Period: From Wednesday, August 12 to Friday, August 14, 2026.

Researchers, as part of a major international collaboration, reported evidence hinting at the existence of a never-before-seen but predicted exotic bound state known as an η′-mesic nucleus. These valuable findings will be published this month in Physical Review Letters.

Physicists have theorized that under certain conditions, short-lived particles called mesons – which only exist for less than ten-millionth of a second – can become temporarily trapped inside a nucleus, forming an exotic bound system. Measuring mesic nuclei could help scientists understand how the strong nuclear force, which binds atomic nuclei together, behaves and how the vacuum structure changes in extremely high-density environments.

“One particle of particular interest is the η′ meson,” says senior author Kenta Itahashi. “It is unusually heavy compared with related particles, and physicists expect that its mass changes when it exists inside nuclear matter. Observing this phenomenon would provide valuable information about how particle masses are generated in the universe.”

To search for the η′-mesic nuclei, the international collaboration carried out a high-precision experiment using a powerful particle accelerator in GSI Helmholtzzentrum für Schwerionenforschung, Germany. They utilized a beam of high-energy protons bombarded on a carbon target to produce η′-mesic states. The energetic proton beam excites the carbon nucleus, producing η′ mesons, which form a bound state with the carbon nucleus with a certain probability. The excitation energies of the carbon nuclei were measured by analyzing the energy of deuterons –the simplest atomic nucleus made of one proton and one neutron– produced forward in the reaction using a high-resolution spectrometer, Fragment Separator (FRS). Researchers employed a special detector called WASA, which was originally developed and constructed in Uppsala, Sweden, to selectively measure high-energy protons that get out from the target, looking for signs that an η′ meson had been created and captured inside the nucleus, otherwise known as decay signals. 

“With our new experimental setup combining the FRS and the WASA, we can identify structures in the data that match theoretical signatures of η′-mesic nuclei,” explains lead author Ryohei Sekiya. “Our analysis suggests that these bound states were indeed formed.”

The resultant excitation spectrum of the carbon nucleus measured in the experiment is displayed in Fig, indicating possible formation of the η′-mesic nuclei. The team’s findings indicate that the mass of the η′ meson may decrease inside nuclear matter, supporting theoretical predictions and providing rare experimental insight into how the properties of particles change in super high-density environments.

“Our measurements provide important new clues about how mesons behave in nuclear matter,” says Itahashi. “This brings us closer to answering deep, fundamental questions about how matter acquires mass, as well as how the vacuum structure changes inside atomic nuclei.”

Future experiments are planned to increase the precision of measurements and search for additional decay signals that could confirm the existence of η′-mesic nuclei. As researchers continue their search, each new result furthers our understanding of the fundamental physical laws that govern the universe.

Excitation-energy spectrum of the carbon-11 nucleus obtained in the present experiment. The excitation energy on the horizontal axis is defined such that zero corresponds to the production of an η′ meson at rest in vacuum. Negative values correspond to bound states of the η′ meson and the nucleus. The circles represent the experimental data, and the vertical bars indicate statistical uncertainties. The solid curve shows the theoretical spectrum that best reproduces the experimental data, while the dotted curve represents the estimated contribution from background processes. The two observed peak structures suggest the existence of η′ meson bound states in an inner (blue) and outer (blue) nuclear orbits in the carbon-11 nucleus.

The article, “Excitation Spectra of the ¹²C(p,d) Reaction near the η’-Meson Emission Threshold Measured in Coincidence with High-Momentum Protons,” will be published in Physical Review Letters at https://doi.org/10.1103/6vsl-ng7x
This article has been selected for “Featured in Physics” at https://journals.aps.org/prl/highlights.

Related links

• Department of Physics, Nuclear Experiment Group

• Eurkalert!
• AlphaGalileo
• Asia Research News
• ResOU（Research at Osaka University）website

Please click the link below for more details.
Graduate Admissions

This award was established in June 2022 to encourage graduate students who have achieved outstanding research results and other notable achievements each academic year.

This year, four graduate students received this award.

Congratulations!

However, in a recent study published in PNAS, researchers from The University of Osaka have revealed that circadian clock oscillation in cyanobacteria is controlled by factors intrinsic to one of the proteins that controls it, in a manner that is unaffected by environmental conditions.

Even the smallest, photosynthetic organisms have internal clocks, including cyanobacteria. These microorganisms are vital for aquatic environments, agriculture, and biotechnology. Given their vitality, it is even more important to ensure the correct timing of biological processes for photosynthesis during the day, and respiration at night.

Cyanobacteria are known to possess the simplest known circadian clock, involving only three primary proteins: KaiA, B, and C. It was these proteins that were the focus of the investigation.

“Though the cyanobacterial circadian clock is very simple, and can be reconstructed with three proteins, we still wanted to understand how these simple elements work together,” says lead author, Kumiko Ito-Miwa. “It is critical to understand how the reliability of the circadian rhythm is maintained under different environmental conditions, as it affects an incredibly wide variety of cellular processes.”

To do this, the researchers examined more than 20 mutations in the KaiC clock protein, with disturbed clock periods ranging from 15 to 60 hours. Through this, they were able to demonstrate that the circadian clock could maintain accurate timekeeping both in vitro and in vivo, regardless of environmental changes, through properties inherent to the clock proteins. This included the activity of ATPase, an enzyme responsible for producing chemical energy, which allows cells to perform their duties in various processes.

“The activity of this protein, which acts as the pacemaker of the cyanobacterial clock, did not change in response to different environmental conditions. This property, which appears to be innate to the protein itself, is likely critical for preserving circadian timing despite environmental changes,” explains Kumiko Ito-Miwa, lead author, building on a concept originally proposed and long pursued by Takao Kondo.

The findings suggest that the environment inside cyanobacterial cells may fine-tune the circadian clock to align it with Earth’s 24-hour cycle, offering significant insight into the fundamental question of how living organisms measure time.

Rhythms of KaiC period mutants in cells and in vitro (representative examples).
Even when temperature or light intensity changes, the circadian period of the wild type (normal strain), as well as those of short- and long-period mutants, remains largely unchanged. Moreover, compared with the in vitro clock, the intracellular clock shifts slightly toward the Earth’s 24-hour cycle: short-period mutants lengthen slightly, whereas long-period mutants shorten slightly.

The article, “Intrinsic period stability of the cyanobacterial circadian oscillator
across in vitro and in vivo conditions,” was published in PNAS at DOI: https://doi.org/10.1073/pnas.2526714123

Related links

• Life and Planetary Evolution Science (LIPES) Group

• Eurkalert!
• AlphaGalileo
• Asia Research News
• ResOU（Research at Osaka University）website

Light-regulated enzymatic degradation and spatial patterning through localized irradiation

The article, “Light-Programmable Polyester Networks with Movable Cross-Links for On-Demand Enzymatic Degradation,” will be published in ACS Nano at DOI: https://doi.org/10.1021/acsnano.5c19646

Related links

• Professor Takashima Yoshinori (Researcher Directory)

• Polymeric Materials Design Laboratory

• Eurkalert!
• AlphaGalileo
• Asia Research News
• ResOU（Research at Osaka University）website

In a study recently published in Nucleic Acids Research, researchers from The University of Osaka discovered that the loss of heterochromatin can kickstart genetic changes, potentially resulting in the development of diseases like cancer.

The model showed that RNA-loops (R-loops) accumulate at clusters of repetitive DNA called pericentromeric repeats in response to a process called transcriptional pausing–backtracking–restart (PBR). These accumulated R-loops are then changed into Annealing-induced DNA-RNA-loops (ADR-loops), leading to gross chromosomal rearrangements (GCRs) at constricted parts of a chromosome.

“Previously, we showed that loss of Clr4, the H3K9me2/3 methyltransferase, or its regulatory protein Rik1, increased transcription and abnormal chromosome formation in fission yeast,” explains lead author, Ran Xu. “However, the molecular link between transcription dynamics and GCRs remains poorly defined.”

Heterochromatin forms at pericentromeric repeats. Previous research showed that heterochromatin could prevent GCRs at centromeres by blocking pericentromeric transcription. However, the present study expanded on past findings by providing insights into the mechanism by which GCRs are generated, including through pericentromeric transcription.

The researchers demonstrated that loss of Clr4 can spark an increase in R-loop levels at pericentromeric repeats. After overexpressing the enzyme RNase H1 in cells lacking the clr4 gene, the research team observed reductions in both R-loops and GCRs.

Further experiments highlighted the importance of Tfs1/TFIIS and Ubp3, which are necessary to restart transcription, in R-loop accumulation and GCRs. In cells lacking Clr4, a type of protein called Rad52 built up at pericentromeric repeats. This promoted the development of GCRs, and cells carrying a mutated version of this protein had fewer GCRs because single-strand annealing (SSA), a DNA repair process, was inhibited.

“These data suggest that, when heterochromatin is lost, transcriptional PBR cycles accumulate R-loops at pericentromeric repeats, and Rad52-dependent single-stand annealing converts R-loops into ADR-loops followed by Polδ-dependent break-induced replication (BIR), encouraging GCRs related to disease,” concluded Xu.

This study could have key insights for treating genetic diseases caused by GCRs, like cancer. Although further research is needed to translate these findings into human applications, drugs targeting Rad52 or other genes and proteins involved in GCR accumulation might emerge as key disease treatments.

DNA-RNA Immunoprecipitation (DRIP)-Seq data showing accumulation of R-loops in the heterochromatin-deficient clr4∆ mutant.

The article, “Transcriptional PBR cycles at pericentromeric repeats cause gross chromosomal rearrangements through Rad52-dependent ADR-loop formation,” was published in Nucleic Acids Research at DOI: https://doi.org/10.1093/nar/gkaf1455

Related links

• Professor Takuro Nakagawa (Researcher Directory)

• Laboratory of Molecular Genetics

• Eurkalert!
• AlphaGalileo
• Asia Research News
• ResOU（Research at Osaka University）website

Conference: 2025 NSYSU × UOsaka Science Joint Symposium
Date: Saturday, December 6, 2025, 9:00 AM – 5:00 PM
Organizers: College of Science, National Sun Yat-sen University; Graduate School of Science, The University of Osaka
Venue: International Conference Hall, College of Science, National Sun Yat-sen University

On December 2, we paid a courtesy visit to National Taiwan Normal University and met with three professors: Prof. Wen-Chung Kao, Dean of the College of Interdisciplinary Industry-Academia Innovation, Prof. Cheng-Hung Lin, and Prof. Hui-Ling Sung. We exchanged views on future educational and research collaboration as well as the possibility of holding an AI Workshop using semiconductors and discussed specific approaches for organizing the workshop. We plan to continue discussions toward organizing the workshop.

Furthermore, on December 3, we visited Realtek Semiconductor and Eink, where we received detailed explanations about each company’s business overview and research and development, and viewed their product and technology displays. These visits provided valuable insights for us to design and develop the “Global Researcher Development Intensive Program in Taiwan,” scheduled for the next fiscal year, particularly for shaping the program including company visits.

The following five members from The University of Osaka’s Graduate School of Science visited National Taiwan Normal University:

KONDO Tadashi, Dean, Professor
KUBO Takashi, Associate Dean, Professor
KOSHINO Mikito, Professor
FUJII Ryoko, Assistant Head of Administration
YOKOI Nozomi, Administrative Staff

This visit gave us a great chance to build closer ties with National Taiwan Normal University.

See the back number.