D. L'Hôte

Global variables and the dynamics of relativistic nucleus-nucleus collisions

Abstract Various global variables providing a simple description of high multiplicity events are reviewed. Many of them are calculated in the framework of an intra-nuclear cascade model, which describes the collision process as a series of binary on-shell relativistic baryon-baryon collisions and which includes inelasticity through the production of Δ-resonances. The calculations are first made for the Ar + KCl system at 0.8 GeV/A, with global variables including either all the nucleons or only the participant nucleons. The shape and the orientation of the ellipsoid of sphericity are particularly investigated. For both cases, on the average, the large axis of the ellipsoid is found to point in the beam direction. This result is discussed in comparison with hydrodynamics predictions and in relation with the mean free path. A kind of small “bounce-off effect” is detected for intermediate impact parameters. The possibility of extracting the value of the impact parameter b from the value of a global variable is shown to depend upon the variation of this variable with b and upon the fluctuation of the global variable for a given impact parameter. A quality factor is denned to quantify this possibility. No current global variable seems to be more appropriate than the number of participant nucleons for the impact parameter selection. The physical origin of the fluctuations inside the intranuclear cascade model is discussed and the possibility of extracting useful information on the dynamics of the system from the fluctuations is pointed out. The energy dependence of our results is discussed. Some results of the calculations at 250 and 400 MeV/A are also presented for the same system Ar + KC1.

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.

Coherent pions in charge-exchange reactions☆

Abstract Data are presented on the 12 C ( 3 He , t π + ) reaction at 2 GeV bombarding energy. In coincidence with forward scattered tritons, pions are momentum-analysed in a large acceptance detector. For events where the target is left in its g.s. or low-lying states, the angular correlation between the momentum transfer vector and the momentum of the pion is sharply peaked at small relative angles.

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.

Do nuclei flow inside the intranuclear cascade model

Abstract An intranuclear cascade calculation of Ca + Ca and Nb + Nb at 400 MeV/A is performed and analysed in terms of global variables. Our results are in qualitative agreement with the recent plastic ball data. The ability of the cascade dynamics to generate a collective flow and the effects of the fluctuations are underlined.

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.

Properties and causes of the collective flow

Abstract The properties of the collective flow occurring in the course of nucleus-nucleus collisions are investigated within the intranuclear cascade (INC) model. Contrary to our previous work, we here use a version of the cascade where the Fermi motion of the spectators is frozen until they interact for the first time. It is shown that there is an intrinsic flow within this version of the cascade, which is essentially due to the participants. The mass dependence of the flow is studied by looking at the Ca + Ca, Nb + Nb and Au + Au systems at 400 MeV/ A The flow is shown to increase with the mass of the system. It decreases, for a given system, when one goes to larger energies. These features are in qualitative agreement with experiment. A quantitative comparison is attempted for the Nb + Nb case. We discuss the problems encountered with application to cascade events of a filter, which fully accounts for the efficiency of the detection system. With a crude filter, we obtain for large multiplicities a peak in the flow angle distribution. The flow angle is too small in the cascade. The uncertainty of the comparison procedure is emphasized. We turn to the causes of the flow. It is shown that in the cascade, the flow arises from the work done by the pressure built inside the compressed central region on the outer layers of the system. However, it is argued on very general grounds that the flow must be reduced by the viscosity forces. It is indicated that this effect is probably present in the cascade, but that, very likely, other off-equilibrium effects further reduce the flow. Off-equilibrium effects in general are found to be responsible for the qualitative features of the mass and energy dependence of the flow.

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.