Multiparticle production in lepton and hadron processes is studied by means of the Gluon Dominance Model (GDM) which is based on the Quantum Chromodynamics (QCD) and phenomenological scheme of hadronization. The model describes the multiplicity distributions and their moments very well. It has revealed an active role of gluons in multiparticle production and confirmed the fragmentation mechanism of hadronization in e + e − annihilation and its change to a recombination mechanism in hadron and nucleus interactions. The GDM explains the shoulder structure of multiplicity distributions. The hadron-pion ratio obtained by GDM has turned out to be in agreement with the experimental RHIC data of Au+Au peripheral collisions. Besides, development of GDM allows one to study the multiplicity behavior of pp annihilation at tens of GeV. The mechanism of soft-photon production and estimates of their emission region have been offered in the framework of this model. The experimental data (project "Thermalization", U-70, IHEP) indicated the cluster nature of multiparticle production revealed by using GDM.
Experimental studies of collective phenomena are carried out on U-70 accelerator in Protvino. These phenomena can be discovered since the high parton density can form in the high multiplicity region. The collective behavior of secondary particles can manifest in the BoseEinstein condensation of pions, Vavilov-Cherenkov gluon radiation, excess of soft photon yield and other unique phenomena. The revealed peaks in the angular distribution are interpreted as the gluon radiation. The search for Bose-Einstein condensation is continued. The gluon dominance model has shown a good agreement with the multiplicity distribution at high multiplicity and confirmed the guark-gluon medium formation.
Over 30 years there has been no comprehensive understanding of the mechanism of soft photons (energy smaller than 50 MeV) formation. Experimental data indicate an excess of their yield in hadron and nuclear interactions in comparison with calculations performed in QED. For a more thorough study of this phenomenon at the Nuclotron (a superconducting accelerator in JINR), preliminary measurements have been carried out with using an electromagnetic calorimeter based on BGO crystals. These results are consistent with the world data. In JINR, in connection with the building of a future accelerator complex NICA, it has become possible to carry out such studies in pp, pA and AA interactions at energies up to 25 A GeV. Our group develops the conception of an heterogeneous electromagnetic calorimeter as “spaghetti” and “shashlik” types based on gadolinium gallium garnet (GaGG) crystals with a low threshold for registration of photons. The first tests of prototypes of them manufactured at JINR on the basis of the GaGG and a mixture of tungstate and copper as an absorber are reported.
Project Thermalization is aimed to study the proton-proton interaction with high multiplicity of secondary particles. The region of high multiplicity is especially actual at present. We expect the manifestation of the secondary particle collective behavior at this region. The experimentally measured topological cross section was corrected for apparatus acceptance and detection efficiency. These data are in good agreement with gluon dominance model. The comparison with other models is also done and shows no essential deviations. There is evidence that Bose-Einstein condensation can formed at high total multiplicity region.
Abstract.The results of E-190 experiment (project Thermalization) with 50 GeV proton beam irradiation of SVD-2 setup are presented. MC simulation has shown the linear dependence of number of photons detected in electromagnetic calorimeter and the average number of neutral pions. Multiplicity distribution of neutral pion, N 0 , for total number of particles in the event, N tot = N ch + N 0 , are obtained with corrections on the setup acceptance, triggering and efficiency of the event reconstruction. The scaled variance of neutral pion fluctuations, ω = D/ < N 0 >, versus total multiplicity is measured. The fluctuations increase at N tot > 18. According to quantum statistics models this behavior can indicate a pion condensate formation in the high pion multiplicity system. This effect has been observed for the first time.
The stages of development and the current status of the versatile "Spectrometer with a Vertex Detector" setup designed for physics experiments at the U 70 accelerator of the Institute for High Energy Physics (Protvino) is described. The main detectors of the setup are the vertex detector based on silicon microstrip detectors, the wide aperture magnetic spectrometer based on multiwire proportional chambers, and the lead glass hodoscope γ detector. In the setup, there is a fast two level trigger system for selecting required particle interactions. The key characteristics of the setup systems are presented, and the physical results obtained on it are briefly listed. * To study the dependence of the cross sections on the atomic number of the nucleus, a passive target-a lead foil 220 µm thick-is located at a point with Z coordinate of +8 mm, and the passive target-a carbon plate 500 µm thick-is located at a point with Z coordinate of +16 mm.
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