Meson exchange interactions at low and intermediate energies: colour screening effects

We consider the colour screening effects of constituent quarks and the exchange of effective gluons in the interaction of hadrons. To this end we propose two different processes derived from fundamental properties of the underlying microscopic theory (QCD). Sum rules for diagrams providing colour screening are also given.     

Wave-spreading effects in nuclear scattering at intermediate energies

Based on the Fresnel approach discussed in a previous article the S-matrix for nuclear elastic scattering is factorized into an infinite product, the n-th term of which is essentially given by (n ? 1)-fold commutators of the interaction potential at n different z-coordinate positions. A subsequent cumulant expansion expresses the Fresnel phase shift as an infinite sum of many-body clusters. In constrast to eikonal-type expansions each term of this sum contains contributions of all orders in the inverse wave number. Because of this feature the present expansion is useful for scattering from light nuclei over a wide range of energies and scattering angles.     

Search for dibaryon production in 4Hep interactions at intermediate energies

With the aid of the 2-m liquid-hydrogen bubble chamber constructed at the Institute of Theoretical and Experimental Physics (ITEP, Moscow), ⁴Hep interactions are studied at primary alpha-particle momenta of 2.7 and 5 GeV/c (the respective kinetic energies of primary protons in the ⁴He rest frame are T _p=220 and 620 MeV). The effective-mass spectra of two nucleons from the reactions ⁴Hep → dppn and ⁴Hep → pppnn are analyzed. The effective-mass spectrum of the two-proton system produced in the quasielastic-charge-exchange reaction p ⁴He → n _F(pp)d at T _p=620 MeV (here, n _F stands for the fast neutron in the ⁴He rest frame) shows a narrow peak, which is indicative of the existence of a dibaryon whose mass and width are M _2p =2008±13 MeV and Γ _2p =0±5 MeV, respectively. In the mass spectra of the two-proton system from the reactions p ⁴He → n _F(pp)d and p ⁴He → p _F(pp)(nn), narrow threshold peaks are also found at a mass of M _2p ?1878–1879 MeV.     

Clusterisation and isospin effects in heavy-ion collisions at intermediate energies

We study the multifragmentation phenomenon in heavy-ion collisions by varying the spatial constraint criterion in minimum spanning tree (MST) clusterisation procedure. Within the framework of isospin-dependent quantum molecular dynamics (IQMD) model, the role of isospin-dependent spatial constraint, i.e. iso-MST version, is investigated on different fragment observables in various isobaric pair of reaction systems varying in the entrance channel isospin (N / Z) content. The fragment observables such as persistence, gain, average yield of free nucleons, light and intermediate mass fragments are slightly sensitive to the isospin-dependent spatial constraint criterion particularly in heavier reaction systems. For a given isobaric pair of reaction systems, the fragment production, however, remains indifferent to isospin content of the colliding nuclei.     

Isospin effects on subthreshold kaon production at intermediate energies

We show that in collisions with neutron-rich heavy ions at energies around the production threshold K0 and K+ yields probe the isospin-dependent part of the nuclear equation of state at high baryon densities. In particular, we suggest the K0/K+ ratio as a promising observable. Results obtained in a covariant relativistic transport approach are presented for Au+Au collisions at 0.8-1.8A GeV. The focus is put on the equation of state influence which goes beyond the collision-cascade picture. The isovector part of the in-medium interaction affects the kaon multiplicities via two mechanisms: (i) a symmetry potential effect, i.e., a larger neutron repulsion in n-rich systems, and (ii) a threshold effect, due to the change in the self-energies of the particles involved in inelastic processes. Genuine relativistic contributions are revealed that could allow one to directly "measure" the Lorentz structure of the effective isovector interaction.     

Study of deuteron breakup in light targets at intermediate energies

Reaction cross-sections and proton removal cross-sections in deuteron-induced reactions with carbon and beryllium targets are studied at intermediate energies. The cross-sections calculated in eikonal approximation show a good agreement with experimental data. The sensitivity of the cross-sections to the target structure, the nucleon-target interaction potential, and the spatial distribution of nucleons in the deuteron is discussed. Received: 30 May 2001 / Accepted: 2 October 2001     

On screening effects in pair production at intermediate and high energies

The effect of screening in atomic field pair production is calculated in Born approximation without approximation as to energy or angle and compared with the results of previous calculations based on the high energy approximation. The difference between the two calculations is comparable to the errors in recent experiments. The effect of nuclear excitation on the total pair production cross section is also investigated, but is found to be extremely small.     

Transient phenomena at intermediate energies

Considering the experimental energy behaviours of elastic K⁺p, pp, πp, polarizations, we show that in helicity-flip amplitudes non-leading contributions, non-identifiable with absorptive corrections, Regge-Regge cuts or resonance tails, remain non-negligible up to ～ 10 GeV/c. The interpretation of such contributions appears uncertain.     

Experiments on the study of the deuteron-proton interactions at intermediate energies at Internal Target at Nuclotron

The experimental program on the study of deuteron-proton interaction at Internal Target at Nuclotron is discussed. Recent results obtained for dp-elastic scattering and dp-breakup reactions at intermediate energies are presented. The status of the DSS setup upgrade is reported.     

Simple regge pole model for neutron-proton elastic scattering at high energies

It is shown that, by a phenomenological choice of residue functions, the most recent measurements of the differential cross section, the total cross section and the slope fornp elastic scattering from 70 to 400 GeV/c can be explained on simple Reggepole model. The predictions are also made regarding the energy dependence of dip and bump structure.     

Relativistic effects in exclusive pd breakup scattering at intermediate energies

The relativistic Faddeev equation for three-nucleon scattering is formulated in momentum space and directly solved in terms of momentum vectors without employing a partial wave decomposition. Relativistic invariance is achieved by constructing a dynamical unitary representation of the Poincaré group on the three-nucleon Hilbert space. The exclusive breakup reaction at 508 MeV is calculated based on a Malfliet–Tjon type two-body interaction and the cross sections are compared to measured cross sections at this energy. We find that the magnitude of the relativistic effects can be quite large and depends on the configurations considered. In spite of the simple nature of the model interaction, the experimental cross sections are in surprisingly good agreement with the predictions of the relativistic calculations. We also find that although for specific configurations the multiple scattering series converges rapidly, this is in general not the case.     

Positron-helium elastic scattering at intermediate energies

Total cross sections for elastic positron-helium collisions have been computed and compared with other theoretical results. Integrated cross sections over small angular regions extending from zero angle, for which a set of experimental estimates is available, are also presented.     

Alpha-nucleus elastic scattering at intermediate energies

Elastic scattering of 288,340,480 and 699 MeV Alpha-particles was measured on ²⁰⁸Pb, ¹¹⁶Sn and ⁵⁸Ni. The data were analysed in terms of a phenomenological optical model. The optical potentials obtained were found to vary consistently with the target nucleus and the incident energy. The radial zone where the potentials are well determined was studied in detail. The data for ²⁰⁸Pb were also analysed with a folding model. The energy dependence of the strong-absorption radius and of the reaction cross section shows that the nuclear surface becomes slightly transparent for incident energies above 150 MeV per nucleon.     

Deuteron-proton elastic scattering at intermediate energies

The deuteron-proton elastic scattering has been studied in the multiple-scattering expansion formalism. Primary attention has been given to such relativistic problems as a deuteron wave function in a moving frame and transformation of spin states due to Wigner rotation. Parametrization of the nucleon—nucleon t matrix has been used to take the off-energy-shell effects into account. The vector, A _y , and tensor, A _yy , analyzing powers of the deuteron have been calculated at two deuteron kinetic energies: 395 and 1200 MeV. The obtained results are compared with the experimental data. The text was submitted by the authors in English.     

Evolution of intranuclear collisions at intermediate energies

The experimental data on energy and multiplicity distributions of protons and momentum spectra of deuterons emitted in the reactions of pions and protons of energy ranging from 2.5 to 200 GeV with several nuclei are compared to the calculations based upon two different model approaches: DCM Monte Carlo [3] code and the tube-fireball (TF) model [4]. The results of this confrontation are discussed within the framework of the intranuclear interaction mechanism assumed. A detailed study of the intranuclear scenario presented by the DCM Monte Carlo program gives grounds to build simplified analytic models like the TF which may turn out to be very useful in many practical problems as being of a predictive nature. The text was submitted by the authors in English.     

Measurement of spin observables in neutron-proton elastic scattering

The spin correlation parameters and the analyzing power have been measured in free np elastic scattering at kinetic energies of 260, 315, 380, 460 and 535 MeV in the c.m. angle range from 60° to 164°. The experiment was performed at the Paul-Scherrer-Institut (PSI) using a polarized neutron beam with continuous energy incident on a polarized proton target. These data will contribute significantly to the determination of the isospin I=0 nucleon-nucleon amplitudes. Received: 24 May 2000 / Published online: 31 August 2000