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.


The common methodologies among four experiments with theoretical research activities are described in Section “Plan for Fostering Early-career Researchers”.

Plan for Fostering Early-career Researchers

Support for self-reliant early-career researchers, Their roles, Unique ideas, and Initiatives.

　To foster the leaders of neutrino science in the next generation by supporting independent research by early-career researchers and taking maximum advantage of the overseas exchange program, we will conduct three initiatives, as shown in Figure 3. Through these initiatives, the early-career researchers will naturally play major roles in the experimental projects, bridging Japanese and foreign researchers and enhancing synergies among different experimental projects. They will also exchange many ideas which could lead to the emergence new ideas for Elucidating Universe’s Creation from quantum fluctuations.


[1] Advanced technology initiative:　We will form three consortia, each focusing on key technologies enabling a leap in the scientific reach of neutrino-science experiments. The first consortium is for (a) data analysis using artificial intelligence and machine learning (AI/ML). All of the participating experimental projects have ongoing development for analyses using AI/ML. The second is for (b) the electronics and data acquisition systems. All of our experimental projects share a common aspect of data acquisition: the acquisition is contiguous without relying on external triggers, unlike particle collider experiments or optical telescopes in cosmology. This puts emphasis on high-throughput electronics and real-time processing, e.g., using FPGAs, in common. The third is for (c) developing cutting-edge detector technologies. Neutrino detectors (SK and HK) share large-aperture photo-multiplier tube (PMT) technologies with neutrino telescopes (IceCube). Dark matter detectors also share a similar PMT technology. Dark matter detectors and CMB telescopes share technologies of large cryogenic systems. Superconducting detector technologies and sub-Kelvin cryogenic systems of CMB telescope receivers find synergy with technologies that may enable future dark matter experiments.
　As described above and shown in Figure 2, each of the three consortia encompasses a technology shared among all the experimental projects in our program. The consortia will enable rapid and efficient progress in technology development by sharing methodologies and taking advantage of synergy among projects. Beyond these three, we may add more consortia to this initiative’s portfolio as our program evolves.
　We will implement this initiative by hosting “technology schools” and workshops, each of which is a few days to a week of focused meetings. The schools and workshops will train the early-career researchers to deepen their expertise and prepare themselves to play active roles in international setups. At the same time, they provide opportunities for the researchers to see how the technologies they develop are being used in other experimental programs outside their own. This will prepare the young researchers to acquire a broader view of neutrino science beyond their own experimental projects. 　We actively invite young theorists to participate in this initiative, too. For example, some of the research using AI/ML can be synergistic with theoretical research areas. Theoretical research can also bridge multiple experimental areas and offer a bird's-eye view of neutrino science.
　Senior members will initially lead the organization of the schools and workshops. We will then gradually increase the involvement of the early-career researchers in organization. This builds upon our success in the Grant-in-Aid for Scientific Research on Innovative Areas “Exploration of Particle Physics and Cosmology with Neutrinos” (PI: Nakaya; FY 2018-2022), where postdocs and graduate students organized young researchers’ workshops on their own. Many postdocs and graduate students who participated in these workshops have continued and further developed their careers as independent researchers in Japan and abroad. We will continue on this success. With young researchers getting together beyond the existing projects and thinking outside the box, this may lead to unexpected innovation and ideas that seed a completely new experimental project by the end of our seven-year program.

[2] Overseas exchange initiative:　The worldwide network of our scientific collaboration offers unparalleled opportunities for early-career researchers to conduct research abroad. There are a large number of competent and highly motivated young scholars in our community, who deserve to grab the opportunities. Table 4 provides our planned implementation of the initiative in the first couple of years, which is designed to take advantage of the training by our advanced technology initiative. PIs of foreign partner institutions will serve as mentors for the young scholars. Young scholars will have a great chance to be educated by their foreign mentors not only about science, but also about different cultures and attitudes. While this plan builds upon our extensive track record of international exchanges supported by other funding sources, we could not afford to fulfill all the needs due to the limited budget, which resulted in the lack of stability in the funding level. Support through this program will not only significantly increase the budget, but more importantly, will provide unparalleled stability over a seven-year period, allowing for more efficient use of other funding sources and the universities’ strategic partnership programs with foreign institutions.
　This initiative will also indirectly enhance the researchers’ visit in the opposite direction. We have many success stories where a Japanese early-career researcher visited a foreign institution and developed strongly tied collaboration with young researchers there, leading to these foreign researchers’ visit to Japan – some of them later took a postdoc or senior position in Japan. With steady and sustaining support for the exchange of young researchers, this program will boost such “opposite flow,” foreign researchers coming to Japan.

[3] Project transfer initiative:　The next-generation neutrino science leaders must have broad views and insight, not limited to a single experimental project, in addition to strong international recognition and leadership. Through this initiative, we will encourage the young researchers to switch to another project when they graduate their Ph.D. programs. We will establish a “trans-project fellowship.” This fellowship will fund a postdoc who works on an experimental project different from the one during her/his Ph.D. program. We will also provide cross-project “matching” opportunities between PIs and early-career researchers from different experimental projects during the schools and workshops of the advanced technology initiative. This will promote cross-project host-postdoc matching not only for the trans-project fellowship but also for the other postdoc opportunities, such as JSPS PD fellowships. Thereby, we will highly leverage the trans-project fellowship to encourage young researchers to experience multiple experimental projects.
　The postdocs who have benefited from this project transfer initiative will be well-positioned to take leadership of the advanced technology program, since they know how a common technology is used in different experimental projects, including the similarity and differences in their use cases.

　We designed the three initiatives to make this positive feedback happen – the young scholars who benefited from these initiatives will in turn become hosts and promoters of the younger researchers in the next generation through these initiatives. The seven-year duration of this program will enable multiple cycles of positive feedback through these initiatives.