Framework and Significance of International Joint Research

Origin of the Universe studied with neutrinos

　In the 20th century, physics made remarkable progress with Quantum Mechanics (which developed into Quantum Field Theory) and the Theory of Relativity. Today, based on Quantum Field Theory and Relativity, the Standard Model of particle physics and the Standard Cosmology have been well established. However, there is a big and fundamental question as to how our universe was created, which has not been answered by the Standard Models. Connecting particle physics and astrophysics by using neutrinos, we study how the universe has been evolved from the "quantum fluctuations" in the early universe. We will attack the outstanding problems in the study of the early universe: inflation, the dark matter, the origin of the asymmetry between matter and antimatter, and the massive black holes. Those problems are addressed by the most-advanced neutrino experiment Hyper-Kamiokande (HK), the CMB telescope Simons Observatory, the dark matter experiment XENON, and the cosmic neutrino telescope IceCube.
　We, a group of this proposal, discovered neutrino mass in the Super-Kamiokande (SK) experiment, resulted in the Nobel Prize in Physics awarded to Prof. Takaaki Kajita. Today, we capture a hint of symmetry violation between a particle and its antiparticle (CP violation) in neutrinos, which could provide clues to why our universe consists of only matter without antimatter. By advancing neutrino physics continuously, we aim to unravel the mysteries of how the universe was created.

Global Neutrino programs with Japanese scientists

　Japanese neutrino scientists have earned a worldwide reputation for their discovery of neutrino oscillations and exploration of neutrino astronomy. Japan's strength is that it has the world's most advanced facilities, such as the Super-Kamiokande detector and the J-PARC neutrino beam, and can contribute to the global neutrino program. For example, 500 intentional researchers from 13 countries are participating in the J-PARC T2K experiment. The construction of the next large neutrino detector, Hyper-Kamiokande, began in 2020, and the data taking is scheduled to start in 2027. In addition to Japan-based experiments, Japanese scientists have made significant contributions to the development of neutrino programs overseas, including the discovery of high-energy cosmic neutrinos at IceCube and the development of a cosmic microwave radiometer for inflation and neutrino mass detection. The members proposing this research have established strong collaboration as the team of Grant-in-Aid for Scientific Research on Innovative Areas “Exploration of Particle Physics and Cosmology with Neutrinos” which has ended in 2022. It is also essential to understand unknown massive constitutes in our universe, called Dark Matter. Including genius theorists, we propose collaborative research addressing the fundamental question “how the universe was created from the quantum fluctuations.

International science projects proposed with supports by this program

　We are proposing to conduct the following science projects with a theory group under this program with neutrinos (ν): [a] ν CP Violation, [b] Dark Matter, [c] CMB, and [d] ν Astronomy with IceCube. The projects with the science targets, the experiments and international collaborations are defined in Table 1. 　The overview of the projects and the research objectives are illustrated in Figure 1. By conducting the four international projects together with theoretical studies, we will address the fundamental questions of our universe. The strong collaboration between the projects is expected to make significant progress on “Elucidating Universe’s Creation with Neutrinos based on International Science Collaborations” over the next 7 years under this program. In addition, the collaborations are essential to advance the common methodologies among experiments, such as Data analysis using AL/ML, Electronics and DAQ, Cutting-edge detector technology, that will be shown in Table 2, and to foster young scientists toward the common scientific goal. Keeping steady and solid progress within the framework of international collaborations, the large-scale and long-term support are essential and are required for the long-term constructions and measurements with the training of young researchers since international projects typically last five years or longer.

Research Plan

Background of the proposed research

　Through our neutrino research, we are challenging a fundamental question “How is the universe created?”. The proposed research is illustrated in Figure 1. The universe is composed only of matters without antimatters, which is global evidence of CP violation. We study neutrino CP violation by T2K, SK, and HK experiments. Unknown constitutes of the universe will be searched for by XENON experiments in this program. Inflation and neutrino mass are important for understanding the evolution of the universe, which are addressed by CMB observations centered on SO. Finally, to understand the current universe, neutrino astronomy with IceCube (and SK and HK) will provide essential observations for high-energy cosmic objects and supernovae. The four projects proposed in Table 1 provide key pieces of evidence to elucidate Universe’s Creation.

Research Objectives and methodologies

　Four experimental projects defined in Table 1, the common methodologies, and research objectives in this proposal are summarized in Figure 2.