Mark I. Gorenstein on QCD Critical Point Research, Fluctuations, and Theoretical Physics in Wartime Ukraine
Author(s): Scott Douglas Jacobsen
Publication (Outlet/Website): The Good Men Project
Publication Date (yyyy/mm/dd): 2026/04/18
Mark I. Gorenstein is a Ukrainian theoretical physicist at the Bogolyubov Institute for Theoretical Physics in Kyiv, where he heads the Department of High-Density Energy Physics. His work focuses on relativistic heavy-ion collisions, fluctuation observables, hadron-resonance-gas models, and the search for the QCD critical point. He co-authored the subensemble acceptance method that connects measured cumulants to grand-canonical susceptibilities, and the quantum van der Waals extension of the HRG model. A Humboldt Research Award laureate with longstanding collaborations in Frankfurt (FIAS/Goethe University), he remains active in Kyiv. Recent work analyzes the chemical freeze-out curve and its implications for critical-point searches.
In this interview with Scott Douglas Jacobsen, Gorenstein discusses current theoretical challenges, promising beam-energy ranges for critical-point searches, the war’s impact on Ukrainian science, and the international collaborations sustaining fundamental research under crisis conditions.
Scott Douglas Jacobsen: What problem in heavy-ion theory is currently on your mind?
Mark I. Gorenstein: During last years the main theoretical problem in my mind is the QCD critical point. Does it exist? If yes, in what part of the phase diagram it is placed? What are the possible experimental signal? At the moment all these questions are the open problems.
Jacobsen: What does the sub-ensemble acceptance method correct in fluctuation analyses?
Gorenstein: The sub-ensemble acceptance method gives a general procedure for the statistical model corrections of the baryon number fluctuations due to the global conservation law. For heavy colliding nuclei the total baryon number (a sum of protons and neutrons) is about 400. This number calculated as a number of baryons minus antibaryons remains constant at all stages of nucleus-nucleus collision. In experiment, the baryon number is measured event-by-event in the part of the momentum space. The average baryon number in this part is typically about 10-20% of the total number, i.e. about 40-80. Just this accepted part of the baryon number does fluctuate event-by-event. An increase of these fluctuations is the expected signal of approaching to the critical point of the strongly interacting matter. However, the global conservation of baryon number 400 influences fluctuations in the accepted part. The sub-ensemble gives the rules to calculate this influence. It makes the measured fluctuation signals significantly more transparent.
Jacobsen: What are the main limits of quantum van der Waals models?
Gorenstein: Van der Waals invented his famous equation in 1873. In 1910 he obtained the Nobel Prize for this discovery. However, only recently we succeeded to extend this model to nuclear matter taking into account the Fermi statistics of protons and neutrons. The resulted model explains the 1st order liquid-gas phase transition in the nuclear matter and appears to be quite useful in a description of the hadron gas, i.e., the mixture of interacting baryons, antibaryons, and mesons. The main limits of this model – it says nothing about quarks and gluons. These hadron constituents form the quark-gluon plasma at high temperature and/or baryonic density. A description of the hadron-quark transformation requires new more sophisticated QCD based model approaches.
Jacobsen: Which measurements would most reduce uncertainty about the Quantum Chromodynamics critical point?
Gorenstein: Event-by-event fluctuations of the baryon number and electric charge are probably most straightforward measurements of the QCD critical point signals.
Jacobsen: Which beam-energy range looks most promising for a critical-point search?
Gorenstein: Today we think that intermediate collision energy of 3-6 GeV per nucleon pair in the center of mass system of colliding nuclei is most promising for the QCD critical point search. This is the experimental region for several collaborations: STAR at RHIC (USA), NA61/SHINE at SPS (CERN), and HADES at GSI (Germany).
Jacobsen: How has the war changed day-to-day research and mentoring activities?
Gorenstein: The day-to-day research continues despite the war. This is likely a special situation for theoretical investigations. They can be conducted under different conditions. I see the two main changes in our department. Before the war it was not easy for Ukrainian senior scientists to have the long term positions in the Western Universities. During the war this becomes much easier. Two of our senior scientists (above 60) are abroad already 3 years – in Italy and in Poland. On the contrary, before 2022 young scientists had good chances to find the positions for PhD students and post-doctors in Europe and USA. Now this is forbidden for men under 60 by the Ukrainian law.
Jacobsen: Which international collaboration has been reliable for your team since 2022?
Gorenstein: For our department there were several places for collaboration: theoretical gropes of Prof. Horst Stoecker (FIAS, Frankfurt) and Prof. Volodymyr Vovchenko (Houston University, USA), and experimental grope of NA61/SHINE Collaboration (CERN). This international collaboration has been reliable since 2022.
Jacobsen: What immediate support would help Ukrainian theory groups stay productive?
Gorenstein: Personal support from Prof. Horst Stoecker (Germany) and Prof. Shin Nan Yang (Taiwan National University, Taipei) for our scientists was very important in 2022.
The Simons Foundation has made a great contribution supporting Ukrainian scientists in theoretical physics and mathematics. Each scientist at our institute obtains a monthly salary supplement of 200 $. This is indeed important as a monthly salary of our scientists in Kyiv is about 250-400 $.
Jacobsen: Thank you for the opportunity and your time, Mark.
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