Relativistic nuclear physics at JINR: from the synchrophasotron to the NICA collider

We describe the development of relativistic nuclear physics at the Joint Institute for Nuclear Research (JINR) from the first experiments to our time and review the current state of the problem. The Nuclotron-based Ion Collider fAcility (NICA) at JINR and its status are described. Two goals of the project — experimental studies of dense nuclear (baryonic) matter and particle spin physics — are combined in the project based on a common experimental method: the investigation of collisions of nuclei at relativistic energies. The first problem is discussed here, and the second will be addressed in a dedicated publication. Such experiments were started at JINR in the 1970s at the Synchrophasotron proton synchrotron, and they are the main focus of the NICA project. Fundamental and applied research in other areas of science and technology that can be implemented at the NICA facility is also discussed. The accelerator facility under construction at JINR will allow performing experimental studies in particle physics at parameters and under experimental conditions that were previously inaccessible. With NICA, particle physics research in a previously inaccessible range of experimental parameters and conditions becomes possible: heavy-ion beams will be collided at center-of-mass energies in the range 4–11 GeV at luminosities up to $10^{27}$ cm$^{-2}$ s$^{-1}$. These studies will be supplemented with experiments using a beam of exracted nuclei incident on a fixed target. A short description is given of the detectors under construction for these studies.