Four-body calculations of elastic scattering in H–H̄ collisions

We present a nonadiabatic treatment of the hydrogen-antihydrogen system. The technique used to describe H- H? collisions is based on the coupled rearrangement channels method. Within this approach the total, nonadiabatic wave function of the system is divided into two parts: an inner and an outer one. To describe the inner part a set of square-integrable 4-body functions is used. These functions are obtained by a diagonalization of the total Hamiltonian projected on a chosen L ² subspace, they explicitly contain components of various arrangement channels expressed in terms of corresponding Jacobi coordinates. The outer part of the total wave function reflects its asymptotic character. Our procedure leads to the system of non-local integro-differential equations that are solved iteratively and simultaneously determine both the shape of the outer part of the wave function and the coefficients in the four-body expansion of the inner part. Using this formalism we perform the one-channel calculation of the elastic scattering to obtain the S-matrix and nonadiabatic scattering length.     

Charge transfer and charge transfer with excitation of a target ion in slow collisions of α particles with helium atoms. II

The cross sections of single- and double-electron capture in ³He²⁺-He collisions in the energy interval of incident α particles of 3–120 keV were obtained. The cross sections were calculated for the Coulomb trajectory of motion of nuclei in the context of the close-coupling equation method using the basis set of two-electron adiabatic molecular states. Introduction of additional requirements that must be satisfied by the numerical solutions of the system of coupled differential equations made it possible to obtain a good correspondence between the calculated values of total cross sections of single- and double-electron capture and the experimental data in the whole range of the considered collision energies.     

Study of hard scattering processes in multihadron production from γγ collisions at LEP

The production of multihadronic states in collisions at LEP has been studied with the DELPHI detector. The analyzed data correspond to an integrated luminosity of about 32pb^–1, collected in the LEP runs of 1990–1992. Minimum bias data and a sample of events with jets at highp _T have been selected under the requirement that no scattered electron or positron is observed. The two data sets have been compared to Monte Carlo predictions. The non-perturbative contribution described by the vector meson dominance Model and direct production from pointlike photons described by the quark parton model were found to be insufficient to reproduce the data. It has been necessary to include a third interaction component, which is due to perburbative hard scattering of the partonic constituents of the photon. Several parametrisations of the quark and gluon densities of the photon have been tested. The interplay with the cut in jet transverse momentum, which is necessary for the separation of the perturbative and non-perturbative regions, is discussed. The data favour parametrisations with rather soft partonic content of the photon.     

Single-spin asymmetry for charged hadrons produced in proton-nucleus collisions at 40 GeV for c.m. production angles in the range 40°–79°

The transverse single-spin asymmetry for charged hadrons (π ^±, K ^±, p, $\bar p$ ) produced in proton-nucleus collisions was measured for c.m. production angles in the range 40°–79°. The measurements were performed with the FODS-2 setup by using a 40-GeV polarized proton beam originating from the accelerator of the Institute for High Energy Physics (Protvino) and hitting carbon and copper nuclear targets. The data in question were obtained in the polarized-proton-fragmentation region (0.0 < x _F < 0.7, 0.6 < p T < 2.5 GeV/c). In agreement with data obtained at other energies, the single-spin asymmetry for π ^± mesons is significant at high x _F. For the first time, a sizable analyzing power, which changes sign at x _F = 0.43, is observed for protons. The dependence of the analyzing power on the target-nucleus mass is insignificant.     

Multiple ionization of argon in coincidence with projectile ions in 60–120 MeV Si q+-Ar collisions

A projectile ion-recoil ion coincidence technique has been employed to study the multiple ionization and the charge transfer processes in collisions of 60–120 MeV Si ^q+ (q = 4−14) ions with neutral argon atoms. The relative contribution of different ionization channels, namely; direct ionization, electron capture and electron loss leading to the production of slow moving multiply charged argon recoil ions have been investigated. The data reported on the present collision system result from a direct measurement in the considered impact energy for the first time. The total ionization cross-sections for the recoil ions are shown to scale as q ^1.7/E _p ^0.5 , where E _p is the energy in MeV of the projectile and q its charge state. The recoil fractions for the cases of total- and direct ionizations are found to decrease with increasing recoil charge state j. The total ionization fractions of the recoils are seen to depend on q and to show the presence of a ‘shell-effect’ of the target. Further, the fractions are found to vary as 1/j ² upto j = 8+. The average recoil charge state 〈j〉 increases slowly with q and with the number of lost or captured electrons from or into the projectile respectively. The projectile charge changing cross-sections σ _qq′ are found to decrease with increasing q for loss ionization and to increase with q for direct-and capture ionization processes respectively. The physics behind various scaling rules that are found to follow our data for different ionization processes is reviewed and discussed.     

Gluon multiple scattering and the transverse momentum dependence of J/ψ production in nucleus-nucleus collisions

A J/ψ meson is mainly formed by gluon fusion for 200 GeV pp collisions. In reactions with nuclei the gluons (g) scatter off other nucleons (N) before fusing to a J/ψ, which thenshows additional transverse momentum p_T. Fitting the value of the parameter σ_gN〈p²_T〉_gN to describe the gluon multiple scattering effect in proton-nucleus data, we can reproduce the p_T distribution of J/ψ production in 200 GeV/A nucleus-nucleus collisions. The origin of the p_T distribution is traced to soft gluon radiation via the Sudakov form factor.     

Forward–backward emission of target evaporated fragments in high energy nucleus–nucleus collisions

The multiplicity distribution, multiplicity moment, scaled variance, entropy and reduced entropy of target evaporated fragments emitted in forward and backward hemispheres in 12 A Ge V4 He, 3.7 A Ge V16 O,60 A Ge V16 O, 1.7 A Ge V84 Kr and 10.7 A Ge V197Au-induced emulsion heavy target(Ag Br) interactions are investigated. It is found that the multiplicity distribution of target evaporated fragments emitted in both forward and backward hemispheres can be fitted by a Gaussian distribution. The multiplicity moments of target evaporated particles emitted in the forward and backward hemispheres increase with the order of the moment q, and the secondorder multiplicity moment is energy independent over the entire energy range for all the interactions in the forward and backward hemisphere. The scaled variance, a direct measure of multiplicity fluctuations, is close to one for all the interactions, which indicate a correlation among the produced particles. The entropy of target evaporated fragments emitted in both forward and backward hemispheres are the same within experimental errors.     

Simulation of energy scan of pion interferometry in central Au＋Au collisions at relativistic energies

We present a systematic analysis of two-pion interferometry for the central Au＋Au collisions at √SNN=3, 5, 7, 11, 17, 27, 39, 62, 130 and 200 GeV/c with the help of a multiphase transport （AMPT） model. Emission source-size radius parameters Rlong, Rout, Rside and the chaotic parameter A are extracted and compared with the experimental data. Transverse momentum and azimuthal angle dependencies of the HBT radii are also discussed for central Au＋Au collisions at 200 GeV/c. The results show that the HBT radii in central collisions do not change much above 7 GeV/c. For central collisions at 200 GeV/c, the radii decrease with the increasing of transverse momentum PT but are not sensitive to the azimuthal angle. These results provide a theoretical reference for the energy scan program of the RHIC-STAR experiment.     

Probing the QCD equation of state with thermal photons in nucleus–nucleus collisions at RHIC

Thermal photon production at mid-rapidity in Au+Au reactions at is studied in the framework of a hydrodynamical model that describes efficiently the bulk identified hadron spectra at RHIC. The combined thermal plus NLO pQCD photon spectrum is in good agreement with the yields measured by the PHENIX experiment for all Au+Au centralities. Within our model, we demonstrate that the correlation of the thermal photon slopes with the charged hadron multiplicity in each centrality provides direct empirical information on the underlying degrees of freedom and on the form of the equation of state, s(T)/T³, of the strongly interacting matter produced in the course of the reaction. PACS 12.38.Mh, 24.10.Nz, 25.75.-q, 25.75.Nq     

Single-electron charge transfer in collisions of CF 2 ++ with Ar and Ne at collision energies 3–5 eV

Non-dissociative, single-electron charge transfer processes between the dication CF ₂ ⁺⁺ and Ar and Ne were investigated in crossed beam scattering experiments at collision energies of 3.0 and 4.55eV (c.m.). The reaction with Ar produces CF ₂ ⁺ in its ground state and, to a smaller extent, in the excited CF ₂ ⁺ (²_u) state. The reaction with Ne occurs only with an excited state of CF ₂ ⁺⁺ lying about 4.4 eV above the ground state and present in a small amount in the reactant dication beam; its molecular product is CF ₂ ⁺ in the ground state.     

The second Born approximation of electron–argon elastic scattering in a bichromatic laser field

We study the elastic scattering of atomic argon by electron in the presence of a bichromatic laser field in the second Born approximation. The target atom is approximated by a simple screening potential and the continuum states of the impinging and emitting electrons are described as Volkov states. We evaluate the S-matrix elements numerically. The dependence of differential cross-section on the relative phase between the two laser components is presented. The results obtained in the first and second Born approximations are compared and analysed.     

Measurement of the yields of positively charged particles at an angle of 35° in proton interactions with nuclear targets at an energy of 50 GeV

Momentum spectra of cumulative particles in the region of high transverse momenta (P _T ) in pA → h ⁺ + X reactions were obtained for the first time. The experiment in which this was done was performed at the SPIN setup (Institute for High Energy Physics, Protvino) in a beam of 50-GeV protons interacting with C, Al, Cu, and W nuclei. Positively charged particles were detected at a laboratory angle of 35° and in the transverse-momentum range between 0.6 and 3.7 GeV/c. A strong dependence of the particle-production cross section on the atomic number was observed. A comparison with the results of calculations based on the HIJING and UrQMD models was performed in the subcumulative region.     

Production of γγ+2 jets from double parton scattering in proton-proton collisions at the LHC

Cross sections for the production of pairs of photons plus two additional jets produced from double parton scattering in high-energy proton-proton collisions at the LHC are calculated for the first time.The estimates are based on the theoretical perturbative QCD predictions for the productions of γγ at next-to-next-to-leading-order,jet+jet and γ+jet at next-to-leading-order,for their corresponding single-scattering cross sections.The cross sections and expected event rates for γγ+2 jets from double parton scattering,after typical acceptance and selections,are given for proton-proton collisions with the collision energy s~(1/2)=13 TeV and integrated luminosity of 100 fb~(-1) planned for the following years,and also s~(1/2)=14 TeV with 3000 fb~(-1) of integrated luminosity as the LHC design.     

Multiplicity distributions of shower particles and target fragments in 7Li–Em collisions at 3 A GeV/c

Multiplicity distributions of shower particles and target fragments in ⁷Li–Em (emulsion) collisions at 3 A GeV/c are experimentally studied. In the framework of the multisource thermal model, the multicomponent Erlang distribution is used to describe the experimental multiplicity distributions of shower particles, grey fragments, black fragments, and heavily ionized fragments. The correlations between these multiplicities are experimentally reported. With the increase of impacting centrality (or the target fragment multiplicity), a saturation phenomenon for shower particle multiplicity is observed in the experiment.     

Simulation of the graphite–diamond transition in an isentropic process

An equation of state for graphite and diamond has been derived in wide density and temperature ranges. A set of equations for the graphite–diamond phase transition has been presented. Hugoniots for graphite and diamond have been calculated. Numerical simulation data for the graphite–diamond transition in the isentropic compression process using a metallic z-pinch with diamond saving have been reported.     

Modulation instability of an intense laser beam in an unmagnetized electron–positron–ion plasma

The modulation instability of an intense circularly polarized laser beam propagating in an unmagnetized, cold electron–positron–ion plasma is investigated. Adopting a generalized Karpman method, a three-dimensional nonlinear equation is shown to govern the laser field. Then the conditions for modulation instability and the temporal growth rate are obtained analytically. In order to compare with the usual electron–ion plasmas, the effect of positron concentration is considered. It is found that the increase in positron-to-electron density ratio shifts the instability region towards higher vertical wave numbers but does not cause displacement along the parallel wave number direction, and the growth rate increases as the positron-to-electron density ratio increases.     

Production of projectile and target-vacancy in near-symmetric collisions of 60–100 MeV Cu~(9+) ions with thin Zn target

We report studies on both target and projectile K-shell ionization by collisions of Cu~(9+)ions on the thin Zn target in the energy range of 60–100 Me V. In this work, the relative ratio for the production of the target to projectile K-vacancy is measured. The result shows that it almost remains stable over this energy range and has good consistency with the predictions by vacancy transfer via the 2pσ–1sσ rotational coupling, which gives experimental evidence for K-vacancy sharing between two partners. Furthermore, the discussion for comparisons between the experimental ionization cross sections and the possible theoretical estimations is presented. These comparisons suggest that the experimental data agree well with those predicted by the Binary–Encounter approximation(BEA) model but are not in good agreement with the modified BEA calculations. It allows us to infer that the direct ionization(and/or excitation) is of importance to initial K-vacancy production before 2pσ–1sσ transitions in the present collision condition.     

Photoluminescence properties of exciton–exciton scattering in a GaAs/AlAs multiple quantum well

We report on the photoluminescence (PL) properties of a GaAs (20 nm)/AlAs (20 nm) multiple quantum well under high-density-excitation conditions at excitation energies near the fundamental exciton energies. The biexciton-PL band is dominant in a relatively low-excitation-power region. The PL originating from exciton–exciton scattering, the so-called P emission, suddenly appears with an increase in excitation power. The excitation-energy dependence of the intensity of the P-PL band indicates that the excitation energy higher than the fundamental heavy-hole exciton by the energy of the longitudinal optical (LO) phonon is the most efficient for the P PL. This suggests that the LO-phonon scattering plays an important role in the relaxation process of excitons leading to the P PL. The appearance of the P-PL band remarkably suppresses the intensity of the biexciton-PL band; namely, the exciton–exciton scattering process prevents the formation of biexcitons. Furthermore, we have confirmed the existence of optical gain due to the exciton–exciton scattering process with use of a variable-stripe-length method.