V. Antonenko

Centrality Dependence of Neutral Pion Production in 158A GeV

The production of neutral pions in 158AGeV Pb+Pb collisions has been studied in the WA98 experiment at the CERN SPS. Transverse momentum spectra are studied for the range 0.3 GeV/c<mT-m0<4.0 GeV/c. The results for central collisions are compared to various models. The centrality dependence of the neutral pion spectral shape and yield is investigated. An invariance of the spectral shape and a simple scaling of the yield with the number of participating nucleons is observed for centralities with greater than about 30 participating nucleons which is most naturally explained by assuming an equilibrated system.

^{208}

Abstract The multiplicity of inclusive photons has been measured on an event-by-event basis for 158· A GeV Pb induced reactions on Ni, Nb, and Pb targets. The systematics of the pseudorapidity densities at midrapidity ( ρ max ) and the width of the pseudorapidity distributions have been studied for varying centralities for these collisions. A power law fit to the photon yield as a function of the number of participating nucleons gives a value of 1.12±0.03 for the exponent. The mean transverse momentum, 〈 p T 〉, of photons determined from the ratio of the measured electromagnetic transverse energy and photon multiplicity, remains almost constant with increasing ρ max . Results are compared with model predictions.

Pb+

Abstract Inclusive spectra of protons emitted in the interaction of 12C-projectiles at an energy of 3.6 GeV/nucleon and C, Cu, Sn and Pb target nuclei were measured. The analysis of the experimental data suggests that in all these interactions the main contribution to proton cross sections in the investigated kinematic region is due to emission from a moving source with a rapidity which differs greatly from the target nucleus rapidity, yt, as well as from that of the effective fireball. The properties of this source such as the excitation energy depend only slightly on the combination of the colliding nuclei.

^{208}

An event-by-event analysis of the azimuthal angular correlation with respect to the reaction plane has been carried out for K+ and pi(+) emission near mid-rapidity in 158 A . GeV Pb + Pb collisions. In semi-central collisions, K+ mesons are found to be preferentially emitted out of the reaction plane, while pi(+) mesons are emitted in the reaction plane. The results suggest that the kaon emission is influenced by in-medium potential effects in addition to collective flow effects

Pb Collisions

New accurate data on the neutron-proton spin-dependent total cross section difference ΔσL(np) at the neutron beam kinetic energies 1.4, 1.7, 1.9 and 2.0 GeV are presented. A number of physical and methodical results on investigation of an elasticnp→pn charge exchange process over a few GeV region are also presented. Measurements were carried out at the Synchrophasotron and Nuclotron of the Veksler and Baldin Laboratory of High Energies of the Joint Institute for Nuclear Research.

Systematics of inclusive photon production in 158-A-GeV Pb induced reactions on Ni, Nb, and Pb targets

Abstract Inclusive spectra of protons emitted in the interaction of 3 He ions with an energy of 4.9 GeV/nucleon with nuclei of C, Cu and Pb were measured. The analysis of these data as well as of those available on nucleus-nucleus interactions at other energies (0.8, 2.1 and 3.6 GeV/nucleon) and for other combinations of colliding nuclei (from 3 He + C to Ar + Pb) has shown that in the target nucleus fragmentation domain the invariant cross section is described by a certain universal function if we go over into the appropriate moving frame. It is natural to interpret the latter as a real physical object, i.e. a moving source. This source turns out to be highly heated (the average energy being ⪆ 90 MeV/nucleon), at the same time its inner properties (the excitation energy and distribution form) do not depend either on the collision energy or on the projectile-target combination. On the other hand, the source velocity and the number of nucleons involved in it are systematically dependent on the collision conditions.

Data implying formation of a moving source in collisions of carbon-12 nuclei with various target nuclei at 3.6 GeV/nucleon

Abstract For nucleus-nucleus collisions at 3.6 GeV/nucl., we have measured correlations between a charged particle emitted in the range from 10° to 18° and identified a charged fragment (proton, deutron or pion) emitted at a certain angle in the range of 45°–130°. The following projectile-target combinations were studied: C+Al (45°, 60°, 80°, 130°); C+Cu (45°, 60°, 80°); C+Pb (45°, 60°, 80°, 130°); He+Al (45°, 80°, 130°); He+Pb (45°, 80°, 130°). In all the studied reactions, correlated particle emission has been observed.

Elliptic emission of K+ and π+ in 158 A·GeV Pb + Pb collisions

The first results of the measurement of 0o of the cross-section ratioRdp=0.50±0.02 in charge-exchange (np) reaction on H2/D2 targets at the neutron beam kinetic energie 1.0 GeV are presented. As a result the value of the ratio of non spin-flip to spin-flip contribution innp→pn charge-exchange at 0ornp→pn/nf/fl=0.33±0.03 was obtained.

Measurements of energy behaviour of spin-dependentnp—observables over 1.2–3.7 GeV energy region Dubna “Delta-Sigma” Experiment

New experimental results on ratio Rdp of the quasielastic charge-exchange yield at the outgoing proton angle θp,lab = 0° for the nd → p(nn) reaction to the elastic np → pn charge-exchange yield, are presented. The measurements were carried out at the Nuclotron of the Veksler and Baldin Laboratory of High Energies of the JINR (Dubna) at the neutron-beam kinetic energies of 0.55, 0.8, 1.0, 1.2, 1.4, 1.8, and 2.0 GeV. The intense neutron beam with small momentum spread was produced by breakup of deuterons which were accelerated and extracted to the experimental hall. In both reactions mentioned above the outgoing protons with the momenta pp approximately equal to the neutron-beam momentum pn,beam were detected in the directions close to the direction of incident neutrons, i.e., in the vicinity of the scattering angle θp,lab = 0°. Measured in the same data-taking runs, the angular distributions of the charge-exchange-reaction products were corrected for the well-known instrumental effects and averaged in the vicinity of the incident-neutron-beam direction. These corrected angular distributions for every of nd → p(nn) and np → pn charge-exchange processes were proportional to the differential cross sections of the corresponding reactions. The data were accumulated by Delta-Sigma setup magnetic spectrometer with two sets of multiwire proportional chambers located upstream and downstream of the momentum analyzing magnet. Inelastic processes were considerably reduced by the additional detectors surrounding the hydrogen and deuterium targets. The time-of-flight system was applied to identify the detected particles. The accumulated data treatment and analysis, as well as possible sources of the systematic errors are discussed.

Universal proton rapidity distributions in high energy nucleus-nucleus collisions

New accurate data of the neutron-proton spin-dependent total-cross-section difference ΔσL(np) at the neutron-beam kinetic energies 1.39, 1.69, 1.89, and 1.99 GeV are presented. In general, these data complete the measurements of energy dependence of ΔσL(np) over the Dubna Synchrophasotron energy region. Measurements were carried out at the Synchrophasotron of the Veksler and Baldin Laboratory of High Energies of the Joint Institute for Nuclear Research. The quasi-monochromatic neutron beam was produced by breakup of extracted polarized deuterons. The deuteron (and hence neutron) polarization direction was flipped every accelerator burst. The initial transverse (with respect to beam momentum) neutron polarization was changed to a longitudinal one and longitudinally polarized neutrons were transmitted through the large proton longitudinally polarized target. The target polarization direction was inverted after one to two days of measurements. Four different combinations of the beam and target parallel and antiparallel polarization directions, both oriented along the neutron-beam momentum, were used at each energy. A fast decrease in −ΔσL(np) with increasing energy above 1.1 GeV and a structure in the energy dependence around 1.8 GeV, first observed from our previous data, seem to be well revealed. The new results are also compared with model predictions and with phase-shift analysis fits. The ΔσL quantities for isosinglet state I = 0, deduced from the measured ΔσL(np) values and known ΔσL(pp) data, are also given. The results of the measurements of unpolarized total cross sections σ0tot(np) at 1.3, 1.4, and 1.5 GeV and σ0tot(nC) at 1.4 and 1.5 GeV are presented as well. These data were obtained using the same apparatus and high-intensity unpolarized deuteron beams extracted either from the Synchrophasotron or from the Nuclotron.