Figure 1: Conceptual diagram of positive muon cooling and acceleration. Cooling the positive muons, whose directions and velocities are initially spread out, enabled efficient radio-frequency acceleration.

Overview

The muon is an elementary particle similar to the electron. Muons produced artificially with accelerators are used in a wide range of fields, including materials science, particle physics, and the transmission imaging of large structures. However, ordinary muon beams produced at accelerator facilities have widely spread directions and velocities, and cannot be accelerated as they are.
A research group of KEK, the Japan Atomic Energy Agency, Okayama University, Nagoya University, Kyushu University, Ibaraki University, and Niigata University achieved the world's first demonstration of muon cooling ( *3) and acceleration, using a method in which muons are first cooled to a nearly-at-rest state and then accelerated with a radio-frequency accelerating cavity (press release of May 2024, "World's first cooling and acceleration of muon - The first muon accelerator finally coming to a reality -"). This result was featured on the cover of Physical Review Letters and was selected for the journal's 2025 collection of notable papers, receiving high international recognition.
This time, a research group of the J-PARC Center, KEK, the Japan Atomic Energy Agency, the University of Tokyo, Nagoya University, and RIKEN carried out a muon cooling and radio-frequency acceleration experiment in June 2026 in the experimental area (H2 Area) of the dedicated muon beamline (H-Line), newly constructed at MLF for the muon g-2/EDM experiment ( *4, *5), the transmission muon microscope, and the development of core technologies for artificial, transportable muon beams. As a result, muons were successfully accelerated to a kinetic energy of 0.3 MeV (approximately 8% of the speed of light). Compared with the 2024 demonstration (0.1 MeV, 0.05 muons per second), the group achieved three times the energy and 200 times the intensity (approximately 10 muons per second) in the area where the actual experiments will be performed.
Because this achievement was obtained in the area where the actual experiments will be performed, the research has advanced significantly from the "demonstration" stage of muon acceleration to the "implementation" stage of producing beams for actual experiments. It marks an important step toward the operation of the world's only muon accelerator. The group aims to achieve acceleration to 4 MeV (approximately 30% of the speed of light) around 2027-28, establishing the key technologies for future artificial, transportable muon beams. Ultimately, the muons will be accelerated to even higher energies (above 200 MeV, more than about 90% of the speed of light) for high-precision tests of the Standard Model of particle physics through precision measurements of the muon g-2 and EDM, as well as for applied research in materials science and engineering with the transmission muon microscope.

Research Group

Particle and Nuclear Physics Division, J-PARC Center
Materials and Life Science Division, J-PARC Center
Accelerator Division, J-PARC Center
Muon-Neutron Group, Institute of Particle and Nuclear Studies, KEK
Muon Science Section, Institute of Materials Structure Science, KEK
Accelerator Laboratory, KEK
Department of Physics, Graduate School of Science, The University of Tokyo
J-PARC Center, Japan Atomic Energy Agency
Kobayashi-Maskawa Institute for the Origin of Particles and the Universe, Nagoya University
Laboratory of High Energy Particle Physics, Department of Physics, Graduate School of Science, Nagoya University
RIKEN Center for Advanced Photonics

Background: From Demonstration to Implementation

The world's first demonstration of muon cooling and acceleration in 2024 was performed in a general-purpose experimental area (S2 Area) of the MLF muon facility. The actual muon g-2/EDM experiment, however, will be conducted in the experimental area (H2 Area) of the dedicated beamline (H-Line), which delivers one of the world's highest-intensity pulsed muon beams. The H2 Area was completed in Japanese fiscal year 2024 and began operation in fiscal year 2025.
The research group relocated the muon cooling apparatus (the cooled muon source) from the S2 Area to the H2 Area, and prepared and commissioned a newly built ultraviolet laser system that strips electrons efficiently, as well as the radio-frequency accelerating system for the cooled muons. In this experiment, these devices were combined to perform the entire sequence from muon cooling to acceleration for the first time in the actual experimental area.

What Did We Find?

The positive muon beam delivered to the H2 Area (approximately 30% of the speed of light, kinetic energy 4 MeV) was injected into a silica aerogel target to form muonium (a neutral atom consisting of a positive muon and an electron). By irradiating the muonium with a special ultraviolet laser (Figure 2) to strip the electrons, the group obtained positive muons cooled to a nearly-at-rest state (0.002% of the speed of light, kinetic energy 25 meV). These muons were then injected into the radio-frequency accelerating system and successfully accelerated to a kinetic energy of 0.3 MeV (approximately 8% of the speed of light) (Figure 3).
The data show that the intensity of the accelerated muons was approximately 10 muons per second, 200 times that of the 2024 demonstration (0.05 muons per second) (Figure 4). This is a major advance in both beam quality and intensity toward the realization of the world's first accelerated-muon facility.

Figure 2: The newly built laser system, which can strip electrons efficiently.

Figure 3: he apparatus installed in the H2 Area (from back to front: the muon cooling apparatus, the accelerating system, and the beam diagnostics).

Figure 4: Signal of muons accelerated to 0.3 MeV, measured at the exit of the accelerating system.

How Will the World Change?

There are many accelerator facilities in the world, but no muon accelerator yet exists. With this achievement, the construction and commissioning toward the world's first muon accelerator have started in its dedicated experimental area. The group will successively connect accelerating cavities, further accelerating the muons from the 0.3 MeV achieved this time to reach 4 MeV (approximately 30% of the speed of light) around 2027-28, thereby establishing the fundamental technologies required for artificial, transportable muon generation. Ultimately, the group aims to use a highly directional muon beam accelerated to energies above 200 MeV for ultra-precise tests of possible cracks in the Standard Model of particle physics (the muon g-2/EDM experiment). Accelerated muon beams are also expected to find a wide range of applications, such as the transmission muon microscope, which can observe materials with nanometer resolution, and the transmission imaging of large structures.

Glossary

Acknowledgments

This work was carried out under the J-PARC MLF experimental programs 2011MS06 and 2025MS06. This work was supported by the JST K Program JPMJKP24J4, and by JSPS KAKENHI 20H05625, 22K21350, 24H00023, 24K03211, 25K24694, and 26K21728.

Contacts

  Figure 1: Conceptual illustration of the quantum behavior of muons in water. The blue sphere (muon) replaces a hydrogen atom in a water molecule to form "MuOH" molecule. Below freezing temperatures, the muon interacts with the surrounding water molecules (represented by wavy lines) through an invisible force. The thickness of the wavy lines indicates the strength of this interaction.

Summary

  When muon is injected into ice, it replaces a hydrogen atom in a water molecule to form a unique molecule called MuOH. We discovered that the muon's spin within this molecule exhibits 'quantum coherence,' synchronizing its quantum waves with the nuclear spins of the surrounding hydrogen atoms. This phenomenon accounts for the long-standing mystery behind the signal variations observed below freezing point.

Overview

  We have discovered a new quantum mechanism within ice. When muons are injected into water, they replace one of the hydrogen atoms in a water molecule (H₂O) to form a unique species known as "MuOH". Our findings reveal that the muon spin within this molecule exhibits 'quantum coherence'-a state in which its quantum waves synchronize with the nuclear spins of surrounding hydrogen atoms. This interaction accounts for the signal variations observed in ice-based muon experiments, resolving a mystery that has persisted for decades.

Research Group

  High Energy Accelerator Research Organization (KEK), Institute of Materials Structure Science: Amba Datt Pant (Researcher), Akihiro Koda (Professor), Katsuhiko Ishida (Researcher), Jumpei G. Nakamura (Engineer), Shoichiro Nishimura (Assistant Professor), Masatoshi Hiraishi (Researcher), Koichiro Shimomura (Professor) University of Göttingen, Germany: Burkhard Geil (Professor) Tribhuvan University, Nepal: Anjan Dahal (Researcher), Anup Shrestha (Researcher), Hari Shankar Mallik (Assistant Professor)

A Message from the Researcher

  Amba Datt Pant, Researcher, KEK : After years of dedicated effort, our team has successfully uncovered a quantum effect in water, resolving a mystery that has persisted for over 50 years. This discovery marks a pivotal step in the use of muons to study hydrated materials and biological systems, paving the way for innovative applications in hypoxia imaging.

Research Background and Objectives

  Muon beams act as a 'specialized microscope' for probing the microscopic properties of matter and life. They hold great promise for exploring biological phenomena-such as the functions of proteins and DNA-and for advancing medical applications. We are currently exploring the possibility of utilizing the high-intensity muon beam at J-PARC for cancer research (KAKENHI Grants 21K15583 and 26H01510). However, before we can achieve this, we must first establish a thorough understanding of what muons can reveal about the fundamental building blocks of life, such as water and amino acids. Although water is ubiquitous, it exhibits diverse characteristics depending on temperature and pressure. Despite over 50 years of research into the behavior of muons in water, the underlying microscopic mechanisms and their quantum nature have remained poorly understood. Therefore, we chose to begin our research with the most fundamental substance of all: water.

The Research Motivation

  The turning point in our research occurred when we observed that a fraction of the muon signal from aqueous biological samples closely resembled that of pure water. This realization led us to prioritize a fundamental study of water itself before attempting to analyze more complex biological systems. We meticulously verified these results through repeated experiments and subsequently developed a model to explain the underlying quantum behavior.

Key Challenges We Overcame

  The greatest challenge was re-envisioning this established field from a new perspective. We had to move beyond conventional thinking and adopt a rigorous quantum mechanical approach to truly understand how muon signals behave in both water and ice. Developing a theoretical model that incorporates these quantum interactions-and accurately explains our experimental data-was a significant hurdle that we successfully overcame through persistent effort.

What We have Discovered?

  When a muon beam is injected into water, the charge state of the muons changes depending on whether the water is in a liquid or solid state. In liquid water, muons capture an electron to form "muonium" (a hydrogen atom analog where the proton is replaced by a muon), and muon bound to surrounding molecules (Figure 2). In ice, however, we have observed that the muonium structure is slightly distorted, and that unique "MuOH" molecule, as well as other distinct muon-related species.

  Figure 2: Schematic illustration showing the difference in muon states in water and ice, as observed in conventional experiments.

  Signals (Figure 3) obtained by applying a magnetic field (20 G) to muons and measuring their spin evolution over time reveals a distinct difference between liquid water and ice. In liquid water, a coherent oscillation signal, analogous to the wobbling of a spinning top, persists for a long duration. In contrast, in ice, the oscillation decays rapidly (relaxation), accompanied by a slight shift in the oscillation period (frequency shift).
  Despite over 50 years of research, these signal decays and frequency shifts in ice remained poorly understood. We proposed a model in which the muon spins within MuOH molecules 'interact quantum-mechanically' with the surrounding water molecules. Using this model, we successfully and consistently explained the experimental data without the need for ad-hoc mathematical adjustments. Furthermore, we confirmed the validity of this model through experiments with heavy water (D₂O), leading to the discovery of a 'quantum effect' of muons in ice.

  Figure 3: Muon spin rotation signals in liquid water (red circles) and ice (black circles). In liquid water, coherent oscillations persist, whereas in ice, the signal undergoes rapid decay (relaxation) and a frequency shift. This behavior is attributed to quantum-mechanical interactions of muon spins.

How Will This Change the Scientific World?

  Water is essential for life on Earth. Gaining a deeper understanding of the microscopic quantum behavior within water is critical for advancing both biology and physics. This discovery unveils a fundamental scientific mechanism, offering new insights that are expected to drive innovation across a wide range of research fields.

Acknowledgments

  The muon experiments were performed at the Materials and Life Science Experimental Facility of the J-PARC under a user program (Proposals 2022B0087, 2023B0217 and 2024A0304) at MLF, J-PARC. This work is supported by Grant-in-Aid for Scientific Research of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, (KAKENHI Grant No. 21K15583). The spin-polarization function was developed with partial support from KAKENHI (Grant No. 22K05275).

Paper information

Title	Origin of muon spin relaxation and frequency shift in frozen water explained by spin-dipole quantum coherences
Authors	Amba Datt Pant, Akihiro Koda, Burkhard Geil, Katsuhiko Ishida, Anjan Dahal, Anup Shrestha, Hari Shankar Mallik, Jumpei G. Nakamura, Shoichiro Nishimura, Masatoshi Hiraishi, and Koichiro Shimomura
Journal	Physical Review B (published online on July 7, 2026)
DOI	https://doi.org/10.1103/jvjm-bn2q
Other related recent papers	1. Amba Datt Pant, Akihiro Koda, Burkhard Geil, Katsuhiko Ishida, Roshan Pudasaini, Kazuaki Kuwahata, Masanori Tachikawa, Stephen P. Cottrell, Jumpei G. Nakamura, Shoichiro Nishimura, and Koichiro Shimomura, Muon Species in Frozen D2O Observed with Zero-field Muon Spin Precession, Journal of the Physical Society of Japan 95,1 (2026) 014603. 2. Amba Datt Pant, Akihiro Koda, Burkhard Geil, Katsuhiko Ishida, Rajendra Adhikari, Kazuaki Kuwahata, Masanori Tachikawa, and Koichiro Shimomura, Formation and structure of MuOH in ice studied by muon spin rotation, Physical Review B 110, 10 (2024) 104104.

Contact

  The T2K experiment in Japan and the NOvA experiment in the United States conducted a joint analysis and published their first results in the journal Nature. Both are long-baseline neutrino oscillation experiments using accelerators, and by leveraging their different baselines and energy conditions, they achieved precision measurements of neutrino oscillations. As a result, they succeeded in reducing the uncertainty in the differences between neutrino masses to below 2%. Although the ordering of the three neutrino masses is still unknown, their results show that depending on this ordering, the magnitude of CP symmetry violation-a difference in behavior between particles and antiparticles-would be strongly constrained. This achievement marks an important step toward uncovering CP symmetry violation in neutrinos and the origin of the matter-antimatter asymmetry in the universe. The joint analysis combined ten years of T2K data collected since 2010 and six years of NOvA data collected since 2014, and it also demonstrates the strength of collaboration between two international experiments that are competitive yet complementary.

Fig. 1 : T2K in Japan (left) and NOvA in the United States (right) are both long-baseline experiments: they each shoot an intense beam of neutrinos that passes through both a near detector close to the neutrino source and a far detector hundreds of kilometers away. Both experiments compare data recorded in each detector to learn about neutrinos' behavior and properties.

Context

  When the universe began, physicists expect there should have been equal amounts of matter and antimatter. But if that were so, the matter and antimatter should have perfectly canceled each other out, resulting in total annihilation.
  And yet, here we are. Somehow, matter won out over antimatter - but we still don't know how or why.
  Physicists suspect the answer may lie in the mysterious behavior of abundant yet elusive particles called neutrinos. Specifically, learning more about a phenomenon called neutrino oscillation - in which neutrinos change types, or flavors, as they travel - could bring us closer to an answer.
  The international collaborations representing two neutrino experiments, T2K in Japan and NOvA in the United States, recently combined forces to produce their first joint results, published today in the journal Nature. This initial joint analysis provides some of the most precise neutrino-oscillation measurements in the field.
  "These results are an outcome of a cooperation and mutual understanding of two unique collaborations, both involving many experts in neutrino physics, detection technologies and analysis techniques, working in very different environments, using different methods and tools," says T2K collaborator Tomáš Nosek.

Different experiments, common goals

  Despite their ubiquity, neutrinos are very difficult to detect and study. Even though they were first seen in the 1950s, the ghostly particles remain deeply enigmatic. Filling in gaps in our knowledge about neutrinos and their properties may reveal fundamental truths about the universe.
  T2K and NOvA are both long-baseline experiments: they each shoot an intense beam of neutrinos that passes through both a near detector close to the neutrino source and a far detector hundreds of miles away. Both experiments compare data recorded in each detector to learn about neutrinos' behavior and properties.
  NOvA, the NuMI Off-axis νe Appearance experiment, sends a beam of neutrinos 810 kilometers from its source at the U.S. Department of Energy's Fermi National Accelerator Laboratory near Chicago, Ill., to a 14,000-ton liquid-scintillator detector in Ash River, Minnesota.
  The T2K experiment's neutrino beam travels 295 kilometers from Tokai to Kamioka - hence the name T2K. Tokai is home to the Japan Proton Accelerator Research Complex (J-PARC) and Kamioka hosts the Super-Kamiokande neutrino detector, an enormous tank of ultrapure water located a kilometer underground.
  Since the experiments have similar science goals but different baselines and different neutrino energies, physicists can learn more by combining their data.
  "By making a joint analysis you can get a more precise measurement than each experiment can produce alone," says NOvA collaborator Liudmila Kolupaeva. "As a rule, experiments in high-energy physics have different designs even if they have the same science goal. Joint analyses allow us to use complementary features of these designs."
  As long-baseline experiments, NOvA and T2K are ideal for studying neutrino oscillations, a phenomenon that can provide insight into open questions like charge-parity violation and the neutrino mass ordering. Two experiments with different baselines and energies have a better chance of disentangling the two effects than one experiment alone.

Interrogating neutrino oscillations

  The mystery of neutrino mass ordering is the question of which neutrino is the lightest. But it isn't as simple as placing particles on a scale. Neutrinos have miniscule masses that are made up of combinations of mass states. There are three neutrino mass states, but, confusingly, they don't map to the three neutrino flavors. In fact, each flavor is made of a mix of the three mass states, and each mass state has a different probability of acting like each flavor of neutrino.
  There are two possible mass orderings, called normal or inverted. Under the normal ordering, two of the mass states are relatively light and one is heavy, while the inverted ordering has two heavier mass states and one light.
  In the normal ordering, there is an enhanced probability that muon neutrinos will oscillate to electron neutrinos but a lower probability that muon antineutrinos will oscillate to electron antineutrinos. In the inverted ordering, the opposite happens. However, an asymmetry in the neutrinos' and antineutrinos' oscillations could also be explained if neutrinos violate CP symmetry - in other words, if neutrinos don't behave the same as their antimatter counterparts.
  The combined results of NOvA and T2K do not favor either mass ordering. If the neutrino mass ordering is found to be normal, NOvA's and T2K's results are less clear on CP symmetry, requiring additional data to clarify. However, if future results show the neutrino mass ordering is inverted, the results published today provide evidence that neutrinos violate CP symmetry, potentially explaining why the universe is dominated by matter instead of antimatter.
  "Neutrino physics is a strange field. It is very challenging to isolate effects," says Kendall Mahn, co-spokesperson for T2K. "Combining analyses allows us to isolate one of these effects, and that's progress."
  The combined analysis does provide one of the most precise values of the difference in mass between neutrino mass states, a quantity called Δm232. With an uncertainty below 2%, the new value will enable physicists to make precision comparisons with other neutrino experiments to test whether the neutrino oscillation theory is complete.

What's next

  These first joint results do not definitively solve any mysteries of neutrinos, but they do add to physicists' knowledge about the particles. Plus, they validate the impressive collaborative effort between two competing - yet complementary - experiments.
  The NOvA collaboration consists of more than 250 scientists and engineers from 49 institutions in eight countries. The T2K collaboration has more than 560 members from 75 institutions in 15 countries. The two collaborations began active work on this joint analysis in 2019. It combines 6 years of data from NOvA, which began collecting data in 2014, and a decade of data from T2K, which started up in 2010. Both experiments continue to take data, and efforts are already underway to update the joint analysis with the new data.
  "The joint analysis work has benefited both collaborations," says Patricia Vahle, co-spokesperson for NOvA. "We have a much better mutual understanding of the strengths and challenges of the different experimental setups and analysis techniques."
  NOvA and T2K are the only currently operating long-baseline neutrino experiments. Their initial combined results lay a foundation for forthcoming neutrino experiments that will answer the questions around neutrinos unambiguously.
  The Fermilab led Deep Underground Neutrino Experiment is under construction in Illinois and South Dakota in the U.S. With its longer baseline of 1,300 kilometers, DUNE will be more sensitive to neutrino mass ordering and could give physicists a conclusive answer shortly after it turns on in the early years of the next decade.
  In Japan, Hyper-Kamiokande, the successor to Super-Kamiokande, is currently under construction in an underground mine in Kamioka, Hida City, Gifu Prefecture, with experiments scheduled to begin in 2028. Hyper-Kamiokande will conduct highly sensitive searches for CP symmetry violation through high-statistics measurements made possible by a detector about eight times larger and an intense neutrino beam.
  Many physicists hope these next-generation neutrino experiments can come together - as NOvA and T2K have already done - to make progress on their shared scientific goals to learn more about neutrinos and their unusual properties.
  "As shown in this very analysis, there are no truly 'rivaling' experiments because they all share a common goal of scientific study of a phenomenon," says Nosek. "Collaborating is naturally important for the transfer of knowledge, know-how and experience, and for sharing resources, ideas and tools. The T2K-NOvA collaboration is not merely a sum of T2K and NOvA collaborations. It is much, much more."

Fig. 2: The T2K collaboration (left) and the NOvA collaboration (right).

Publication Details

Media contacts for further inquires

  A research group led by Professor Yasutaka Kitagawa of Osaka University, Professor Kei Wada of Miyazaki University, and Professor Masaki Unno of Ibaraki University (in collaboration with researchers from Tokyo University of Pharmacy and Life Sciences, Kurume University, CROSS, and JASRI) has revealed a mechanism for controlling the potential of an "electron carrier" protein in the redox reaction that all organisms need to obtain energy. Based on experiments using the Ibaraki Biological Crystal Diffractometer (iBIX) at the Materials and Life Science Experimental Facility (MLF) in the Japan Proton Accelerator Research Complex (J-PARC), the precise 3D structure of the protein including hydrogen atoms was determined, and theoretical calculations using this data visualized the electronic structure of the iron-sulfur cluster. As the results, it was revealed, for the first time, that the electric potential of the iron-sulfur cluster changes dramatically depending on the presence or absence of a single hydrogen atom at an amino acid side chain, a so-called "nano-switch" mechanism.
  This study was published in the online edition of the international scientific journal eLife^*1 on November 15, 2024 (Reviewed Preprint).

Background

  In living organisms, electrons are constantly being transferred between substances. Donating electrons to a substance is called "reduction"; drawing electrons from a substance is called "oxidation". These repeated reactions are called "redox reactions". Respiration and photosynthesis are typical examples of redox reactions in living organisms.
  In living organisms, various proteins assist in redox reactions, some of which contain clusters of iron and sulfur (iron-sulfur clusters), and these iron-sulfur clusters play important functions in the transfer of electrons between proteins. Ferredoxin, a small protein containing iron-sulfur clusters, is thought to be present in almost all living organisms and is a typical example of an "electron carrier". The history of ferredoxin research is old; it began 60 years ago. Various types of ferredoxins with various iron-sulfur clusters which contain different numbers of constituent irons and sulfurs have been discovered (Figure 1).

 Figure 1. Examples of typical iron-sulfur clusters. The yellow-green spheres are sulfurs, and the gray spheres are irons. There are cases in which some of these are present in combination in a single protein molecule. The letters Cys represent sulfurs of cysteines in the protein.

  While water flows from high to low by nature, electrons flow from low to high potential energy ( "potential" or "electrostatic potential" ). The potentials (redox potentials) of ferredoxins, which have various types of iron-sulfur clusters, are diverse and wide-ranging. Ferredoxins raise and lower the redox potentials of their iron-sulfur clusters like an elevator, sometimes giving electrons to other proteins and sometimes withdrawing them from others. However, how this is controlled remains unclear.
  Although theoretical calculations based on a method called "density functional theory"^*2 can be used to study the electronic structure^*3 of ferredoxin, it requires an accurate 3D structure of ferredoxin including the hydrogen atoms for accurate calculations. However, since it is extremely difficult to determine the position of hydrogen atoms in a protein molecule, theoretical calculations for most proteins, including ferredoxin, have conventionally used "assumed" positions of hydrogen atoms. If the assumed positions of the hydrogen atoms differ from the exact ones, the most fundamental assumptions of the theoretical calculations would break down and the obtained conclusions would be meaningless. The research group, therefore, set out to experimentally determine the positions of hydrogen atoms in ferredoxin, and to elucidate the electronic structure of the iron-sulfur cluster based on experimental facts.

Research Methods

  Currently, X-ray crystallography (Nobel Prize in Chemistry 1962) and cryo-electron microscopy (Nobel Prize in Chemistry 2017) are commonly used to analyze the 3D structure of proteins with resolutions at the atomic levels, however, these methods are not suitable for identifying the smallest hydrogen atom. In addition, even the protein structure prediction algorithm (AlphaFold), which won this year's (2024) Nobel Prize in Chemistry, cannot predict the exact locations of hydrogen atoms to date. Therefore, in this study, we used neutron crystallography^*4, which can identify hydrogen atoms with the same degree of clarity as other atoms in proteins.
  In this research, after overcoming the high hurdle of growing very large ferredoxin crystals, we patiently collected data using neutrons at the Ibaraki Biological Crystal Diffractometer (iBIX) ^*5 in the Materials and Life Science Experimental Facility (MLF) at the Japan Proton Accelerator Research Complex (J-PARC) ^*6 in Tokai-mura, Ibaraki Prefecture The data was collected using neutrons.
  Furthermore, using the experimentally determined precise structure, the electrons in the iron-sulfur cluster were investigated by theoretical calculations based on quantum mechanics and quantum chemistry (density functional theory). The results obtained were verified by using various mutants of ferredoxin, in which one amino acid in ferredoxin was altered by gene manipulation, as a sample, and by conducting experiments in a chamber where oxygen was excluded to the utmost limit to prevent oxidation of the iron-sulfur clusters by air.

Results

  In this study, the 3D structure of ferredoxin, which has four iron and four sulfur clusters in the molecule ([4Fe-4S]-type clusters; Figure 1), was determined by neutron crystallography, and the exact positions of atoms including hydrogen around the iron-sulfur cluster were experimentally determined (Figure 2). The actual positions of hydrogen atoms around the iron-sulfur cluster were found to be different from the previously predicted position (Figure 3). Based on the exact positions of the hydrogen atoms, the electronic structure around the iron-sulfur cluster was calculated theoretically, and it was found, for the first time, that electrons originating from the iron-sulfur cluster are distributed not only around the iron-sulfur cluster but also to aspartic acid 64 (Asp64) at a distance of more than 1 nm (nanometer = 0.000001 mm) away from the cluster. (Figure 3 and Figure 4). (1 nm is too long a distance in a protein molecule for direct interaction.) Interestingly, this electron distribution was observed only in the absence of a hydrogen atom (-COO-) in the side chain (carboxy group: -COOH) of Asp64, while in the presence of a hydrogen atom (-COOH), the electrons were distributed only around the iron-sulfur cluster (Figure 4). In fact, by measuring the rate at which the iron-sulfur cluster is oxidized and the redox potential, we proved that Asp64 has a significant effect on the reactivity of the iron-sulfur cluster. Although ferredoxin contains multiple aspartic acids, only Asp64 showed such a phenomenon. Furthermore, in ferredoxins from various microorganisms, aspartic acid residues in similar 3D positions were found to affect the electronic structure of the iron-sulfur cluster.
  In this study, we elucidate for the first time in the world the existence of a "nano-switch mechanism" in which the presence or absence of a single hydrogen atom in the aspartic acid side chain changes the electronic state of the iron-sulfur cluster (Figure 5). This nano-switch mechanism has also been shown to be conserved in archaea and is believed to be widely used in the biological world.

Figure 2. Left: The overall structure of [4Fe-4S]-type ferredoxin containing hydrogen atoms, which was successfully analyzed using neutrons in this study. Hydrogen atoms are highlighted by gray spheres. Right: An X-ray image of the overall structure of the same ferredoxin that was previously known.

Figure 3. Structure around the iron-sulfur cluster. Hydrogen bonds are indicated by dotted lines. For example, threonine 63 was initially thought to be hydrogen bonded to the iron-sulfur cluster (circled in red), however, neutron crystallography showed that the hydrogen atom of its -OH was in the other direction, forming hydrogen bonds with the main chain of threonine 10.

Figure 4. Structure around the iron-sulfur cluster. Hydrogen bonds are indicated by dotted lines. For example, threonine 63 was initially thought to be hydrogen bonded to the iron-sulfur cluster (circled in red), however, neutron crystallography showed that the hydrogen atom of its -OH was in the other direction, forming hydrogen bonds with the main chain of threonine 10.

Figure 5. A schematic drawing of the electron transfer mechanism by ferredoxin that revealed in this study.

Future Expectations

  Iron-sulfur clusters in protein molecules are involved in various reactions that play fundamental roles in biological activities. The present redox potential control switch mechanism can be applied to the control of those reactions. For example, in proteins that detect oxygen (O₂) and nitric oxide (NO) in vivo, it is the iron-sulfur cluster that detects very small amounts of gases. Also, in many microorganisms, including pathogenic bacteria, [4Fe-4S]-type iron-sulfur clusters in proteins play essential roles in energy acquisition. More recently, ferredoxin and iron-sulfur clusters have been found to play important roles in cancer cells. The findings of this research will not only deepen our scientific understanding of biological reactions but also provide a major clue for the future development of ultra-sensitive sensors of O₂ and NO and novel drugs (e.g., anti-cancer drugs, antibiotics against pathogens, etc.).

Terminologies

Research Funds

  These results were obtained through the following research projects and financial supports.
  ● Ministry of Education, Culture, Sports, Science and Technology Grants-in-Aid for Transformative Research Areas (A), and Japan Society for the Promotion of Science (JSPS), Grant in Aid for Scientific Researches (B) and (C)
  ● Research Grant from Takeda Science Foundation, Enzyme Research Grant from the Japan Foundation for Applied Enzymology, and Ibaraki Prefecture Leading Research Project Fund (Sendo-kenkyu)

Paper Information

Title	Protonation/deprotonation-driven switch for the redox stability of low potential [4Fe-4S] ferredoxin
Authors	Kei Wada*, Kenji Kobayashi†, Iori Era†, Yusuke Isobe, Taigo Kamimura, Masaki Marukawa, Takayuki Nagae, Kazuki Honjo, Noriko Kaseda, Yumiko Motoyama, Kengo Inoue, Masakazu　Sugishima, Katsuhiro Kusaka, Naomine Yano, Keiichi Fukuyama, Masaki Mishima, Yasutaka Kitagawa*, Masaki Unno* *corresponding authors †equally contributed
Journal	eLife
Publish date	Nov. 15th, 2024 (Reviewed Preprint)
DOI	10.7554/eLife.102506

Researchers Information

  Yasutaka Kitagawa
     Graduate School of Engineering Science, Osaka University
  Kei Wada
     Department of Medical Sciences, University of Miyazaki
  Masaki Unno
     Graduate School of Science and Engineering, Ibaraki University
  Masaki Mishima
     Department of Molecular Biophysics, Tokyo University of Pharmacy and Life Sciences
  Masakazu Sugishima
     Department of Medical Biochemistry, Kurume University School of Medicine
  Katsuhiro Kusaka
     Neutron Science and 20 Technology Center, Comprehensive Research Organization for Science and Society (CROSS)
  Naomine Yano
     Structural Biology Division, Japan Synchrotron Radiation Research Institute (JASRI)

Electromagnets at J-PARC's Main Ring

50 words summary

  We surpassed our initial target, achieving a beam power of 760 kW, a crucial indicator at the Japan Proton Accelerator Research Complex (J-PARC), while concurrently achieving significant energy savings. This success paves the way for the Hyper Kamiokande project, with expectations of a drastic enhancement of the T2K experiment.

Overview

  The Japan Proton Accelerator Research Complex (J-PARC) in Tokai Village, Ibaraki Prefecture, is a high-intensity proton accelerator facility jointly constructed and operated by the High Energy Accelerator Research Organization (KEK) and the Japan Atomic Energy Agency (JAEA). J-PARC conducts cutting-edge research across a wide range of research, including elementary particle physics, atomic nuclei physics, condensed matter physics, chemistry, material science, and biology. The administrative entity overseeing its operations is called the J-PARC Center.

  The proton beam undergoes staged acceleration through three accelerators: a linear accelerator (LINAC), a circular accelerator called the Rapid-Cycling Synchrotron (RCS), capable of accelerating particles up to 3 GeV (gigaelectron volts, a unit of kinetic energy), and another circular accelerator called the Main Ring. Through these accelerators, proton beams are supplied to experimental facilities, including the Neutrino Experimental Facility.

  Accelerated protons collide with targets, generating particles (neutrons, K mesons, and neutrinos) used in experiments. Experiment sensitivity depends on the total produced particles, directly proportional to incident protons. Higher sensitivity requires more protons. Achieving this relies on 'beam power' (*1), the number of accelerated protons per unit time, measured in kilowatts (kW).

  The Main Ring is a synchrotron circular accelerator initially designed with a target performance of 750 kW. Through numerous beam adjustments and improvements, the beam power reached 500 kW in 2019. Substantial upgrades to the accelerator were implemented, and since the fiscal year 2022, it has been in acceleration tuning operation. On December 25, 2023, it successfully achieved a record-breaking beam power of 760 kW, as shown in Graph 1.

Graph 1: Annual Maximum Beam Power of the Main Ring

  We have implemented a technique that employs large-capacity capacitors for the power supplies for the electromagnets, utilizing magnetic force to guide the beam into a circular orbit. This innovative approach efficiently repurposes the energy stored in the magnets, enabling us to achieve a beam power of 760 kW with the same power consumption as before the Main Ring enhancement. This translates to an approximately 1.5 times increase in beam power without an increase in power consumption.

  At the Neutrino Experimental Facility, which utilizes protons accelerated in the Main Ring, the apparatus for generating neutrinos has been strengthened to endure high beam power, with improvements in radiation shielding. On December 25, 2023, stable and continuous production of neutrinos was achieved at a record-breaking beam power of 760 kW.

A word from the researchers

  Professor IGARASHI Susumu from the Accelerator Division at J-PARC Center：  Through the efforts of each team member and excellent teamwork in equipment upgrades and beam tunings, we have successfully reached a long-awaited goal that involved the dedication of many individuals. I am deeply moved and relieved by this accomplishment. Looking ahead, I am committed to continuing our research and striving for even higher achievements.

Why did you start this research?

  One of the key experiments utilizing the J-PARC Main Ring beam is the T2K experiment (Tokai to Kamioka Long-baseline Neutrino Oscillation Experiment, *2). This experiment aims to investigate the fundamental properties of neutrinos, elementary particles. Protons accelerated up to 30 GeV in the Main Ring, are extracted and directed to the neutrino experimental facility, where they irradiate a target.

  The neutrinos, created by converging the secondary particles generated there, are observed by the Super-Kamiokande detector located 295 km away in Kamioka-cho, Hida City, Gifu Prefecture. To precisely measure the changes caused by the long-distance flight of neutrinos, which have minimal interactions with matter, a substantial number of neutrinos are needed. This, in turn, requires a high beam power of protons, the particles generate neutrinos.

Overview of T2K Experiment

Where did you put in the effort?

  The Main Ring, operating as a synchrotron-type accelerator, receives a 3 GeV beam from the preceding accelerator, accelerates it to 30 GeV, and emits a single pulse toward the experimental facility. The electromagnets' magnetic field is then adjusted back to 3 GeV for the next beam injection, and this cycle is repeated. To enhance the beam power in the Main Ring, the following modifications are necessary.

  The T2K Collaboration has started data taking using the enhanced neutrino beam and new neutrino near-detectors from December 2023. The KEK/J-PARC center has upgraded the main ring accelerator and the neutrino beamline to increase the beam power. T2K has also upgraded its neutrino production instruments. The stable operation of neutrino beam has been successfully achieved at a record high beam intensity (about 710 kW), an increase of about 40% compared to before the upgrade. Furthermore, on December 25^th, the continuous operation of neutrino beam has been successfully achieved at 760kW, which is greater than the initial design beam power.

  The pulsed electromagnet (electromagnetic horn) system, the heart of the neutrino generator, was also upgraded. The current applied to the electromagnetic horn has been increased from 250 kA to 320 kA. This allowed us to increase the neutrino intensity by about 10%. In addition, T2K installed new neutrino detectors that can measure neutrino interactions with even higher precision than before. The newly installed detectors consist of SuperFGD, which detects tracks around a neutrino interaction point inside the detector, High-Angle TPC, which measures momentum of particles emitted over a wide range of angles, and Time-of-Flight, which can detect incoming or outgoing particles and identify particles. Neutrino event candidates were successfully observed during a technical run of the new detectors after the start of beam operation. In 2020, the T2K gave the first hints that the symmetry between matter and antimatter could be violated in neutrino oscillations. With these enhancements, T2K will continue to lead the world in advancing the understanding of neutrino properties and unraveling the mystery of the absence of antimatter in the universe.

  Fig. 1 : Illustration of the upgraded neutrino production devices (left) and the new neutrino detectors (right).

Context

  T2K is an experiment to study neutrino oscillations ^(*1) by sending neutrinos produced at the Japan Proton Accelerator Research Complex (J-PARC) in Tokai, Ibaraki, to the neutrino near-detectors and the Super-Kamiokande detector in Kamioka, Hida, Gifu, about 300 km away. The T2K experiment started taking data in 2010 and directly observed electron neutrino appearance for the first time in the world in 2013. In 2014, we started measurements using an anti-neutrino beam to verify CP violation ^(*2) , and in 2020 we strongly restricted possible values of the neutrino CP phase ^(*3) for the first time. To obtain evidence of the CP violation, a more precise measurement is required to eliminate 0 and ±180 degrees from the possible range of CP phase with a high degree of confidence. However, in order to realize this, it is necessary to produce more neutrinos and to obtain a deeper understanding of the interactions between neutrinos and nucleus.

Points of improvement

  The T2K Collaboration, an international collaborative experiment involving approximately 570 researchers from 78 research institutions in 14 countries, has launched a new phase of the experiment using an enhanced neutrino beam and new near-detectors. Neutrinos are produced from decayed pions or other particles produced in interactions between proton beams and a graphite target. The KEK/J-PARC Center upgraded the J-PARC main ring accelerator, including the power supply for the main magnet, to increase the repetition rate of the proton beam from 2.48 seconds to 1.36 seconds, which supplies more protons to the neutrino production target. The T2K experimental group upgraded, modified, and exchanged instruments in the neutrino beam facility such as targets, electromagnetic horns, and beam monitors. The beam commissioning was started in November 2023. The stable production of neutrino beam has been successfully achieved at a record high beam power (about 710 kW), an increase of about 40% compared to before the upgrade. Furthermore, on December 25^th, the continuous operation of neutrino beam has been successfully achieved at 760kW, which is greater than the initial design beam power. The heart of the neutrino generator is the electromagnetic horns (Fig.2). The current applied to the three electromagnetic horns was increased from 250kA to 320 kA by upgrading the power supply and other components, thereby improving the focusing efficiency of parent particles such as pions produced at the target. This improves the quality of the neutrino beam delivered to the Super-Kamiokande detector while increasing the number of neutrinos observed by about another 10%.

  Fig. 2: Upgraded 2^nd electromagnetic horn with improved cooling capacity to enable neutrino production by high-intensity proton beams.

  In addition, the T2K Collaboration has started observation using new neutrino near-detectors(Fig.3) at the Neutrino Monitor Building located 280 m downstream of the neutrino production target. By October 2023, three new types of detectors have been installed. A new detector, SuperFGD with a mass of approximately 2 tons of sensitive volume, is located at the center of the upgraded detectors. It has an innovative structure consisting of approximately 2 millions 1cm3 cubes each with 3 holes, made of plastic scintillator. Approximately 56,000 optical fibers penetrate the cubes from three directions and photodetectors are at the ends of the fibers.

  Charged particles can be observed in high resolution from three projections and 3D tracks can be reconstructed. Below the SuperFGD, the first High Angle Time Projection Chambers (HATPC) has been installed. The HATPC is a gaseous detectors composed by two field cages that produce a uniform electric field and that uses, as readout system, resistive Micromegas modules. HATPC is an innovative detector that allow for an excellent reconstruction of the track trajectory and hence of the momentum of particles emitted by neutrino interactions in the SuperFGD. Finally, the detectors surrounding the SuperFGD and HATPC are Time-of-Flight detectors. It is used to determine the direction of particles and particle identification. T2K started measurements with the upgraded neutrino beam in December 2023 and succeeded in observing neutrino event candidates from the newly acquired data (Fig. 4, 5).

  Fig. 3: A photo of the new detectors.

 Fig. 4: An event display of a neutrino interaction candidate in the SuperFGD with a track entering the bottom HATPC while another track is entering the original detectors.

 Fig. Fig. 5: The beam timing structure observed with one of the new detectors, Time-of-Flight.

Outlook

  With these improvements, the T2K experiment enters a new phase with an enhanced neutrino beam and novel new detectors. The J-PARC accelerator and neutrino experimental facility are undergoing an upgrade plan to further increase the output power to 1.3 MW (= 1300 kW) while supplying beams for the T2K experiment. Together with the upgraded neutrino production instruments such as the electromagnetic horns with improved focusing efficiency, it will be possible to observe about three times as many neutrino interactions (per unit time) as before, and to reduce the error originating from statistical variations (statistical error) in the observed data. In addition, the new detector can detect large angle scattering in the neutrino interactions, which was not possible with our original detector. This will enable a better understanding of the neutrino-nucleus interactions and therefore reduces the systematic errors. Furthermore, the Super-Kamiokande detector has also improved its detector performance with a much higher neutron detection efficiency by loading gadolinium in the water. The T2K experiment will significantly improve the sensitivity of the measurements through these improvements and proceed to verify the differences in neutrino and antineutrino behavior. The J-PARC high-intensity proton accelerator and neutrino experimental facility are expected to play a key role in the next generation of neutrino research. The new phase of the T2K experiment is an important step toward the next generation of experiments and T2K is expected to continue to lead the world in neutrino research unraveling the mystery of the missing antimatter from our universe.

Media contacts for further inquires

  Figure 1: Experimental apparatus for muonic helium atom spectroscopy measurements in the D2 area of J-PARC MLF MUSE (left), and view of the experimental apparatus being assembled from above (right).

80 words summary

  Precision spectroscopy measurements of the muonic helium atom energy structure were performed at J-PARC MLF MUSE D-line more precisely than previous measurements done 40 years ago, improving the current world record by a factor of 1.5. This is an important milestone for the coming measurements planned at the H-line with higher muon beam intensity and under a high magnetic field that will permit one to improve further the precision a hundred times and determine more precisely the negative muon mass.

Overview

  A muonic helium atom, which is composed of an ordinary helium atom with one of its two electrons replaced by a negative muon^※1, is a special atom that is not found in nature. It has a small energy level structure, called hyperfine structure^※2, that results from the interaction between the remaining electron and the negative muon (due to the intrinsic properties of their respective spin).
  Precision spectroscopy measurements of the muonic helium atom hyperfine structure are the only available experimental results for three-body muonic atoms (i.e., helium nucleus, electron, and negative muon) that can be used to verify and improve the current theory of particle physics for three-body atomic system and quantum electrodynamics (QED), which accurately describes how matter and light interact at the quantum level. It can also be used to determine the mass of the negative muon to test CPT invariance by comparing the masses of positive and negative muons (second-generation leptons).
  The hyperfine structure of muonic helium atoms has only been measured twice in the 1980s at the Paul Scherrer Institute in Switzerland (directly at zero magnetic field) and Los Alamos National Laboratory in the United States (indirectly at high magnetic field).
  In this study, we succeeded in measuring the hyperfine structure of muonic helium atoms directly at zero magnetic field using the Muon Science Facility (MUSE) D-line at the Materials and Life Science Experimental Facility (MLF) of the Japan Proton Accelerator Research Complex (J-PARC^※3) with a precision 3 times better than the previous direct measurement.
  The result obtained is also 1.5 times more precise than the previous indirect measurement at high field improving the current world record and establishing a highly precise spectroscopic method. It was also performed for the first time with methane admixture used as an electron donor to form neutral muonic helium atoms efficiently, the prerequisite to measuring the hyperfine structure.
  This technique will soon be used at the H-line, which offers about ten times higher muon beam intensity than at the D-line and the possibility of longer measurement time. This will allow us to measure the hyperfine structure of muonic helium atoms at high magnetic field a hundred times more precisely and determine the negative muon mass with greater precision.

※1 Muon
  A muon is an elementary particle with properties similar to the electron but with a mass about 200 times greater. Both positively and negatively charged muons exist. They can be found naturally in our surroundings, raining down to Earth from space as cosmic rays. However, muons decay in a very short time (about 2 microseconds lifetime). Negative muons can orbit nuclei the same way electrons do to produce a "muonic atom" with different characteristics from ordinary atoms because muons are much heavier.

※2 Hyperfine structure (HFS)
  A small energy structure of an atom caused by the interaction between electrons and the nuclei. It is currently used in the cesium atomic clock to define the length of a second.

※3 J-PARC
  J-PARC is a large-scale research facility jointly operated by the High Energy Accelerator Research Organization (KEK) and the Japan Atomic Energy Agency (JAEA) in Tokai-mura, Ibaraki Prefecture, Japan. J-PARC is a multi-purpose and multidisciplinary facility that is unique in the variety of secondary-particle beams produced and put to use in cutting-edge research across a wide range of scientific fields, from academic research in particle physics, nuclear physics, condensed matter physics, chemistry, materials science, and biology, to applied research in industrial fields.

Research Group

Institute for Materials Structure Science (IMSS), High Energy Accelerator Research Organization (KEK)	P. Strasser (lecturer), R. Iwai (research fellow), S. Kanda (assistant professor), S. Nishimura (special assistant professor), and K. Shimomura (professor).
Department of Physics, Nagoya University	S. Fukumura (graduate student), S. Kawamura (graduate student), M. Kitaguchi (associate professor), H. M. Shimizu (professor), and H. Tada (graduate student).
Graduate School of Science, The University of Tokyo	H. A. Torii (associate professor).
Graduate School of Arts and Sciences, The University of Tokyo	S. Seo (graduate student).

A word from the researchers

  There is nothing like good teamwork to achieve new scientific breakthroughs and reach exciting new frontiers.

Why did you start this research?

  The hyperfine structure of muonic helium atoms can be precisely measured to determine the mass of negatively charged muons, one of the fundamental physical constants^※4, and by comparing it with theoretical predictions, we can also verify the current theory of particle physics. However, there has been no precise measurement of the hyperfine structure of muonic helium atoms since their first measurements back in the 1980s.
  In recent years, however, the MuSEUM^※5 collaboration at J-PARC has developed a technique for measuring the hyperfine structure of muonium. Muonium and muonic helium are very similar (Fig. 2), both hydrogen-like atoms. They have almost equal hyperfine structures, but these are inverted because of the opposite charge of the muon. The same microwave magnetic resonance technique has the potential to measure the hyperfine structure of muonic helium atoms with a precision up to nearly 100 times greater than the current level. That is how this research was initiated.

※4 Fundamental physical constants
  Physical quantities that express the fundamental properties of the natural world and are universal, and immutable throughout the Universe.

※5 MuSEUM Collaboration
  Abbreviation for "Muonium Spectroscopy Experiment Using Microwave". An experiment to investigate the energy structure of a special type of atom called muonium, in which an electron orbits around a positively charged muon. Muonium does not exist in nature.

  Figure 2: Comparison between a muonium atom (left) and a muonic helium atom (right).

Where did you get your inspiration?

  We noted that precisely determining the mass of negatively charged muons using muonic helium atoms allows for testing the CPT theorem^※6 by combining it with other experimental results on the mass of positively charged muons (antiparticles).
  Additionally, the gas used in the experiment has distinctive characteristics. To form neutral muonic helium atoms, it is necessary to mix helium gas with a small amount of foreign gas that acts as an electron donor. Indeed, when the muon is captured by a helium atom, it quickly ejects both electrons and forms a singly charged muonic helium ion. Subsequently, it cannot capture an electron from neighboring helium atoms because its electron binding energy is much smaller, like hydrogen. In previous experiments performed in the 1980s, the noble gas xenon was used as an electron donor because it is easily ionizable. This time, we used methane (CH₄) because it is more efficient, absorbs fewer muons than xenon, which has a higher atomic number, and is less expensive.

※6 CPT theorem
  The CPT theorem says that the fundamental symmetry of physical laws under any combination of charge conjugation (C), parity inversion (P), and time reversal (T) holds for all physical phenomena. This means that any particle and its antiparticle must have the same mass, the same magnetic moment (with opposite sign), and the same lifetime.

  Figure 3: Diagram showing the relationship between this research and other ongoing experiments at J-PARC and other research facilities.

Where did you put in the effort?

  The precision of measuring the hyperfine structure of muonic helium atoms is mainly determined by the number of muonic helium atoms we can produce. On the other hand, muons captured by a helium atom decay in about 2 microseconds, so the precision of the measurement is essentially determined by the muon beam intensity and the measurement time. To improve the measurement precision as much as possible over a total beamtime of 15 days (carried out in three different cycles), we focused on optimizing the measurement conditions (Fig. 4) and studying different and more advanced analysis methods. We could measure the muonic helium hyperfine structure resonance curve with He + CH₄ (2%) at three different pressures (Fig. 5) and extrapolate to zero pressure to determine the muonic helium atom hyperfine structure frequency (Fig. 6).

  Figure 4: Schematic view of the experimental setup to measure the hyperfine structure of muonic helium atoms at zero magnetic field. (© 2023 American Physical Society)

  Figure 5: Muonic helium hyperfine structure resonance curve measured at zero field with He + CH₄ (2%) at (a) 3.0, (b) 4.0, and (c) 10.4 atm, respectively. The red solid lines represent the fitting results. (© 2023 American Physical Society)

  Figure 6: Muonic helium atom hyperfine structure frequency as a function of the He + CH₄ (2%) gas pressure (red circle). The red solid line shows the linear extrapolation to determine the hyperfine frequency at zero pressure. Previous results from Orth et al. (green diamond) and Gardner et al. (blue square) with the linear extrapolation (blue dashed line) measured with He + Xe(1.5%) are also shown for comparison.

What did you find out?

  We succeeded in measuring the hyperfine structure of muon helium atoms with the world's highest precision and established the measurement technique using pulsed muons at J-PARC. The measurement results also show that the measurement precision can be improved by a factor of 100 using this technique and the high-intensity muon beams from MLF MUSE H-line at J-PARC. We are currently working on further technological developments to improve the measurement precision.

How would that change the world?

  Compared to positively charged muons, negatively charged muons are measured less precisely in terms of mass, and the difference in properties due to the opposite charge has not yet been well studied. Suppose that more precise measurements of the hyperfine structure of muonic helium atoms are made based on this experiment. In this case, the difference in properties due to the charge, i.e., the difference between particles and antiparticles, could be clarified.
  In addition, the theory of complex atoms composed of three or more particles is not well developed at present. Precise measurements of muonic helium atoms, which are composed of three particles (helium nucleus, electron, and muon), can be expected to provide a significant boost to verify and improve the current theory of particle physics.

Acknowledgments

  The muon experiment at the Materials and Life Science Experimental Facility (MLF) of J-PARC was performed under a user program (Proposals No. 2020B0333, No. 2021B0169, No. 2022A0159). This work was supported by the JSPS KAKENHI Grant No. 21H04481.

Paper Information

  "Improved Measurements of Muonic Helium Ground-State Hyperfine Structure at a Near-Zero Magnetic Field" , Physical Review Letters 131, 253003 (2023).