News Release

New model of quark-gluon plasma solves a long-standing discrepancy between theory and data

Researchers from Japan provide a novel theoretical framework for describing the quark-gluon plasma, which agrees better with experimental data

Peer-Reviewed Publication

Sophia University

Towards a better and more accurate hydrodynamical model of QGP

image: QGP is conventionally described using relativistic hydrodynamic models and studied experimentally through heavy-ion collisions. There has been a long-standing discrepancy between theory and experiment regarding the observation of particle yields in the low transverse momentum region and their absence in the model predictions. Now, researchers from Japan have addressed this issue, proposing a model that pins down the origin of the missing particle yields. view more 

Credit: Tetsufumi Hirano from Sophia University, Japan

Research in fundamental science has revealed the existence of quark-gluon plasma (QGP) – a newly identified state of matter – as the constituent of the early universe. Known to have existed a microsecond after the Big Bang, the QGP, essentially a soup of quarks and gluons, cooled down with time to form hadrons like protons and neutrons – the building blocks of all matter. One way to reproduce the extreme conditions prevailing when QGP existed is through relativistic heavy-ion collisions. In this regard, particle accelerator facilities like the Large Hadron Collider (LHC) and the Relativistic Heavy Ion Collider have furthered our understanding of QGP with experimental data pertaining to such collisions.

Meanwhile, theoretical physicists have employed multistage relativistic hydrodynamic models to explain the data, since the QGP behaves very much like a perfect fluid. However, there has been a serious lingering disagreement between these models and data in the region of low transverse momentum, where both the conventional and hybrid models have failed to explain the particle yields observed in the experiments.

Against this backdrop, a team of researchers from Japan, led by theoretical physicist Professor Tetsufumi Hirano of Sophia University, undertook an investigation to account for the missing particle yields in the relativistic hydrodynamic models. In their recent work, they proposed a novel “dynamical core-corona initialization framework” to comprehensively describe high-energy nuclear collisions. Their findings were published in the journal Physic Review C on 18 November 2022 and involved contributions from Dr. Yuuka Kanakubo, doctoral student at Sophia University, (Present affiliation: postdoctoral research fellow at the University of Jyväskylä, Finland) and Assistant Professor Yasuki Tachibana from Akita International University, Japan.

“To find a mechanism that can account for the discrepancy between theoretical modeling and experimental data, we used a dynamical core-corona initialization (DCCI2) framework in which the particles generated during high-energy nuclear collisions are described using two components: the core, or equilibrated matter, and the corona, or nonequilibrated matter,” explains Prof. Hirano. “This picture allows us to examine the contributions of the core and corona components towards hadron production in the low transverse momentum region.”

Alongside, the researchers conducted heavy-ion Pb-Pb collision simulations on PYTHIA (a computer simulation program) at an energy of 2.76 TeV to test their DCCI2 framework. Dynamical initialization of the QGP fluids allowed the separation of core and corona components, which were made to undergo hadronization through “switching hypersurface” and “string fragmentation,” respectively. These hadrons were then subjected to resonance decays to obtain the transverse momentum (pT) spectra.

“We switched off the hadronic scatterings and performed only resonance decays to see a breakdown of the total yield into core and corona components, as hadronic scatterings mix up the two components in the late stage of reaction,” explains Dr. Kanakubo.

The researchers then investigated the fraction of core and corona components in the pT spectra of charged pions, charged kaons, and protons and antiprotons for collisions at 2.76 TeV. Next, they compared these spectra with that obtained from experimental data (from the ALICE detector at LHC for Pb-Pb collisions at 2.76 TeV) to quantify the contributions from corona components. Lastly, they investigated the effects of contributions from corona components on the flow variables.

They found a relative increase in corona contributions in the spectral region of approximately 1 GeV for both 0-5% and 40-60% centrality classes. While this was true for all the hadrons, they found almost 50% corona contribution to particle production in the spectra of protons and antiprotons in the region of very low pT (≈ 0 GeV) .

Furthermore, results from full DCCI2 simulations showed better agreement with the ALICE experimental data compared when only core components with hadronic scatterings (which neglect corona components) were compared. The corona contribution was found to be responsible for diluting the four-particle cumulants (a flow observable) obtained purely from core contributions, indicating more permutations of particles with corona contribution.

“These findings imply that the nonequilibrium corona components contribute to particle production in the region of very low transverse spectra,” highlights Prof. Hirano. “This explains the missing yields in hydrodynamic models, which extract only the equilibrated core components from experimental data. This clearly shows that it is necessary to extract the nonequilibrated components as well for a more precise understanding of the properties of QGP.”

While these findings certainly contribute to the expansion of our knowledge of the universe, their subsequent applications to applied research is expected to benefit our lives in the future as well.


Title of original paper:

Nonequilibrium components in the region of very low transverse momentum in high-energy nuclear collisions


Physical Review C




Yuuka Kanakubo1, Yasuki Tachibana2, Tetsufumi Hirano1


1Sophia University

2Akita International University


About Sophia University

Established as a private Jesuit affiliated university in 1913, Sophia University is one of the most prestigious universities located in the heart of Tokyo, Japan.  Imparting education through 29 departments in 9 faculties and 25 majors in 10 graduate schools, Sophia hosts more than 13,000 students from around the world.

Conceived with the spirit of “For Others, With Others,” Sophia University truly values internationality and neighborliness, and believes in education and research that go beyond national, linguistic, and academic boundaries. Sophia emphasizes on the need for multidisciplinary and fusion research to find solutions for the most pressing global issues like climate change, poverty, conflict, and violence. Over the course of the last century, Sophia has made dedicated efforts to hone future-ready graduates who can contribute their talents and learnings for the benefit of others, and pave the way for a sustainable future while “Bringing the World Together.”



About Akita International University

Established in 2004, Akita International University (AIU) is a leader in international higher education in Japan, recognized among the top 10 universities in the country. With nearly 60% of its faculty and 25% of its student body hailing from overseas, AIU teaches all content courses in English and requires all undergraduate students to spend one year studying abroad at one of over 200 exchange partners in over 50 countries, ensuring an unrivaled international education.

Through its Applied International Liberal Arts curriculum, AIU develops global leaders with outstanding intelligence and character who serve global society with a sense of responsibility and passion, by challenging students to apply their studies to global issues represented in our local community. As a public university, AIU promotes research, industry collaboration, and project-based learning designed to generate new value for our Akita community, Japan, and the world.



About Professor Tetsufumi Hirano from Sophia University

Dr. Tetsufumi Hirano is a Professor affiliated with the Department of Engineering and Applied Sciences at Sophia University, Japan. A theoretical physicist, Prof. Hirano conducts high-energy nuclear physics research at the university’s Hadron Physics Group. He completed his doctoral studies in 2001 from Waseda University, Japan, before embarking on his career as an academician and researcher. His research interests lie in understanding the intricacies of the QGP, core-corona initialization, and relativistic hydrodynamics, among others. Prof. Hirano has published more than 120 papers with over 5,000 citations to his credit.


Funding information:

This study is supported by JSPS KAKENHI Grant No. 20J20401.

Media contact:

Office of Public Relations, Sophia University

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