M.C. Lemaire

Medium Effects in Pion Production

Abstract Pion production is studied in the framework of the intranuclear cascade model, for three different systems: proton-nucleus, heavy-ion and pion-nucleus collisions. The aim of this work is to investigate the sensitivity of the pion yield to modifications of the input data describing the properties of the delta and pion formation, destruction and propagation due to the surrounding nuclear medium. For these systems both the target mass dependence at one incident energy and the energy dependence for few selected targets have been considered. It is found that the three systems behave quite differently. The sensitivity of the heavy-ion case is rather large and medium corrections can remove the original discrepancy between standard cascade results and experiments. The possible lack of absorption in the standard model for pion production is also discussed.

Study of decay and absorption of the Δ resonance in nuclei with a 4π detector

Charge exchange reactions have been investigated at Saturne with various projectiles (3He, \(\vec d\), 12C,16O, 20Ne, 40Ar) and incident energies between.5 and 1.1 GeV/nucleon1. These experiments have given clear indication of the selectivity of charge exchange reactions for spin isospin excitations. In particular, a strong excitation of the Δ resonance shows up for energy transfers higher than the pion mass. The most striking common feature of all these charge exchange reactions is a shift of about 70 MeV between the positions of the Δ bump on nuclei and on hydrogen targets (fig.1). This result has been interpreted by theoreticians as a collective effect in the spin longitudinal channel2 and has been quantitatively reproduced by recent calculations3. An alternative qualitative explanation is the projectile excit ation4.

Measurement of a Baryon azimuthal emission pattern in Ne+(NaF, Nb, Pb) collisions at 800 MeV per nucleon

Abstract Triple-differential cross sections of proton-like particles were obtained for Ne+NaF, Ne+Nb and Ne+Pb reactions at 800 MeV per nucleon from quasi-exclusive measurements. Their dependence upon the azimuthal angle with respect to the reaction plane is analyzed. In addition to the usual flow pattern, another component of the azimuthal dependence is observed. It can be parametrized using the second order parameters in Fourier series expansion. The dependence of this component upon transverse momentum, rapidity and impact parameter (multiplicity) is investigated.

The Diogene 4 pi detector at Saturne

Diogene, an electronic 4 pi detector, has been built and installed at the Saturne synchrotron in Saclay. The forward angular range (0 degree-6 degrees) is covered by 48 time-of-flight scintillator telescopes that provide charge identification. The trajectories of fragments emitted at larger angles are recorded in a cylindrical 0.4-m3 Pictorial Drift Chamber (PDC) surrounding the target. The PDC is inside a 1-T magnetic field; the axis of the PDC cylinder and the magnetic field are parallel to the beam. Good identification has been obtained for both positive and negative pi mesons and for hydrogen and helium isotopes. Multiplicities in relativistic nucleus-nucleus reactions up to 40 have been detected, limited mainly by the present electronics.

Nuclear collective flow from gaussian fits to triple differential distributions

Abstract In order to study the nuclear collective flow, the triple differential momentum distributions of charged baryons are fitted to a simple anisotropic gaussian distribution, within an acceptance which removes most of the spectator contribution. The adjusted flow angle and aspect ratios are corrected for systematic errors in the determination of the reaction plane. This method has been tested with Monte Carlo simulations and applied to experimental results and intranuclear cascade simulations of argon-nucleus collisions at 400 MeV per nucleon.

Proton-proton correlations at small relative momentum in neon-nucleus collisions at E/A=400 and 800 MeV

Proton-proton small angle correlations have been measured in neon-nucleus collisions, using the 4 pi detector Diogene, at 400 and 800 MeV per nucleon incident energies. Values of the size of the emitting region are obtained by comparison with the Koonin formula, taking into account the biases of the apparatus. The dependence of the density on target mass and incident energy is also analysed.

Possible observation of medium effects using a pion correlation technique

Abstract Correlations of oppositely charged pions produced in interactions of protons and light nuclei with different nuclear targets are investigated. The invariant-mass distributions are analyzed in order to search for new conjectured nuclear effects. Strong and electromagnetic final-state interactions between pions and the Coulomb interaction between pions and the target nucleus are taken into account. An admixture of pion pairs coming from resonance decays is estimated. An enhancement in the low invariant-mass region is observed only for heavy target systems. A shadowing effect due to pion absorption inside the nuclear medium is considered as a possible source of this pattern. Azimuthal π + π − correlations and pion yield dependence on the impact parameter, as well as transport calculations, seem to confirm this assumption.

Collective flow and pion production at diogene

Abstract Recent results from the large solid angle detector “DIOGENE” at Saturne (Saclay), for Ne + A and Ar + A reactions at 400, 600 and 800 MeV per nucleon are presented. Several points are selected: 1) The particles azimuthal emission pattern (with respect to the reaction plane) exhibiting preferential out-of-plane emission at c.m. rapidities, 2) The presence of an appreciable amount of collective flow for asymmetric systems and even for (rather) small projectile masses, 3) The first measurements of “flow” and multiplicities for both positive and negative pions, 4) The first measurement of deltas in central relativistic heavy ion collisions.

Exclusive measurements of light fragment production at forward angles in Ne-Pb and Ne-NaF collisions at E/A=400 MeV and 800 MeV.

Emission of light fragments at small angles is studied in relativistic heavy ion collisions using the Diogene plastic wall for both symmetrical and non-symmetrical target-projectile systems with 400 MeV per nucleon and 800 MeV per nucleon incident neon nuclei. Efficiency of multiplicity measurements in the small angle range for the selection of central or peripheral collisions is confirmed for asymmetric systems. Differential production cross sections of Z = 1 fragments show evidence for the existence of two emitting sources. The apparent temperature of each source is obtained from comparison with a thermodynamical model.

Charged pion anisotropy in relativistic nucleus-nucleus collisions

Abstract The momentum distributions with respect to the reaction plane for charged pions emitted in Ne-nucleus and Ar-nucleus collisions have been measured with the 4 π detector Diogene at the Saturn synchrotron. The reaction plane is estimated from the baryon momenta, using a standard technique for flow analysis. The pions exhibit a non-zero flow. For asymmetric systems, there is a preferential emission in the direction of the lighter (projectile) nucleus. Such a behaviour contradicts the predictions of INC calculations and is underestimated by QMD calculations which include mean field effects. It can be interpreted with a simple geometrical model in terms of absorption by the spectator nuclear matter.