E. S. Kokoulina

Spectrometer with a vertex detector for experiments at the IHEP accelerator

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 trigger of events with a high multiplicity of charged particles at the SVD-2 setup

The Thermalization project is currently under way on the extracted proton beam of the U-70 accelerator at the Institute for High Energy Physics. The main goal of this experiment is to study the collective behavior of secondary particles produced in a multiparticle pp interaction at beam energy E = 50–70 GeV. The region of high charged-particle multiplicities nch ≥ 10 is investigated. In this region, a significant portion of energy in the center-of-mass system is dissipated on production of secondary particles, which results in formation of a hadronic system. At the initial instant of evolution, the density of this system is high, which may be responsible for the collective behavior of the secondary particles. The investigations are performed at the SVD-2 wide-aperture magnetic spectrometer. A multichannel trigger scintillation hodoscope used to select rare events with a large number of charged particles exceeding the predetermined value is described. The main flow of low-multiplicity events is suppressed by the trigger system by a factor of ∼102.

High-multiplicity study

Theoretical and experimental problems of multiparticle production are analyzed in the high-multiplicity region. Interesting collective phenomena may be revealed in this region. Preliminary results of the Thermalization Project are being reported.

A drift tracker of the SVD-2 setup

The design and the characteristics of a drift tracker for the SVD-2 setup at the U-70 accelerator of the Institute for High Energy Physics (Protvino, Russia) are described. The drift tracker has been developed to improve the quality of track reconstruction in high-multiplicity events. It is composed of 2304 drift tubes with a diameter of 6 mm and a sensitive region 500–900 mm long. The distance between a track and the anode wire is determined from the drift time. The coordinate resolution and the detection efficiency of the tubes have been determined from the data of the accelerator run in 2006.

Evidence for a pion condensate formation in pp interactions at U-70

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.

A Soft Photon Calorimeter for the SVD-2 Experiment

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.

Search for collective phenomena in hadron interactions

New results of the search for collective phenomena have been obtained and analyzed in the present report. The experimental studies are carried out on U-70 accelerator of IHEP in Protvino. It is suggested that these phenomena can be discovered at the energy range of 50–70 GeV in the extreme multiplicity region since the high-density matter can form in this very region. The collective behavior of secondary particles is considered to manifest itself in the Bose-Einstein condensation of pions, Vavilov-Cherenkov gluon radiation, excess of soft-photon yield, and other unique phenomena. The perceptible peak in the angular distribution has been revealed. It was interpreted as the gluon radiation and so the parton matter refraction index was determined. The new software was designed for the track reconstruction based on Kalman Filter technique. This algorithm allows one to estimate more precisely the track parameters (especially momentum). The search for Bose-Einstein condensation can be continued by using the selected events with the multiplicity of more than eight charged particles. The gluon dominance model predictions have shown good agreement with the multiplicity distribution at high multiplicity and confirmed the guark-gluon medium formation under these conditions.

High-energy interactions in the region of extreme multiplicities

Hadron and nuclear interactions at high energies in the region of extreme multiplicities (much higher than mean multiplicities) are reviewed. Collective phenomena such as Bose-Einstein condensation, Cherenkov gluon radiation, clustering, and an excess yield of soft photons may manifest themselves in this region. The phenomenological gluon-dominance model developed for describing extreme multiplicities predicts the restrictions on their values and the type of hadronization mechanism and makes it possible to estimate thee size of the hadronization region. The status of the research within the Thermalization project at the U-70 accelerator of the Institute for High Energy Physics (IHEP, Protvino) is reported, and information about searches for new collective phenomena and about studies of known ones in the region of extreme multiplicities is given.

Proton interactions with high multiplicity

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.

Analysis of high multiplicity events

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.