Research

The Standard Model of particle physics

Matter is divided into atoms, atoms into nuclei and electrons, nuclei into protons and neutrons, and nucleons into quarks. Particles that cannot be further divided are called elementary particles. The Standard Model is an effective theory of elementary particles and the interactions between them. It describes a framework in which Fermi particles, which constitute matter, and Bose particles, which mediate the electromagnetic, strong, and weak forces. Although the validity of the theory has been verified by various experiments, it has unsolved problems and mysteries, suggesting the existence of unknown physics beyond the model.

The Standard Model can explain many phenomena by formulating the nature of elementary particles and the interactions between them. The Higgs mechanism by which matter acquires mass and the breaking of CP symmetry by generational mixing of quarks are also part of the Standard Model. The theoretical framework of the Standard Model was established in 1974, but its experimental verification required a high-energy particle accelerator, which was completed in 2012 with the observation of the Higgs boson by the ATLAS and CMS experiments at LHC.

Unsolved problems in particle physics

The Standard Model has been very successful in various respects, but there are still mysteries to be solved. The existence of dark matter, neutrino masses, and the hierarchy problem are well known. However, there are many other unsolved problems as well. The Standard Model has 17 elementary particles and 19 parameters, but their origins are unknown. Why are there three generations of quarks and leptons? Why do only weak interactions break the CP symmetry?

The Standard Model can predict observables that can be experimentally verified precisely. In particular, the perturbation theory of quantum electrodynamics can calculate physical quantities with extremely high precision. By comparing theoretical and experimental results, we can investigate how accurately the Standard Model can describe nature. Some of the experimental results deviate from the theoretical predictions by the Standard Model, and are considered to be signs of new physics.

標準模型は実験的検証が可能な観測量を高精度で予言することができ、中でも量子電磁力学の摂動論はきわめて高い精度で物理量を計算可能です。理論値と実験値とを精密に比較することで、標準模型がどこまで正確に自然を記述できるのか調べることができます。いくつかの実験結果は標準模型による計算値と一定以上の水準で乖離しており、新物理の兆候と目されているものもあります。

Searches for physics beyond the Standard Model

Phenomena that cannot be explained within the scope of the Standard Model provide clues to new physics beyond the exisring theory. The most obvious example is the observation of new elementary particles, where new particles of various types are being searched for in high-energy collider experiments and underground experiments.

Alternatively, quantum effects allow us to tackle areas that are beyond the reach of direct searches. We can search for events with extremely small probability using high-intensity quantum beams or high-intensity electromagnetic fields. Furthermore, it is possible to explore the slightest break in the Standard Model by comparing the results of precise measurements and calculations.

Muon and its property

Muons are subatomic particles classified as second-generation charged leptons. They are 200 times heavier than the electron and decay with a lifetime of two microseconds due to weak interactions. It was first discovered as a high-energy cosmic ray, and was later artificially produced using particle accelerators. Muons, like electrons and protons, have an internal degree of freedom called spin. Spin is often likened to the particle's rotation. As for muons, their spin can be imagind as a tiny azimuthal magnet.

Production and decay of muons

In experiments using particle accelerators, a proton beam is irradiated on to a target to produce pions, which decay into a muon and a neutrino. The muons emitted in this reaction are spin polarized. Furthermore, when we use the two-body decay of stopped pions, the muon's kinetic energy is monochromatic. These properties of the muon are of great use as particle beams.

Applications of muons

Research using muons can be broadly divided into three directions. (1) experiments to search for unknown physical phenomena by precisely studying the properties of muons as elementary particles, (2) experiments to clarify the magnetic properties of matter and the electronic state of atoms by investigating how muon spin behaves in matter, and (3) experiments to analyze the constituent elements and internal structure of samples by using the interaction between muons and matter. Applications of muons range from basic and applied physics, radiochemistry, life sciences, and archaeological research.

Muons for fundamental physics

When looking for new physics using elementary particles, a particle with a larger mass tends to be more susceptible to unknown phenomena. On the other hand, heavier particles are more difficult to produce and have shorter lifetimes, limiting the possible experiments. Muon mass, lifetime, spin, production method, and decay mode strike a fine balance between the pursuit of precision and a search for undiscovered phenomena.

Muonic atoms

I am particularly interested in investigating the properties of exotic atoms involving muons to search for physics beyond the Standard Model. Atoms containing muons are virtually nonexistent in nature and are obtained in a facility using particle accelerators. Muons are 207 times heavier than electrons, so they exist much closer to the nucleus in an atom than electrons do. This property provides a special and ideal environment to study the interaction between an elementary particle and a nucleus.