FLTR: GSI/FAIR Research Director Professor Karlheinz Langanke, Professor Nu Xu, Professor Taka Otsuka
and Professor Volker Koch - Image credit: GSI/FAIR

13th September 2023. Press release GSI/FAIR. Three world-renown scientists, Profs. Volker Koch, Nu Xu and Takaharu Otsuka, are currently spending long-term research stays at GSI and FAIR and its partner universities in Darmstadt and Frankfurt. They are analyzing and interpreting current experimental data and preparing the first scientific experiments at FAIR in fruitful interdisciplinary cooperation.

Professor Volker Koch and Professor Nu Xu are both from the Lawrence Berkeley Laboratory. Volker Koch holds the professorship for theoretical heavy-ion physics and has been the laboratory’s nuclear physics division head. Nu Xu is professor for experimental heavy-ion physics and the former spokesman of STAR, a flagship experiment at the Relativistic Heavy-Ion Collider (RHIC) at the Brookhaven National Laboratory. Professor Takaharu Otsuka held the chair of theoretical nuclear physics at the University of Tokyo until his retirement. Taka Otsuka and Nu Xu are both recipients of Humboldt Research Awards, while Volker Koch is currently an EMMI Visiting Professor.

GSI and FAIR took the unique opportunity to discuss with these colleagues in an interview the motivation why they have chosen GSI for their long-term stay, and what personally fascinates them from the many science options at FAIR. Despite very different perspectives and different scientific expectations with regard to the FAIR research pillars, the three scientists have one thing in common: the anticipation of outstanding research prospects and decisive advances in knowledge in a unique world leading research infrastructure at FAIR. The whole interview can be read here:

GSI/FAIR: You all three are world-leading scientists and come from prestigious institutions. Why did you choose GSI for your research stays?

Volker Koch: The Rhein-Main-Neckar region is the center of gravity in nuclear science, in particular in my field of interest, which focuses on the properties of the strong force at the high-density and high-energy frontiers as it can be explored in heavy-ion collisions. There is for example the HADES experiment, which has taken exciting data in their latest runs within the FAIR Phase-0 program, which we try to understand now.  It is of great advantage to have many experts on campus and at the neighboring universities with whom we can look at these data from very different angles. In fact, I have missed such a stimulating scientific atmosphere during the pandemic and I every much enjoy the daily discussion taking place here. Of course, we also discuss the future opportunities, in particular the CBM experiment at FAIR, which we hope will answer some of the fundamental questions in our field of research.

Nu Xu: Indeed, the phase diagram of Quantum Chromodynamics, which describes the properties of the strong force as function of temperature and density, has still several open fundamental questions. I was much involved in the preparation and in the execution of experiments of the STAR collaboration where we have tried to explore whether this phase diagram exhibits a critical point like it is familiar to us from the phase diagram of water. Unfortunately, the STAR experiment left a gap in the data, which is needed to answer this question. The place from which we expect the answer is the CBM experiment at FAIR. To prepare this unique and scientifically extremely important experiment I am here.

Takaharu Otsuka: My scientific interest is somewhat different from that of my colleagues as I try to develop models, which describe the many facets of nuclear structure. Here the frontier are exotic unstable nuclei, which for example have a large number of extra neutrons compared to their stable counterparts. These nuclei and their properties are, however, crucial if we want to develop a general model, which describes the many phenomena the nuclear many-body system exhibits. For example, we have learnt in recent years that nuclear magic numbers, which are a cornerstone of nuclear structure whose explanation was awarded a Nobel Prize, are different in exotic from those in stable nuclei. We could recently show that among others the tensor force plays a crucial role in these exotic nuclei. In my career, I have benefitted very much from close contact to experimentalists, which some years ago were my colleagues at RIKEN. Now I think that in the future the NUSTAR experiments at FAIR will have the leading role in understanding many aspects of the structure of exotic nuclei beyond the present reach. In particular, I am interested in the physics, which determines the limit of existence in very neutron-rich nuclei where FAIR opens completely new perspectives. Therefore, I am happy to intensify my collaboration with my theory and experiment colleagues in Darmstadt. I hope that both sides will benefit from these activities.

GSI/FAIR: Professor Xu, you mentioned the STAR experiment at RHIC, which is one example that there are also other facilities worldwide which explore the science which will be in the focus at FAIR. Professor Otsuka, you referred to the Japanese flagship facility RIKEN. Perhaps you can elaborate where you see the advantages of FAIR and perhaps its uniqueness?

NX: The Brookhaven activities are finished leaving important questions unanswered. In my view, CBM is in the position to answer them. Actually, if there were other facilities, which were better advanced than CBM, I would have joined these activities. But there is none. If FAIR can deliver SIS100 beams the CBM collaboration will be ready for data taking. And the CBM experiment has the high-rate capabilities to decide whether a critical point exists in the QCD phase diagram or not.

VK: Indeed, to answer this fundamental science question statistics is the name of the game and CBM has the capability to deliver the required rate of data. This allows actually much more than to prove the existence of the critical point. For example, one can also explore the symmetry energy at densities twice or even three-times the value of saturation density, as it exists inside of heavy nuclei like lead. Such high densities are of crucial importance in many astrophysical environments, like core-collapse supernovae or neutron star mergers. The CBM data will also provide very valuable constraints for the nuclear Equation of State, which governs the structure of neutron stars, which are the most compact objects which one can study directly in the Universe. In fact, there are so many upcoming activities in astrophysics opening the era of multi-messenger exploration of the Universe, which all are intimately related to science, which will be, often for the first time, explored at FAIR. During my stay in Darmstadt, my colleagues and I have developed several new ideas how this complementarity can be optimally explored. I am really looking forward that FAIR will be switched on and the CBM and NUSTAR experiments start. This will be a new game in town, as we say in California.

TO: The FAIR facility offers significantly higher bombarding energies than the other facilities. This allows to explore mass regions in the nuclear chart which are not easily accessible with other accelerators, making the global activities complementary in many aspects. This opens exciting perspectives for my research interest. It is very exciting that FAIR will soon deliver for example first data on the very neutron-rich nuclei, which build the third peak in the astrophysical r-process, which is often referred to as the "gold peak". We have predicted the half-lives for the nuclei in the gold peak and it will be nice to see whether we have been right. Let me stress another important point. Also many activities at FAIR, although unique on the global level, are very complimentary. Take the symmetry energy, which my colleagues Volker Koch and Nu Xu want to study at very high densities. It is also relevant for astrophysical applications to know it at densities at and below saturation. This behavior can be studied with the R3B experiment within the NUSTAR collaboration.

GSI/FAIR: Your home countries have very strong activities in heavy-ion and nuclear structure science. Which role does FAIR play for these communities?

VK: The US Nuclear Physics community is currently preparing its Longe Range Plan, which also addresses the future opportunities of the research on high-density nuclear matter, that is the behavior of the QCD phase diagram at high densities as it will be explored at FAIR. I am not personally involved in the writing team, but I know that the intellectual interest of my theory colleagues in this field is tremendous. Personally, I am also convinced that there will be a growing American participation in CBM. 

NX: I share the view of my colleague Volker Koch concerning the interest in the US. But I like to add, that also in my mother country China there is a very large interest in the CBM physics, carried by six institutions including many postdoctoral and graduate students. The Chinese colleagues have been involved in the STAR experiment at RHIC and bring their expertise now to CBM. To underline the Chinese interest, components of the time-of-flight detector system for CBM have been built in China. They are tested and ready to be employed at FAIR. We need a SIS100 beam.

TO: There is an existing strong interaction between the Japanese and GSI activities in nuclear structure, but also in other FAIR research fields like atomic or biophysics. Some FAIR detectors developed by the NUSTAR collaboration have already been tested and used in experiments at RIKEN. But the exchange is in both directions. One interesting research field at FAIR will be hypernuclei, which is regular nuclei to which a lambda particle, which carries a strange quark, is added. Japan has a long history in hypernuclear research. But now we bring activities to FAIR based on a Memorandum of Understanding signed by RIKEN and GSI/FAIR where we jointly open research on neutron-rich hypernuclei. FAIR provides the SIS100 accelerator and the Super FRS, the equipment to produce such really exotic nuclei, and RIKEN develops and builds a novel detector which allows to study these hypernuclei. RIKEN has in fact very positive experience with such collaborative efforts abroad, for example, with a dedicated hadron physics program at Brookhaven. I am sure that also the RIKEN-FAIR project will be a success.

GSI/FAIR: What is the scientific highlight you personally wish to see delivered by FAIR?

NX: With its high-rate capability and the other available observables, CBM will answer the question whether a critical point exists in the QCD phase diagram, or not. CBM will also constrain the nuclear equation of state to a level that it has a very strong impact on the understanding of astrophysical objects like neutron stars or supernovae. I would like to add that while the high-energy programs at CERN focus on the properties of the quark-gluon plasma – the form of matter as it exists in the very early phase of the Universe, here we concentrate on the properties of matter at high densities. If CERN is the high-energy frontier, FAIR is the high-density frontier. Both programs are complementary to each other and are both necessary for understanding the QCD phase diagram.

VK: The critical point and the equation of state are certainly also on the top of my list. But CBM can do more, perhaps answer questions which we do not even think about now. For example, recent lattice QCD calculations predict that the interaction between two Omega baryons is attractive. CBM with its very high event rate is likely the only experiment, which can check this prediction.

TO: In general, I expect from the NUSTAR experiments at FAIR decisive progress in our general understanding of the nucleus as a many-body system, already from phase 0 experiments and then more once FAIR is operational.  It would be quite exciting to understand the boundaries of nuclear existence as a function of neutron excess but also in the regime of superheavy nuclei, derived from nucleons as the fundamental building blocks and the strong and Coulomb forces acting between them. But I personally would also like to explore whether hypernuclei might be a tool to probe the emergence of nuclear shapes. There are some hints, which have recently emerged that nuclei might have a wider spectrum of geometric shapes than usually assumed.

GSI/FAIR: Thank you very much for this discussion. We wish you a successful stay in Darmstadt and many fruitful returns to GSI and later to FAIR.