Studies of the out-of-plane emission of pions in symmetric heavy-ion collisions

Triple differential cross-sections of midrapidity pions from ²⁰⁹Bi+²⁰⁹Bi collisions were measured with the Kaon Spectrometer at SIS at incident energies of 400, 700 and 1000 AMeV. The azimuthal emission pattern of the pions has been investigated in dependence of beam energy and impact parameter. An enhanced emission of pions perpendicular to the reaction plane is observed. The strength of the anisotropy increases with beam energy and pion transverse momentum. In contrast to the nucleons the anisotropy varies only little with the impact parameter. No difference in the behaviour of positive and negative pions is observed.     

Study of the out-of-plane emission of protons and light fragments in symmetric heavy-ion collisions

Midrapidity protons from²⁰⁹Bi+²⁰⁹Bi collisions were measured with the Kaon Spectrometer at SIS at incident energies of E _Lab /A=400, 700 and 1000 MeV. Additionally, light fragments were analysed at 400 MeV. We have investigated the azimuthal emission pattern of the particles relative to the reaction plane as function of transverse momentum, bombarding energy and impact parameter. We observe an enhanced emission of particles perpendicular to the reaction plane at all bombarding energies. The ratio of the number of particles emitted out-of-plane/in-plane increases strongly with the particles transverse momentum. The anisotropy decreases with increasing beam energy. Composite particles show a much stronger effect than protons.     

Dissipation,mass exchange and the microscopic time scale of heavy-ion collisions

The time scale provided by the nucleon exchange mechanism in heavy-ion reactions is employed to study kinetic energy damping. The experimental kinetic energy lost per nucleon exchanged in Kr- and Xe-induced reactions is observed to decrease linearly with the total kinetic energy loss. These results, which are consistent with energy dissipation and nucleon exchange occuring on a similar time scale, are compared with the predictions of a one-body dissipation mechanism and a diffusion model.     

Azimuthal dependence of collective expansion for symmetric heavy-ion collisions

Detailed studies of the azimuthal dependence of the mean fragment and flow energies in the Au+Au and Xe+CsI systems are reported as a function of incident energy and centrality. Comparisons between data and model calculations show that the flow energy values along different azimuthal directions could be viewed as snapshots of the fireball expansion with different exposure times. For the same number of participating nucleons more transversally elongated participant shapes from the heavier system produce less collective transverse energy. Good agreement with Boltzmann-Uehling-Uhlenbeck calculations is obtained for a soft nuclear equation of state.     

Prompt emission of nucleons in heavy-ion collisions

The mechanism of the production of quasifree promptly emitted particles in heavy ion collisions is studied within a sharp surface model. The particle production cross section is found to be rather low, ～1% of the total reaction cross section at energies slightly above the Coulomb barrier. Arguments on how to observe them in experiments are presented. Comparison between theoretical results and experimental data does not rule out the possibility of this reaction process.     

Volume emission of pions in relativistic heavy-ion collisions

We consider the possibility of volume emission of pions by the Δ(1232) decay in a hot compressed projectile-target residue. We calculate the momentum distribution of the pions created by the two-body Δ(1232) decay. Reabsorption is taken into account assuming a quasi-deuteron model. We argue that low-energy pions created deep inside the hot zone are likely to escape and contribute to the pion inclusive spectrum.     

A two-stage model for fast particle emission in heavy-ion collisions

Using Bertsch's TDHF-motivated trajectory model we combine two physically distinct approaches to describe fast particle emission. In the early stage we calculate particle emission in the spirit of the Fermi-jet mechanism. In the later stage, after neck formation, particles are assumed to be emitted from a rapidly expanding hot zone of appreciably large initial dimension, which is strongly anisotropic in momentum space. We calculate absolute double-differential cross sections for preequilibrium neutron emission and obtain a remarkable agreement with experimental data without introducing free parameters.     

Universal flow-driven conical emission in ultrarelativistic heavy-ion collisions

The double-peak structure observed in soft-hard hadron correlations is commonly interpreted as a signature for a Mach cone generated by a supersonic jet interacting with the hot and dense medium created in ultrarelativistic heavy-ion collisions. We show that it can also arise due to averaging over many jet events in a transversally expanding background. We find that the jet-induced away-side yield does not depend on the details of the energy-momentum deposition in the plasma, the jet velocity, or the system size. Our claim can be experimentally tested by comparing soft-hard correlations induced by heavy-flavor jets with those generated by light-flavor jets.     

K-vacancy creation and positron emission in heavy-ion collisions

The production of K-shell vacancies during a heavy-ion collision is calculated in the adiabatic monopole approximation. For (U, U) collisions we present the cross section for positron emission in the transient supercritical field, based on this K-vacancy production probability.     

Quantum-mechanical treatment of diabatic neutron emission in heavy-ion collisions

The diabatic precompound emission of neutrons is studied in a finite time-dependent two-center shell model for central collisions of ⁹⁰Zr + ⁹⁰Zr and ⁹⁶Zr + ⁹⁶Zr. For laboratory energies between 8 $\text{MeV}\text{u}$ and 20 $\text{MeV}\text{u}$ the neutron multiplicity is found to be significantly enhanced for ⁹⁶Zr + ⁹⁶Zr as compared to ⁹⁰Zr + ⁹⁰Zr. The angular distribution of the emitted neutrons is forward-backward peaked in the center-of-mass system, while the energy spectra show an exponential tail, with a slope parameter T₀ roughly independent of the scattering angle. A close relation between the diabatic emission process, the Fermi-jet mechanism and the exiton model is established.     

Direct versus thermal particle emission in high-energy heavy-ion collisions

Recent measurements of proton inclusive cross sections for heavy-ion collisions at several hundred MeV per nucleon are discussed in a phenomenological model which assumes coexistence of direct and thermal particle emission. The model postulates the existence of an additive twocomponent structure for inclusive spectra. The two components consist of a direct and a thermal one. The direct component is described in an extended impulse approximation and the thermal component in the participant-spectator geometrical model. Their normalizations are fixed by geometrical considerations. With this model we calculate the charged particle and proton inclusive spectra for Ne on U at 250 MeV per nucleon, Ne on U and Ne on Al at 400 MeV per nucleon, and Ne on NaF at 800 MeV per nucleon. A comparison is made with corresponding data. At 250 MeV and 400 MeV per nucleon, we find the postulated two-component structure to be instrumental in reproducing satisfactorily the slopes and the magnitudes of the data in the critical forward angle region. At 800 MeV per nucleon, due to the larger transparency of the nuclei, non-equilibrium statistical components appear to be important.     

The inversion-asymmetry of particle emission source in relativistic heavy-ion collisions

The inversion-asymmetry of the particle emission source in relativistic heavy-ion collision under the Bertsch-Pratt convention is discussed and explicitly exhibited by a Monte Carlo model. The Gaussian source function popularly used in the HBT analysis of relativistic heavy-ion collisions is invalid in this case. An inversion-asymmetric source function is suggested. A method for extracting the inversion-asymmetry degree of the source together with the source size from experimental data is proposed.     

Induced photon emission from quark jets in ultrarelativistic heavy-ion collisions

We study the induced photon bremsstrahlung from a fast quark produced in AA collisions due to multiple scattering in quark-gluon plasma. For RHIC and LHC conditions, the induced photon spectrum is sharply peaked at a photon energy close to the initial quark energy. In this region, the contribution of the induced radiation to the photon fragmentation function exceeds the ordinary vacuum radiation. Contrary to previous analyses [4–7], our results show that, at RHIC and LHC energies, the final-state interaction effects in quark-gluon plasma do not suppress the direct photon production and may even enhance it at p _T ～5–15 GeV.     

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.