief overview of PBHs and the physics of their evaporation. Then, I will introduce my works on evaporating PBHs, about their reformation, isocurvature generation, and dark matter and hotspots. These examples are just a fraction of the phenomenological side of the rich physics of PBHs.

Cosmic inflation is known to address problems in the Big Bang theory and to generate the fluctuations that seed the current cosmological structures. Its existence is strongly suggested by the Planck observation of the cosmic microwave background (CMB). Though the CMB observation implies small fluctuations on large scales, large fluctuations may be realized on a smaller scale and the primordial black holes (PBHs) may be made by their gravitational collapse. PBHs have gotten much attention lately as candidates for dark matter and may be searched for by LISA in future probes. However, the detailed analysis of the PBH formation from inflation requires a lot of assumptions, particularly in the statistics of the primordial curvature perturbations.
In this talk, I introduce a numerical lattice simulation of inflation in the stochastic approach (STOLAS: STOchastic LAttice Simulation). It enables us to know the true statistics of the curvature perturbation and predict the accurate statistic of PBH in each inflation model.
I show examples in some models. I also describe the importance sampling technique to sample efficiently the large perturbation related to PBH formation.

Friday, March 27 (13:30-15:00, room 1)
Lecture 1: Foundations of Machine Learning
Monday, April 6 (13:30-15:00, room 3)
Lecture 2: Multi-Layer Perceptron (MLP)
Friday, April 10 (13:30-15:00, room 3)
Lecture 3: Convolutional Neural Network (CNN)
Tuesday, April 21 (13:30-15:00, room 1)
Lecture 4: Self-supervised and Semi-supervised Learning (optional)
Tuesday, April 28 (13:30-15:00, room 1)
Lecture 5: Transformers and Large Language Models (LLMs)

The Peccei–Quinn (PQ) mechanism is a prominent solution to the strong CP problem. However, it faces the axion quality problem: higher-dimensional, Planck-suppressed operators which explicitly break PQ symmetry can reintroduce the effective QCD theta angle. In composite axion models, where strong dynamics at high energies dynamically break PQ symmetry, certain constructions address this quality problem.
In some cases, hidden interactions distinct from QCD appear to contribute to the axion mass through instanton effects. I demonstrate that while these small instantons enhance the axion mass in a toy model, they do not contribute to the axion mass in an explicit model which addresses the quality problem. This talk is based on the following work: https://arxiv.org/abs/2404.19342″

In this talk, I will discuss my research on non supersymmetric theories and anomalies. In particular, I will revisit the chiral spectra on charged 1- and 5-branes in the 10d non supersymmetric sp(16) string theory and verify anomaly cancellation via inflow. Furthermore, I will show compelling evidence that the global structure of the gauge group is Sp(16)/Z2 along with hints of a possible duality to a non-supersymmetric heterotic theory. Secondly, I will discuss the presence of a discrete topological term in heterotic strings and delve into its relation with anomaly inflow cancellation on the worldvolume of the non-supersymmetric NS5-brane. These insights will allow us to assess the consistency of candidate spectra for the six-dimensional theory living on these defects. Finally I will present a recent study of Type II non-supersymmetric toroidal asymmetric orbifolds with a vanishing cosmological constant at one-loop in string perturbation theory. This talk will be based on 2412.17894, 2507.11610 and 2602.07113.

Multiple studies analyzing CMB polarization have recently reported evidence for cosmic birefringence, which refers to a rotation of the polarization plane of photons during their propagation. This signal violates parity symmetry and suggests the presence of extremely light pseudoscalar fields, such as axion-like particles. Such fields are potential candidates for dark matter or dark energy. In this seminar, we discuss the current observational status of cosmic birefringence, its implications for searches of dark sectors, and future prospects.

The “Proton Radius Puzzle,” initiated by the 2010 muonic hydrogen measurement, highlighted a significant discrepancy (about 7\sigma) with respect to values derived from electron scattering and ordinary hydrogen spectroscopy. While recent measurements, such as the PRad experiment at JLab and some of updated hydrogen spectroscopies, look to favor a smaller proton charge radius, consistent with the muonic result, inconsistencies with earlier electron scattering data remain unresolved. In addition, discrepancies among recent 1S–3S transition hydrogen spectroscopy resultsn suggest the presence of unquoted systematic uncertainties. Consequently, a precise determination of the proton charge radius is still critical to resolve this long-standing issue.
In this seminar, I will report on the current status of our ULQ2 (Ultra-Low Q2) project at the Research Center for Accelerator and Radioisotope Science (RARiS; former ELPH), Tohoku University. Utilizing a 60-MeV electron linac, we have measured the elastic electron–proton scattering cross section in the lowest-ever momentum transfer region, Q^2 = 0.0003–0.008 (GeV/c)^2. A key feature of this project is an absolute cross-section measurement relative to the well-known ^{12}C cross section using a CH_2 target, aiming to control systematic uncertainties at the 10^{-3} level. This approach is expected to provide the least model-dependent determination of the proton charge radius from electron scattering.
In addition to the proton measurement, we have also performed elastic electron–deuteron scattering measurement under the same kinematics, providing the world’s lowest-Q^2 data for the deuteron. A puzzle similar to that of the proton has been pointed out, and our measurement enables a determination of the deuteron charge radius. From this e+d data, we are challenging the determination of the neutron charge radius via electron scattering for the first time by exploiting the fourth moments of the deuteron charge distribution.
I will first review the current status of the Proton Radius Puzzle and recent global experimental efforts. I will then present the details of the ULQ2 project, including the challenges for the neutron charge radius.

We review a recent proposal for a quantum black hole as a certain large N matrix quantum mechanics. In this model, black hole horizon in general relativity is replaced by a fuzzy geometry. The model has past a number of tests: i) the macroscopic mass-size-shape-angular momentum relation for Schwarzschild and Kerr black hole is reproduced. ii) Berkenstein-Hawking entropy is reproduced from a microstates counting. iii) The Hawking radiation is described consistently in terms of quantum mechanical tunneling of the fuzzy sphere via the nucleation of a fuzzy monopole on the horizon. Further directions will be discussed.