H- formation in proton-helium collisions

The double electron capture cross sections in proton-helium collisions are calculated by an approximate integral form of the close-coupling formalism considering three states and indistinguishability of electrons. The results fairly agree with the experimental observations.     

Angular and energy distributions of grey particles in high-energy hadron-nucleus collisions

The angular and energy distributions of grey particles (mostly protons in the energy range 30–400 MeV) emitted in the interactions of high energy hadrons with nuclei are investigated here. Grey particles “g particles” are assumed to result from the intranuclear cascade initiated by the passage of the incident hadron through the target nucleus. Using the pretested hypothesis that these particles arise only from the first two generations of the cascade, we calculate their angular and energy distributions. Various experimental data are successfully described both in shape and absolute magnitude.     

Strangeness enhancement and energy dependence in heavy ion collisions

The canonical statistical model analysis of strange and multistrange hadron production in central A–A relative to p–p/p–A collisions is presented over the energy range from GeV up to GeV. It is shown that the relative enhancement of strange particle yields from p–p/p–A to A–A collisions substantially increases with decreasing collision energy. It is largest at GeV, where the enhancement of and is of the order of 100, 20 and 3, respectively. In terms of the model these results are due to the canonical suppression of particle thermal phase space at lower energies, which increases with the strangeness content of the particle and with decreasing size of the collision fireball. The comparison of the model with existing data on the energy dependence of the kaon/pion ratio is also discussed. Received: 25 October 2001 / Published online: 5 July 2002     

Angular pattern of minijet transverse energy flow in hadron and nuclear collisions

The azimuthal asymmetry of a minijet system produced at the early stage of nucleon–nucleon and nuclear collisions in a central rapidity window is studied. We show that in pp collisions the minijet transverse energy production in a central rapidity window is essentially unbalanced in the azimuth due to asymmetric contributions in which only one minijet hits the acceptance window. We further study the angular pattern of the transverse energy flow generated by the semihard degrees of freedom at the early stage of high energy nuclear collisions and its dependence on the number of semihard collisions in the models both including and neglecting soft contributions to the inelastic cross section at RHIC and LHC energies as well as on the choice of the infrared cutoff. Received: 19 January 1999 / Revised version: 18 February 2000 / Published online: 26 July 2000     

Angular pattern of minijet transverse energy flow in hadron and nuclear collisions

bstract The azimuthal asymmetry of a minijet system produced at the early stage of nucleon-nucleon and nuclear collisions in a central rapidity window is studied. We show that, in pp collisions, the minijet-transverse-energy production in a central rapidity window is essentially unbalanced in azimuth because of asymmetric contributions in which only one minijet hits the acceptance window. We further study the angular pattern of the transverse-energy flow generated by semihard degrees of freedom at the early stage of high-energy nuclear collisions and its dependence on the number of semihard collisions in the models either including or neglecting soft contributions to the inelastic cross section at RHIC and LHC energies, as well as on the choice of infrared cutoff. From Yadernaya Fizika, Vol. 65, No. 5, 2002, pp. 918–929. Original English Text Copyright ? 2002 by Leonidov, Ostrovsky. This article was submitted by the authors in English.     

Angular dependence of double electron capture in collisions of C4+ with He

In charge-transfer collisions of C⁴⁺(1s^{2 1}S) with He (1s^{2 1}S), the process of double electron capture into the ground state C²⁺ is well-known to dominate other channels by an order of magnitude for projectile energies below 10 keV. This work presents a calculation of differential cross-sections resolved in the angle and energy gain variables, based on an ab initio treatment of electronic states, and compares with the measurements published in the literature (projectile energy E=270, 400, and 470 eV). We also briefly discuss the semi-empirical two-state models developed by experimentalists for this process.     

Jet-photon conversion with energy loss in heavy ion collisions

The rate of high energy photons produced from energetic jets during their propagation through the QGP at RHIC and LHC is studied by taking into account the contribution of jet quenching in the medium. It is shown that the jet quenching effect reduces the rate of jet-photon conversion at large transverse momemtum by about 40% at RHIC with √8= 200 AGeV, and by about 80% at LHC with √8 = 5500 AGeV.     

Jet-photon conversion with energy loss in heavy ion collisions

The rate of high energy photons produced from energetic jets during their propagation through the QGP at RHIC and LHC is studied by taking into account the contribution of jet quenching in the medium. It is shown that the jet quenching effect reduces the rate of jet-photon conversion at large transverse momemtum by about 40% at RHIC with √S＝ 200 AGeV, and by about 80% at LHC with √S = 5500 AGeV.     

Peculiarities in charge symmetry of energy loss in ion-atom collisions

Systematic experimental studies on energy loss (?dE/dx) were performed for ions with nuclear charge Z = 2–18 in various gases. The energy range ～0.001 to ～1 MeV/amu was considered. Detailed analysis confirmed the oscillating character of the energy loss dependence on the ionic nuclear charge associated with the ions’ electron shell structure. The symmetry of the energy loss dependence on the nuclear charges of projectile Z and target Z _t is examined.     

Decoherence and energy loss in QCD cascades in nuclear collisions

The paper analyzes the consequences of the assumption that the effects of quantum coherence and the resulting angular ordering in QCD cascades are disrupted within the hot fireball created in ultrarelativistic heavy ion collisions. The effects of decoherence are taken into account by switching off angular ordering for the vertices of MC-generated cascade that lie within the fireball of some given size. We show that decoherence exerts a pronounced influence on the spatiotemporal pattern of a cascade. Related effects due to non-radiative losses of cascade gluons and those of final prehadrons are discussed. We show that decoherence leads to a substantial increase in the yield of cascade particles hadronizing inside the hot zone thus exerting a strong influence on the properties of final hadrons and amplifying the effects of collisional energy losses of cascade partons and final prehadrons that take place inside the fireball.     

Beam energy dependence of spontaneous positron creation in delayed nuclear collisions

We compute positron emission from U+Cm collisions within a quantum mechanical model for delayed nuclear collisions. We demonstrate a striking beam energy dependence of the strength of the spontaneous positron peak.     

Transverse energy dependence of neutron squeeze-out in relativistic heavy ion collisions

We present a microscopic calculation of neutronsqueeze-out in relativistic heavy ion collisions at beam energies betweeen 400 and 1000 MeV/nucleon. After demonstrating the importance of the correct isospin treatment for the neutron to proton ratio, our main emphasis is put on the investigation of the properties of neutronsqueeze-out. Thesqueeze-out ratio increases monotonously with the transverse momentum of the neutrons. This ratio is independent of the incident beam energy if plotted versusp _t /p _proj . Most importantly, we observe a strong dependence on the nuclear equation of state and momentum dependent interaction.Supported by GSI, BMFT and DFG     

Bombarding energy dependence of angular momentum transfer in deep inelastic collisions

Measurements of first and second moments of γ-ray multiplicity distributions from deep inelastic collisions of ⁸⁶Kr + ¹⁵⁴Sm are reported. A global systematics of the angular momentum distributions from deep inelastic reactions with projectile masses ? 40 is presented. The average angular momentum is found to depend linearly on the incident channel average angular momentum, while no simple systematics for the second moment appears obvious. In order to illuminate the question whether the angular momentum transfer process reaches statistical equilibrium in deep inelastic collisions, numerical calculations have been performed on two models: a two-sphere classical model including the collective modes of twisting, bending, wriggling and tilting, and a statistical equilibrium Fermi-gas model. The two-sphere classical model is not able to account for the observed second moments, and neither does the Fermi-gas model give an explanation of the deep inelastic multiplicity data.     

Medium temperature dependence of quark energy loss

By using the reaction operator approach and the single hard scattering approximation scenario, we study the medium temperature dependence of heavy quark radiation energy loss via introducing the medium temperature into QCD running coupling α _s and Debye mass μ _D . Our results indicate that there exist an equilibrum temperature T = 320 MeV in Au-Au collisions withe energy 130 GeV at RHIC, which is agreement with the prediction from hydrodynamic model. We also find that, in the low p _t region, the color coupling effect dominates the heavy quark radiation energy loss, and the energy loss will be dominated by the Debye screening effect when the quark p _t become large.     

Impact parameter dependence of electronic energy loss

Energy-loss moments of the electronic energy loss are calculated as functions of impact parameter for protons of 1 to 7 MeV incident upon Be, Al, Cu, Ag, and Ta targets. The Lindhard dielectric-function formalism is used, in the local-density approximation. The target-atom electron densities are Lenz-Jensen, and the projectile is a point charge. The moments are fitted by a simple algebraic function, and the energy and Z₂ dependence of the parameters of this function are given.     

Scaling of differential cross-section in high energy proton-helium scattering

Possible occurrence of scaling of differential cross-section for high energy hadronnucleus elastic scattering is demonstrated takingp-⁴He scattering as an example and using three well-known scaling variables proposed earlier for hadron-hadron scattering. The available data on differential cross-section ratio betweenE _lab=45 and 393 GeV are found to scale in all the three variables reasonably well and the positions of the dip and the secondary maximum are found to follow the predicted patterns of behaviour as a function of energy. Extrapolating the fits to the available slope-parameter data onto higher energies and using the scaling curves, the positions of the dip and the secondary maximum and the differential cross-section ratio as a function of |t| are predicted for higher energies.