Flow effects in 84Kr induced collisions in emulsion at 0.95 GeV per nucleon

Using emulsion detector the collective flow signals in inelastic interactions of ⁸⁴Kr nuclei with Ag(Br) at 950 MeV/nucleon are studied. A transverse momentum analysis is performed to determine the reaction plane. The bounce-off of spectator fragments is observed. In azimuthal distributions relative to the reaction plane squeeze-out and side-splash of participants are seen. Received: 27 September 1999     

Transparency effects in heavy-ion collisions over the energy range 100–300 MeV/nucleon

Direct measurements of total reaction cross sections between 100 and 300 MeV/nucleon indicate that σ_R reaches a minimum around 300 MeV/nucleon corresponding to the maximum of the (surface) transparency effects in heavy ion collisions. Data are well reproduced by simple microscopic calculations and are in agreement with the new parametrization we proposed earlier.     

Neutron rich nucleon flow in central heavy ion collisions

In reactions of 1.2 GeV²⁰⁸Pb ions with ¹¹⁰Pd nuclei, mass, charge, and energy of the products were measured at various angles. Transfers up to 50 nucleons connected with very low excitation energies of the fragments were observed especially for reactions leading to large mass asymmetry. For central collisions the neutron to proton ratio of the nucleon flow has the unusually high value of 2.5.     

Partial-transparency effects in heavy-ion collisions at energies of the order of 1 GeV/nucleon

A model of heavy-ion collisions at energies E_lab ～ 1 GeV/nucleon taking into account the effects of incomplete statistical equilibrium in highly-excited nuclear matter is presented. The collision process is considered as the interaction of two flows of nucleons decelerating each other. This process is described within the framework of a relativistic kinetic approach employing the Fokker-Planck approximation. Assuming two-flow nonequilibrium the momentum distribution function is represented as a sum of two maxwellian distributions displaced by the average relative velocity of flows. Equations for time evolution of space-averaged velocities and internal energies of flows are derived. These equations contain a single model parameter, i.e. the effective deceleration length λ_d. Using the firestreak model geometry inclusive cross sections of protons and composite particles (d, t) in the reactions Ne + NaF, Ar + KCl and Ar + Pb are calculated at various values of E_lab. In contrast to the firestreak model, a complete stopping of colliding tubes in the centre-of-mass frame was not assumed in the present calculation. Composite particle spectra are calculated on the basis of the coalescence model. The theory is thoroughly compared with the experimental data. The approach suggested allows us to reproduce a two-humped structure in the rapidity distributions of the secondary particles. The experimental data analysis leads to the value λ_d = 8 fm for E_lab = 0.8 GeV/nucleon, which is in good agreement with the estimation based on experimental NN cross sections.     

Interplay between particle-hole excitation and nucleon transfer in deep-inelastic collisions

The interplay between particle-hole excitation and nucleon transfer in deep-inelastic collisions is studied within an independent-particle model in which the relative motion is treated classically. The energy and angular momentum loss are evaluated selfconsistently in the model. The results show that the inclusion of the ph excitation strongly affects the transfer mode giving rise to a drift of the charge centroid towards charge asymmetry. The model predicts a roughly equal excitation energy for the two fragments rather than one proportional to their masses.     

Compound-nucleus effects in nucleon depolarization

Compound-elastic spin-flip is shown to contribute significantly to nucleon depolarization. For example, the measured depolarizations for ¹⁴N(p,p)¹⁴N at 16.15 MeV at angles beyond 90° can be accounted for by compoundelastic scattering alone, without spin-spin or quadrupole spin-flip interactions.     

Preequilibrium nucleon jets in TDHF calculations of medium-energy,heavy-ion collisions

We study collisions of ¹²C and ¹⁹⁷Au projectiles at E_lab/A_proj = 30 to 85 MeV/A on ¹⁹⁷Au targets, using the 3-dimensional TDHF model. Most of the qualitative phenomena observed experimentally are reproduced by the TDHF calculations. The results are compared with recent data from the 1 GeV ¹²C beam at CERN and show some quantitative deviations from experiment. Possible reasons for these discrepancies are discussed and give valuable insight into intermediate energy collision processes.     

Transverse energy distributions in Si-nucleus collisions at 10 GeV/nucleon

Transverse energy distributions have been measured for collisions of 10 GeV/nucleon Si with targets of Al, Cu and Pb using a combination of a NaI wall and a uranium based sampling calorimeter. The measured cross sectionsdσ/dE _T anddE _T /dη are consistent with full stopping and an increase, with increasing values ofE _T , of energy flow into large angles.     

Strange particle production in nuclear collisions at 200 GeV per nucleon

Multiplicities and spectra of strange particles ( andK ^– produced in central³²S+S,³²S+Ag and³²S+Au collisions at 200 GeV per nucleon are presented and compared with data on strange particle production in protonnucleus and nucleon-nucleon interactions. It is shown that strangeness production in³²S+Ag collisions is enhanced by a factor of two, similar to that found previously in central³²S+S collisions.     

J/psi production in Indium-Indium collisions at 158 GeV/nucleon

The NA60 experiment studies muon pair production at the CERN Super Proton Synchrotron. In this Letter we report on a precision measurement of J/psi in In-In collisions. We have studied the J/psi centrality distribution, and we have compared it with the one expected if absorption in cold nuclear matter were the only active suppression mechanism. For collisions involving more than approximately 80 participant nucleons, we find that an extra suppression is present. This result is in qualitative agreement with previous Pb-Pb measurements by the NA50 experiment, but no theoretical explanation is presently able to coherently describe both results.     

Measurement of multiplicity distributions in oxygen-tungsten collisions at 200 GeV per nucleon

Multiplicity distributions of charged particles have been measured in the pseudorapidity region 0.9 <η _lab < 4.9 for oxygen-tungsten collisions at an incident energy of 200 GeV per nucleon. The multiplicity cross-sectiondσ/dN as measured in the backward hemisphere (0.9 <η _lab < 2.9) is found to be very similar in shape to the transverse energy distributiondσ/dE _T published previously by this group, reflecting the particular geometry of nucleus-nucleus collisions. Pseudorapidity distributionsdN/dη are presented and their dependence onE _T is discussed. The average transverse energy per charged particle is studied versusE _T and is found to be constant within our systematic error of approximately 10%.     

Production of nucleon resonances in π+d collisions

Production of nucleon resonances in π⁺d collisions in the energy range 1–5 GeV has been investigated in a model developed earlier as a direct means of testing the dynamic role of N¹ exchanges found in the processes π⁺d → pp and dp → pd. The results which are given in the form of inclusive cross sections for π⁺d → p(N¹, Δ⁺) show that despite the dominance of the isosbars P₃₃ and F₃₇, there are appreciable signals from the more prominent N¹ resonances (F₁₅ and to some extent D₁₃), so as to be amenable to experimental observation. It is further argued that observation of these N¹ should help discriminate the N¹ exchange mechanism from other alternative such as OPE.     

Subthreshold pion production from nucleus-nucleus collisions around 100 MeV/nucleon

Several global variables were tested with the aim to determine the impact parameter in nucleus-nucleus collisions producing pions at incident energies around 100 MeV/nucleon. The experimental set-up includes the MEDEA multidetector, part of which is used as a ⁰ spectrometer, and an additional hodoscope of plastic scintillators to cover very forward angles. A statistical model was used to generate bothinclusive and pion-triggered events. Selection ofwell measured events was made through the measured total parallel momentum. Among the different global variables which were tested, the average parallel velocity was seen to give the best correlation with the impact parameter.     

Gamow states in a realistic two-center potential and nucleon emission in heavy-ion collisions

For a realistic finite-depth two-center potential with fixed spherical Woods-Saxon wells adiabatic bound and Gamow states have been calculated by representing each potential in a harmonic oscillator basis in momentum representation. Numerical results for the real and complex energy eigenvalues of the single-particle states as a function of the distance between the centers of the wells are presented for a neutron in a ¹⁶O + ¹⁶O potential. Due to the neglect of volume conservation and of any shape degrees of freedom the considerations are restricted to separation regions with a small overlap of the density distributions only. The matrix elements for the coupling of adiabatic states due to non-adiabatic effects in the collective relative motion of the potentials during a heavy-ion collision have been calculated for bound-bound as well as bound-continuum transitions. Neglecting the rotational coupling, from the population of Gamow states a differential neutron emission spectrum has been computed, which for a peripheral ¹⁷O + ¹⁶O reaction shows distinct peaks at the position of low-lying asymptotic adiabatic states and a decreasing high-energy tail connected with emission from quasistationary states, pushed up in energy when the collision partners are coming in contact. An appreciable fraction of the particle emission appears as a sequential decay of the excited fragments after separation.     

Higher baryon resonances in carbon-carbon collisions at 4.2 GeV/c per nucleon

The production of the N(1440), the so-called Roper, and N(1520) resonances in high-energy collisions of carbon nuclei with the carbon nucleus, using a 2 m propane bubble chamber, was investigated. Attention was paid to the two-pion decay mode of the higher baryon resonances. From the invariant masses of three-particle states the mass and width of the resonances were obtained. The ratio of the resonances decay to and states was estimated.Received: 1 July 2003, Revised: 29 September 2003, Published online: 20 April 2004PACS: 25.70.Ef Resonances - 14.20.Gk Baryon resonances with S = 0     

Multijet events in soft and semihard hadronic collisions

Various phenomena suggest the importance of a hard component in soft hadronic collisions with large cross sections already at moderate energies below \(\sqrt s \sim 30 GeV\) which involve the valence quarks. We consider Born term models based on one gluon exchange and hadron quark couplings either pointlike or with additional largep _T damping. A resolution dependent multijet analysis-performed in a way similar toe ⁺ e ^?-annihilation-can yield important information about the elements of a perturbative approach. Predictions are presented for multijet rates and properties of 2 and 3 jet events in the low resolution regime. Some results for 2 jet events do also apply for higher energies and high resolution.     

Suppression of soft nuclear bremsstrahlung in proton-nucleus collisions

Photon energy spectra up to the kinematic limit have been measured in 190 MeV proton reactions with light and heavy nuclei to investigate the influence of the multiple-scattering process on the photon production. Relative to the predictions of models based on a quasifree production mechanism, a strong suppression of bremsstrahlung is observed in the low-energy region of the photon spectrum. We attribute this effect to the interference of photon amplitudes due to multiple scattering of nucleons in the nuclear medium.     

The transverse-energy distributions of 32S-nucleus collisions at 200 GeV per nucleon

Transverse-energy distributions have been measured for the collisions of the ³²S nucleus with Al, Ag, W, Pt, Pb, and U target nuclei, at an incident energy of 200 GeV per nucleon. The shapes of these distribution reflect the geometry of the collisions, including the deformation effects. For central collisions, the transverse-energy production in the region −0.1<η_lab<2.9 increases approximately as A^0.5, where A is the atomic mass number of the target. This increase is accompanied by a relative depletion in the forward region η_lab > 2.9. These results are compared with those obtained under similar conditions with incident ¹⁶O nuclei. A comparison is also made with the predictions of a Monte Carlo generator based on the dual parton model. Finally, we give estimates of the energy density reached and its dependence on the atomic mass number of the projectile.     

Search for direct photons from S-Au collisions at 200 GeV/nucleon

The CERES experiment has measured inclusive photon production in S-Au collisions of 200 GeV/nucleon at the CERN SPS. No evidence for direct emission of photons was found. For the kinematic region 2.1<y<2.65 and 0.4 GeV/c<p??<2.0 GeV/c the yield andp??-dependence of the observed photons are well reproduced by hadron decays. Furthermore, their production rate is found to be proportional to the charged particle density. The systematic errors comparing the measured and expected photon yield result in an upper limit of 14% for the emission of direct photons in central S-Au collisions. For a photon source with a yield depending quadratically on the charged particle density the limit can be reduced to 7%.