Substantial experimental and theoretical efforts worldwide are devoted to explore the phase diagram of strongly interacting matter. At LHC and top RHIC energies, QCD matter is studied at very high temperatures and nearly vanishing net-baryon densities. There is evidence that a Quark-Gluon-Plasma (QGP) was created at experiments at RHIC and LHC. The transition from the QGP back to the hadron gas is found to be a smooth cross over. For larger net-baryon densities and lower temperatures, it is expected that the QCD phase diagram exhibits a rich structure, such as a first-order phase transition between hadronic and partonic matter which terminates in a critical point, or exotic phases like quarkyonic matter. The discovery of these landmarks would be a breakthrough in our understanding of the strong interaction and is therefore in the focus of various high-energy heavy-ion research programs. The Compressed Baryonic Matter (CBM) experiment at FAIR will play a unique role in the exploration of the QCD phase diagram in the region of high net-baryon densities, because it is designed to run at unprecedented interaction rates. High-rate operation is the key prerequisite for high-precision measurements of multi-differential observables and of rare diagnostic probes which are sensitive to the dense phase of the nuclear fireball. The goal of the CBM experiment at SIS100 (sqrt(s_NN) = 2.7 - 4.9 GeV) is to discover fundamental properties of QCD matter: the phase structure at large baryon-chemical potentials (mu_B > 500 MeV), effects of chiral symmetry, and the equation-of-state at high density as it is expected to occur in the core of neutron stars. In this article, we review the motivation for and the physics programme of CBM, including activities before the start of data taking in 2022, in the context of the worldwide efforts to explore high-density QCD matter.

he results of the SERP-E-184 experiment at the U-70 accelerator (IHEP, Protvino) are presented. Interactions of the 70 GeV proton beam with C, Si and Pb targets were studied to detect decays of charmed $D^0$, $\overline D^0$, $D^+$, $D^-$ mesons and $\Lambda _c^+$ baryon near their production threshold. Measurements of lifetimes and masses are shown a good agreement with PDG data. The inclusive cross sections of charm production and their A-dependencies were obtained. The yields of these particles are compared with the theoretical predictions and the data of other experiments. The measured cross section of the total open charm production ($\sigma _{\mathrm {tot}}(c\overline c)$ = 7.1 $\pm $ 2.3(stat) $\pm $1.4(syst) $\mu $b/nucleon) at the collision c.m. energy $\sqrt {s}$ = 11.8 GeV is well above the QCD model predictions. The contributions of different species of charmed particles to the total cross section of the open charm production in proton-nucleus interactions vary with energy.

Experimental and theoretical studies of direct photon production in hadronic collisions essentially expand our insights in multiparticle production mechanisms. These photons are useful probes to investigate nuclear matter at all stages of the interaction. Soft photons play a particular role in these studies. Until now we have no explanation for the experimentally observed excess of soft photons. These photons have low transverse momenta \( p_{T} < 0.1\) GeV/c, \( \vert x\vert < 0.01\) . In this domain their yield exceeds the theoretical estimates by 5-8 times. The registration of soft photons at Nuclotron (LHEP, JINR) has been carried out by the electromagnetic calorimeter built by the SVD-2 Collaboration. Soft photon electromagnetic calorimeter was tested at U-70, IHEP (Protvino). For the first time the soft photon yield at interactions of 3.5A GeV/c per nucleon deuterium and lithium beams has been measured. The obtained energy spectra confirm the increased yield of soft photons with their energy less than 50MeV (in the laboratory system) in comparison with theoretical predictions and agree with previous experiments at high-energy interactions. It is planned to continue soft photon study at the future accelerator complex NICA with heavy-ion beams.

First results of a soft photon yield in nucleus-nuclear interactions at 3.5 GeV per nucleon are presented. These photons have been registered at Nuclotron (LHEP, JINR) by an electromagnetic calorimeter built in the SVD Collaboration. The obtained spectra confirm the excess yield in the energy region less than 50 MeV in comparison with theoretical predictions and agree with previous experiments at high-energy interactions.