Interest in studying multiparticle-production processes in lepton and hadron interactions has not faded to date. The development of strong-interaction theory in the form of quantum chromodynamics (QCD) permits describing these interactions at the level of quarks and gluons. The transition from partons to observed hadrons at the hadronization stage is very difficult. The gluon-dominance model describing multiparticle production of secondaries and including the hadronization stage is proposed in the present study. This model is applied to describing electron–positron annihilation. It confirms an active role of the gluon component in the formation of secondaries, is indicative of the fragmentation mechanism of hadronization occurring in a vacuum, and describes experimental data on multiplicity distributions.

The Spin Physics Detector, a universal facility for studying the nucleon spin structure and other spin-related phenomena with polarized proton and deuteron beams, is proposed to be placed in one of the two interaction points of the NICA collider that is under construction at the Joint Institute for Nuclear Research (Dubna, Russia). At the heart of the project there is huge experience with polarized beams at JINR. The main objective of the proposed experiment is the comprehensive study of the unpolarized and polarized gluon content of the nucleon. Spin measurements at the Spin Physics Detector at the NICA collider have bright perspectives to make a unique contribution and challenge our understanding of the spin structure of the nucleon. In this document the Conceptual Design of the Spin Physics Detector is presented.

The latest results on the search for collective phenomena in proton interactions at the U-70 accelerator at IHEP (Protvino) are presented. This long-term experiment has been carried at the SVD-2 setup. Our study was aimed at a high multiplicity region where the series of collective phenomena is predicted. We have received an evidence of the formation of a pion (Bose-Einstein) condensate. We have observed two noticeable peaks in the angular distribution of charged pions interpreted as Cherenkov radiation of gluons by quarks and others phenomena.

(SVD-2 Collaboration) A. G. Afonin, E. N. Ardashev, V. F. Golovkin, S. N. Golovnya, S. A. Gorokhov, A. A. Kiryakov, A. G. Kholodenko, V. V. Konstantinov, L. L. Kurchaninov, I. S. Lobanov, E. V. Lobanova, G. A. Mitrofanov, V. S. Petrov, A. V. Pleskach, M. K. Polkovnikov, V. N. Riadovikov*, V. N. Ronzhin, V. A. Senko,N. A. Shalanda, M. M. Soldatov, Yu. P. Tsyupa, A. P. Vorobiev, V. I. Yakimchuk, and V. N. Zapolskii∗ IHEP, Protvino, Moscow region, Russia.

In the framework of the diagrammatic approach, the total cross section of near-threshold π– photoproduction on the deuteron is calculated. Contributions of the diagrams corresponding to the plane wave impulse approximation as well as to NN- and πN-interactions in the final state have been taken into account. We have compared the theoretical predictions with the results of the recent measurements performed at the MAX IV laboratory of the Lund University. There is good agreement with the data at photon energies from 147 to 155 MeV. However, at 158 and 160 MeV, the theoretical predictions significantly overestimate the measured cross sections.

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 , p A 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.

Multiparticle production is studied experimentally and theoretically in QCD that describes interactions in the language of quarks and gluons. In the experiment the real hadrons are registered. Various phenomenological models are used for transfer from quarks and gluons to observed hadrons. In order to describe the high multiplicity region, we have developed a gluon dominance model (GDM). It represents a convolution of two stages. The first stage is described as a part of QCD. For the second one (hadronization), the phenomenological model is used. To describe hadronization, a scheme has been proposed, consistent with the experimental data in the region of its dominance. Comparison of this model with data on e+e- annihilation over a wide energy interval (up to 200 GeV) has confirmed the fragmentation mechanism of hadronization, the development of the quark-gluon cascade with energy increase and domination of bremsstrahlung gluons. The description of topological cross sections in pp collisions within GDM testifies that in hadron collisions the mechanism of hadronization is being replaced by the recombination one. At that point, gluons play an active role in the multiparticle production process, and valence quarks are passive. They stay in the leading particles, and only the gluon splitting is responsible for the region of high multiplicity. GDM with inclusion of intermediate quark charged topologies describes topological cross sections in pp̅ annihilation and explains initial linear growth in the region of negative values of a secondary correlative momentum vs average pion multiplicity with increasing of energy. The proposed hadronization scheme can describe the basic processes of multiparticle production.

Study of multi-particle production has longer than the semi-centennial history. As it is known, with the growth of energy of accelerators, the new channels of reaction are being opened, the average number of secondary particles is increasing. Physicists are able to accelerate stable particles, such as electrons, positrons, protons, antiprotons, ions (both light and heavy). Rarely, they accelerate kaons and pions. The obtained experimental material stimulates the development of the different theoretical approaches. Since appearance of the modern theory of strong interactions, quantum chromodynamics (QCD), our understanding of multi-particle production is advanced significantly. The language of quarks and gluons is basic one at the explanation of observable phenomena. This review is devoted to the history of appearance and the following development of the gluon dominance model. This model is based on the pQCD and the phenomenological description of the hadronization stage. It permits to describe multiplicity distributions both for lepton and hadron interactions, especially in the high multiplicity region.

Study of high multiplicity events in proton-proton interactions is carried out at the U-70 accelerator (IHEP, Protvino). These events are extremely rare. Usually, Monte Carlo codes underestimate topological cross sections in this region. The gluon dominance model (GDM) was offered to describe them. It is based on QCD and a phenomenological scheme of a hadronization stage. This model indicates a recombination mechanism of hadronization and a gluon fission. Future program of the SVD Collaboration is aimed at studying a long-standing puzzle of excess soft photon yield and its connection with high multiplicity at the U-70 and Nuclotron facility at JINR, Dubna.

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.

Reactions induced by high energy antiprotons on proton on nuclei are accompanied with large probability by the emission of a few mesons. Interesting phenomena can be observed and QCD tests can be performed, through the detection of one or more mesons. The collinear emission from high energy (anti)proton beams of a hard pion or vector meson, can be calculated similarly to the emission of a hard photon from an electron \cite{Kuraev:2013izz}. This is a well known process in QED, and it is called the "Quasi-Real Electron method", where the incident particle is an electron and a hard photon is emitted leaving an 'almost on shell' electron impinging on the target \cite{Baier:1973ms}. Such process is well known as Initial State Emission (ISR) method of scanning over incident energy, and can be used, in the hadron case, to produce different kind of particles in similar kinematical conditions. In case of emission of a charged light meson, $\pi$ or $\rho$-meson, in proton-proton(anti-proton) collisions, the meson can be deviated in a magnetic field and detected. The collinear emission (along the beam direction) of a charged meson may be used to produce high energy (anti)neutron beams. This can be very useful to measure the difference of the cross sections of (anti)proton and (anti)neutron scattering from the target and may open the way for checking sum rules with antiparticles. Hard meson emission allows also to enhance the cross section when the energy loss from one of the incident particles lowers the total energy up to the mass of a resonance. The cross section can be calculated, on the basis of factorized formulas, where the probability of emission of the light mesons multiplies the cross section of the sub-process. Multiplicity distributions for neutral and charged meson production are also given.

A set of evolution equations for correlators of densities of quarks and gluons is considered. Approximate solutions are derived in the framework of the gluon and quark dominance. A new approach to estimate the cross sections of the processes with the interaction of high energy hadrons within the parton model is proposed. This approach is demonstrated in a set of processes of hadron-hadron collisions with QCD subprocesses of 2 -> 2 type. The process with the subprocess gb -> tH(-) was also considered.

Описана конструкция калориметра мягких фотонов установки СВД-2. Апертура прибора 210 × 210 мм. Приведены первые результаты его испытания в ходе физического набора статистики в эксперименте E-190 на ускорителе У-70 ИФВЭ.

The design of the soft photon calorimeter for the SVD-2 experiment is described. The aperture of the calorimeter is 210 × 210 mm. The first results of its testing in the course of data acquisition in the E-190 experiment at the U-70 accelerator of the Institute for High Energy Physics are presented.

An extension of the QED “return to resonance” mechanism to light meson emission (π, ρ) in (anti)proton collisions with a hadronic target (nucleon or nucleus) is proposed. The cross section and the multiplicity distributions are calculated. The collinear emission (along the beam direction) of a charged meson may be used to produce high energy (anti)neutron beams. Possible applications at existing and planned facilities are considered.

The experimental studies of the high multiplicity events in proton interactions are carried out on U-70 accelerator in Protvino. It is suggested that the collective phenomena can be discovered since the high density matter can be formed in this very region. The collective behavior of secondary particles can manifest in the Bose-Einstein condensation of pions, Vavilov-Cherenkov gluon radiation, excess of soft photon yield and other unique phenomena.

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.

A high multiplicity study (project Thermalization, experiment E-190) is carried out on U-70 accelerator at IHEP (Protvino, Russia). This project is aimed at searching for collective phenomena. It is known that pions are mainly produced at the 50 GeV-proton beam accelerator. Their mean energy decreases while multiplicity increasing. Bose-Einstein condensate (BEC) can be formed in this system. The theoretical and experimental studies of BEC have been performed since 70es. Within the framework of an ideal pion gas model M. Gorenstein and V. Begun have shown that sharp growth of fluctuations of the neutral pion number will be a signal of BEC formation with the increase of the total multiplicity (neutral and charged particle sum). SVD-2 Collaboration (JINR, IHEP and SINP MSU) investigated fluctuations of the neutral pion number in pp interactions at 50 GeV/c incident beam on U-70 versus the total multiplicity and has revealed noticeable growth of the scaled variance with the total multiplicity increase. The growth of these fluctuations reaches more than 7 standard deviations for the scaled variance at the total multiplicity about 30 pions as opposed to the tendency for the simulated events. This growth has been observed both in the registered photons and restored neutral pions.

Описаны этапы развития и современное состояние многоцелевой установки “Спектрометр с вершинным детектором”, созданной для проведения физических экспериментов на ускорителе У-70 Института физики высоких энергий (Протвино). Основными детекторами установки являются: вершинный детектор на основе кремниевых микростриповых детекторов, широкоапертурный магнитный спектрометр на основе пропорциональных камер, годоскопический детектор -квантов, собранный из свинцовых стекол. Для отбора нужных взаимодействий частиц в установке имеется быстродействующая двухуровневая система запуска. Приведены основные характеристики созданных систем установки, кратко перечислены полученные физические результаты.