From soft photon study at Nuclotron and U-70 to NICA

Experimental and theoretical studies of direct photonproduction in hadronic collisions essentially expand our insights in multiparticle production mechanisms. These photonsare useful probes to investigate nuclear matterat all stages of the interaction. Soft photonsplay a particular role in these studies. Until now we have no explanation for the experimentally observed excess of soft photons. These photonshave 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 photonsat Nuclotron(LHEP, JINR) has been carried out by the electromagnetic calorimeterbuilt by the SVD-2 Collaboration. Soft photonelectromagnetic calorimeterwas tested at U-70, IHEP (Protvino). For the first time the soft photonyield 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 photonswith 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 photonstudy at the future accelerator complex NICA with heavy-ion beams.