Central Nucleus-nucleus Collisions Research Articles

Quasi-elastic recoil in central nucleus-nucleus collisions

The effect of nuclear elastoplasticity is studied in central collisions of 208 Pb + 124 Sn at bombarding energies between 0.1 and 5 MeV/u above the Coulomb barrier. The results show that for energies < 2 MeV/u a considerable amount of elastic response survives such that with 208 Pb as the projectile the recoiling target-( 124Sn-)like nuclei have velocities at least 15% larger than the beam.

Subthreshold negative pions and energetic protons produced at θcm = 90° in 246 MeV/nucleon 139La+ 139La collisions

The inclusive cross section for subthreshold negative pions and high energy protons produced at θ cm = 90° has been measured for 139La+ 139La at 246 MeV/nucleon. The spectrum of negative pions exhibits no bump or sharp break as has been suggested by theoretical speculation on the formation of a pionic instability in central nucleus-nucleus collisions. The measured spectra for both pions and protons are compared with results of a phase space model and an intranuclear cascade calculation.

Pion production as a test of nuclear matter properties.

Nuclear matter equations of state are constructed which incorporate explicitly the pion and delta degrees of freedom. These are used in relativistic one-dimensional and three-dimensional hydrodynamical models of central nucleus-nucleus collisions to determine the influence of the compressional energy on the observed pion multiplicity. We find a modest dependence on the compressional energy but a significant difference between slab-slab and sphere-sphere collisions having to do with curved shock waves. Thus one-dimensional hydrodynamic models do not appear to be suitable for interpreting pion production at energies of 100 MeV--2 GeV.

Pion production in inelastic and central nuclear collisions at high energy.

A detailed study of pion production in inelastic and central nucleus-nucleus collisions was carried out using a 2 m streamer spectrometer. Nuclear targets mounted inside the streamer chamber were exposed to nuclear beams of 4.5 GeV/c/nucleon momentum. A systematic study of the influence of the central trigger on observed data is performed. The data on multiplicities, rapidities, transverse momenta, and emission angles of negative pions are presented for various pairs of colliding nuclei. Intercorrelations between various characteristics are studied and discussed. The results are compared with predictions of some theoretical models. It is shown that the main features of the pion production in nuclear collisions can be satisfactorily described by a model assuming independent nucleon-nucleon collisions with subsequent cascading process. However, the observed correlation between \ensuremath{\Lambda} and pion characteristics seems to be unexplained by this picture.

Pion production as a probe of the nuclear matter equation of state

It is shown that thermal and chemical equilibrium are approached during the high density stage in central nucleus-nucleus collisions and that the yield of produced pions is determined at that time. A chemical model with Rankine-Hugoniot compression is used to extract a nuclear matter equation of state from the observed pion yield assuming a partition of the internal energy per nucleon into thermal and compressional energy fractions, with only the former part contributing to particle production.

Diabatic single-particle states: A convenient basis for dissipative nucleus-nucleus collisions

The diabatic limit for the quantum-mechanical coupling between collective and single-particle motion is investigated for central nucleus-nucleus collisions. Travelling diabatic single-particle states qualify as a convenient basis within a transport-theoretical approach between 1 MeV/u and 12 MeV/u bombarding energies above the barrier for heavy symmetric systems.

Diabatic emission of neutrons: A probe for the energy-dissipation mechanism in nucleus-nucleus collisions

The precompound emission of neutrons in central nucleus-nucleus collisions is investigated within the framework of dissipative diabatic dynamics. For 92Mo + 92Mo at bombarding energies between 7.5 and 20 MeV/u the differential neutron multiplicities dM n dE n are estimated from the decay of highly excited diabatic single-particle states. The energy spectra have an almost exponential high-energy tail with effective temperatures up to 10 MeV for 20 MeV/u bombarding energy.

The diabatic two-center shell model

Diabatic single-particle representations for the microscopic description of central nucleus-nucleus collisions within the framework of dissipative diabatic dynamics (DDD) are investigated. Based on the scaling condition for diabatic states, two construction methods referred to as the methods of maximum overlap and maximum symmetry, respectively, are studied within a two-center shell model. The conservative diabatic potential and the associated mass tensor for the pure diabatic single-particle motion are calculated as functions of three collective variables for 92Mo+ 92Mo and 86Kr+ 166Er. The calculated diabatic potentials are strongly repulsive and essentially depend only on the distance between the centers of mass in the region of strongly overlapping nuclei. The diabatic mass tensors are smooth functions of the collective variables and are close to the values for irrotational incompressible flow.

Hydrodynamics of hadronic matter in heavy ion collisions

We study central nucleus-nucleus collisions using a model based on hydrodynamics in order to assess the possibility of attaining the quark-gluon plasma phase. We integrate numerically the hydrodynamic equations with source terms and obtain in the central region an energy density which appears to be high enough for the formation of plasma.

Characteristics of Λ andK 0 particles produced in central nucleus nucleus collisions at a 4.5 GeV/c momentum per incident nucleon

Data on Λ's andK0's, produced in central nucleus-nucleus collisions (C−C, C−Ne, O−Ne, C−Cu, C−Zr, C−Pb, O−Pb) at a 4.5 GeV/c momentum per incident nucleon obtained in the streamer chamber spectrometer SKM-200, are presented. Multiplicities, transverse momenta, rapidities and other characteristics are considered and compared with those for inelastic He−Li interactions. The polarization of Λ's was found to be consistent with zero. The upper limit of\(\bar \Lambda /\Lambda \) production ratio was estimated to be ≤10−2 with a 90% confidence level. The results are compared with data of other experiments and some model calculations.

Dependence of the Average Transverse Momentum on the Multiplicities in Nucleus-Nucleus Collisions

It is shown that the value of /D is related to the increase of the average transverse momentum with increasing multiplicity. In this way, the rise is smaller for total nucleus-nucleus events than for p-barp events; however, this rise is very fast for central nucleus-nucleus collisions because of the large value of /D of such collisions. This effect could explain the large value of the average transverse momentum found in cosmic-ray events.

Pion multiplicity in central nucleus-nucleus collisions

Recently, the discrepancy between standard intranuclear cascade model predictions and experimental data for the pion multiplicity in central nucleus-nucleus collisions has been interpreted as a manifestation of compression effects. We explore here alternative effects, involving pion reabsorption by two nucleons, off-mass shell effects and renormalization of the pion sources inside a dense nuclear medium.

Peripheral and central nucleus-nucleus collisions at 4.2 GeV/c per nucleon

Experimental data on interactions of relativistic p, d, He and C nuclei with carbon and tantalum nuclei are presented. The data come from the 2−m propane bubble chamber with an internal tantalum target. It is found that interaction cross sections for carbon projectile fragments with chargesZ=5,6 are 10% greater than expected. In central C+C collisions and in multinucleon interactions of d, He and C with tantalum a contribution is observed from cumulative interactions irreducible to the superposition of NN-collisions taking into account final state interactions, rescattering effects, etc.

Dissipative diabatic dynamics: A shell-model approach to large-amplitude collective nuclear motion

Starting from the discussion of isoscalar giant vibrations as small-amplitude diabatic modes, a time-dependent shell-model approach is introduced which simultaneously treats coherent and incoherent aspects of large-amplitude collective nuclear motion. The coherent coupling of the single-particle motion to the time-dependent mean field is treated in a diabatic approximation. The incoherent aspect is represented by residual two-body collisions which lead to statistical equilibrium within the intrinsic degrees of freedom (local equilibrium). An important consequence of the combined effects from diabatic single-particle motion and dissipative two-body collisions is a significant retardation of the friction force for realistic values of the mean free path of nucleons. The retarded-friction term describes an elastic response of the nucleus for perturbations fast compared to the local equilibration time, and a dissipative response for slow perturbations. This elastoplastic behaviour represents an interesting dynamical property of nuclear matter. A diabatic two-center shell model is introduced and results on diabatic single-particle levels and potentials are given and compared with corresponding quantities from TDHF calculations. Effects on cross-sections of nucleus-nucleus collisions (deeply inelastic reactions, extra-push for capture and fusion) are discussed. A specific finger print for the diabatic single-particle motion is the preequilibrium emission of neutrons from unbound diabatic levels in central nucleus-nucleus collisions.

ΛHyperons Produced in Central Nucleus-Nucleus Interactions at 4.5 GeV/cMomentum per Incident Nucleon

Transverse momenta and rapidities of ..lambda..'s produced in central nucleus-nucleus collisions at 4.5 GeV/cxu (C--C,..., O--Pb) were studied and compared with those from inelastic He-Li interactions at the same incident momentum. Polarization of the ..lambda.. hyperons was found to be consistent with zero (..cap alpha..P = -0.06 +- 0.11 for ..lambda..'s from central collisions). An upper limit of the Lambda-bar/..lambda.. production ratio was estimated to be less than 4.5 x 10/sup -3/. The experiment was performed in a triggered streamer chamber.

Multiplicity and average PT of central nucleus-nucleus collisions

We evaluated the multiplicity and average P T of central nucleus-nucleus collisions using an extension of the Glauber model and theoretical ideas coming from particle-nucleus interactions. The multiplicity rises with the number of nucleons of the projectile A as c 1A+c 2A 1 3 . The average P T decreases with the number of nucleons B of the target. We discuss our results in connection with the possible phase transition between hadron matter and quark-gluon matter.

The influence of precritical scattering on inclusive proton production in relativistic central nuclear collisions

By use of the improved version of our model of transport theory, the effect of precritical scattering on the inclusive cross section for protons produced in central nucleus-nucleus collisions at 800 MeV/N is studied. As a result, we find an enhancement of the inclusive cross section at high transverse momenta by nearly a factor of two, and angular distributions which are more isotropic than for the case without precritical scattering. These features are consistent with the results of a previous investigation of impact parameter integrated inclusive cross sections.

ΛProduction near Threshold in Central Nucleus-Nucleus Collisions

$\ensuremath{\Lambda}$'s produced in central collisions of $^{40}\mathrm{Ar}$+KC1 at 1.8-GeV/u incident energy were detected in a streamer chamber by their charged-particle decay. For central collisions with impact parameters $bl2.4$ fm the $\ensuremath{\Lambda}$ production cross section is 7.6\ifmmode\pm\else\textpm\fi{}2.2 mb. A calculation in which $\ensuremath{\Lambda}$ production occurs in the early stage of the collision qualitatively reproduces the results but underestimates the transverse momenta. An average $\ensuremath{\Lambda}$ polarization of -0.10\ifmmode\pm\else\textpm\fi{}0.05 is observed.

Meson production in central collisions of heavy primaries of the cosmic radiation

Twenty one central nucleus-nucleus collisions of heavy (Z < 3) primaries with nuclei of either the photographic emulsion or lead with energies E < 100 GeV/nucleon have been studied. The total number of minimum tracks in the whole sample was 2564. Among them an extra high-multiplicity event of the type 10 + 518 Ca has been analysed. Twenty six secondary jets have been found and measured, the relative scattering measurements have been performed on 31 tracks. It has been shown that the process of destruction of the incident nucleus in the central collisions is different from the usual evaporation. In these collisions the main part of the incident nucleons interacts. After interaction they are collimated at small angles conserving about 40% of the primary energy. Meson production in central collisions is not in disagreement with the assumption of a superposition of nucleon-nucleon collisions.