Physicists from NRU MEPhI are active participants in experiments at the NICA accelerator complex (Nucleotron-based Ion Collider Facility) at the Joint Institute for Nuclear Research, Dubna. They have already participated in the first physical session of the BM@N experiment (Baryonic Matter at Nuclotron) and are now actively involved in the preparation of the first experiments on the MPD (Multi-Purpose Detector) installation, which are scheduled to begin in late 2025 - early 2026. We are talking about the work of mythologists at the NICA collider with Arkady Taranenko, Associate Professor of the Department of Experimental Methods of Nuclear Physics at MEPhI, Deputy head of the MPD collaboration.
- Arkady Vladimirovich, please tell us about the MEPhI scientific group that works at the NICA collider.
- The group was created in 2015 - after I started working at MEPhI after returning from Stony Brook University in the United States of America. We created a group for the study of relativistic nuclear collisions, and at the end of 2015 we arrived in Dubna, where we began working on the subject of the NICA project. We immediately chose a topic for ourselves, which we have been doing all this time.
- And what is this topic?
- This topic is the study of collective azimuthal fluxes of particles born in collisions of relativistic heavy ions and methods of their measurement. Collective flows are just one of those physical phenomena, the detailed study of which led to the discovery of quark-gluon matter. It was discovered as a result of experiments on the collision of gold nuclei at an energy of 200 GeV in the center of mass system at the RHIC collider at Brookhaven National Laboratory, where I worked before. Now, the study of the properties of this new form of highly interacting matter in collisions of relativistic nuclei is, one might say, a hot topic for research in many accelerator laboratories in the world. Well, since we had a lot of experience measuring collective particle fluxes in other experiments, we chose this direction. In fact, the MEPhI group is the only group in Russia that is engaged in the experimental study of this physical phenomenon and continues to improve measurement techniques. This allowed us to prepare a physical program for measuring collective fluxes in experiments at the NICA accelerator complex and already obtain the first physical results in the BM@N experiment on the Nuclotron, which took place in 2022-2023.
- Has your group already received the first data at the BM@N installation?
- Yes, our group was able to obtain the first physical results of measuring the directional flow of protons in the BM@N experiment to study collisions of xenon collisions on cesium and iodine Xe+Cs(I) at a beam energy of 3.8 AGeV. This was the first physical session of the experiment in 2022-2023 and was preceded by 7 technical sessions. Modern experiments in the field of relativistic nuclear physics are very complex and are being prepared by a large team of physicists, engineers, electronics engineers, programmers and technicians. Currently, the international BM@N collaboration has more than 200 participants from 13 institutes and universities. Data analysis in such experiments is also a complex process. At the beginning, we spent more than a year on a detailed simulation of the installation of the BM@N experiment using various modern nuclear collision generators. The methods of measuring collective flows were tested on reconstructed model data. We have made sure for ourselves that accurate measurements are possible. Then our guys, graduate students and MEPhI staff participated in shifts on the BM@N experiment. Our group took on about 50 shifts of 8 hours each – this is already in sessions with a data set. The dataset session lasted 2 months from December 2022 to February 2023. Then we participated in the calibration of experimental data. This made it possible, using the methods that we have developed, to obtain the first signals of a directed flow of protons. Well, then it was necessary to make a lot of presentations at the meetings of the collaboration, write a note on the analysis of this data, and only then protect your data before the collaboration. Without this procedure, data according to the rules of collaboration cannot be presented to the public. By the way, it was our group that was the first to receive the results in the BM@N experiment, which the collaboration approved. For the first time, the data were shown at the Core-2024 conference by Mikhail Mamaev, a graduate student at the MEPhI National Research University, in July 2024.
- Can I explain briefly and popularly what is valuable in these results?
- Well, first of all, these results show that such complex measurements are possible in an experiment, and the values that we measured are consistent with the general dependence of these fluxes on the energy that other experiments measured. This indicates that the quality of our measurements corresponds to the world. We have taken into account all the taxonomies and, since no one has studied these flows for such a collision system before us, our results, of course, have scientific value, since we can compare flows in different colliding systems for this energy for the first time. That is, we can compare the results of the STAR experiment at the RHIC collider for gold-on-gold collisions with the results of measurements in collisions of xenon on cesium and iodine Xe+Cs(I). In 2025, an energy scan for this system is already planned, and we will get an answer to a very important question: how does the size of a system of colliding nuclei affect the formation of collective flows in the region of maximum baryon density.
- Does MEPhI's contribution relate more to the field of mathematics, data processing, or is there something else?
- Data processing is a rather complicated process, it includes writing programs and learning how various detector subsystems are arranged and how to calibrate them, because the experiment is quite complex. With the reconstructed model data, we can realistically reproduce these signals and thereby prove – first of all, to ourselves - that everything we have done works perfectly at the modeling stage. Only later, after processing the experimental data, we can judge what we have received, learn how to correctly compare our data with the results of other experiments and with model data in order to understand what physics we can extract from our measurements. Thus, this is a comprehensive data processing program that is not limited to writing computer programs or running them, it is a very long, meticulous research process that in the end should lead you to the right result; and you also have to prove that your result is really new, and no one has done it before, and at the same time it is correct.
- And what is the task facing the new experiment that is currently being prepared in Dubna - MPD?
- At higher energies, which were available at the RHIC collider or at the large Hadron collider in CERN, collisions of heavy nuclei such as lead on lead or gold on gold are also being studied. In such collisions at energies of more than 100 GeV per pair of nucleons in the center of mass system, as it was proved about 20 years ago, a new form of matter can arise, which is called quark-gluon matter. It is a matter consisting of quarks, antiquarks and gluons. It is believed that our universe existed in this form a few microseconds after the Big Bang. And, accordingly, when a new form of matter is discovered, it is very interesting to study its properties. By comparing with various models, it has been proved that this form of matter behaves rather like an ideal liquid, that is, it has a very small ratio of shear viscosity to entropy. Further experiments, which are being conducted not only in Dubna, but also in other accelerator centers in the world, are aimed at studying the properties of this new form of matter. In Dubna, we are going to study this form of matter in the field of high baryon densities. That is, when our temperatures are not as high as in the early universe, but similar temperatures and densities may exist, for example, during the collision of neutron stars, which were discovered in 2017. Although the values of density and temperature are similar, the scales of space and time are different: a kilometer for the fusion of neutron stars and a femtometer (10 to minus 15 degrees of a meter) in the case of a collision of heavy ions. Similarly, the durations of collision events differ by 20 orders of magnitude. The time ranges in collisions of relativistic nuclei are about a dozen femtometers. That is, these are very short times. To measure such time ranges, no device can be built that will measure them. But, nevertheless, the temperatures and densities of baryons in our models turn out to be very similar to those that occur in stars, so studying quark-gluon matter will help, among other things, in solving the problems of astrophysics. And we hope that maybe quark-gluon matter has other phase transitions. Just as water has different phase transitions and there is a critical point, so there may be a transition of the first kind, which will be completely different, that is, not the same as that observed at higher energies. This sets the direction of the experiment.
- And now, while this accelerator is not yet completed and the experiment has not yet begun, what is the MEPhI group doing with MPD?
- In MPD, as with any group participating in the experiment, we have, first of all, service work. For example, MEPhI graduate students participated for several months in testing electromagnetic calorimeter modules. In general, each group is obliged to participate in service work, allocating its human resources for this purpose. Also, now the guys will participate in shifts for cooling the superconducting magnet of the MPD installation, which will begin in 2-3 weeks. And then we will participate in the assembly of detectors when the field of the magnet is measured and everyone is convinced that the quality of the magnet, which was ordered in Italy for 20 million euros, corresponds to the passport data. Well, after that, it will be possible to assemble this entire collider experiment.
