Two-photon processes at very high energies

Formulae are presented by which a numerically stable computer program for two-photon processes above $\text{s}\text{= 100}\text{GeV}$ can be constructed. For this purpose a new method for evaluating matrix elements is introduced. It is then shown that two-photon processes should be no background to Drell-Yan signals in pp collisions. A comparison is made between two-photon physics at e⁺e^?, ep and pp collisions.     

State-Selective and Total Single-Capture Cross Sections for Fast Collisions of Multiply Charged Ions with Helium Atoms

The four-body boundary corrected first Born approximation (CB1-4B) is used to calculate the single electron capture cross sections for collisions between fully stripped ions (He²⁺, Be⁴⁺, B⁵⁺ and C⁶⁺) and helium target at intermediate and high impact energies. The main goal of this study is to assess the usefulness of the CB1-4B method at intermediate and high impact energies for these collisions. Detailed comparisons with the measurements are carried out and the obtained theoretical cross sections are in reasonable agreement with the available experimental data.     

Multifragmentation of gold nuclei by light relativistic ions: Thermal breakup versus dynamical disintegration

The multiple emission of intermediate-mass fragments (IMF) is studied for collisions of p, ⁴He, and ¹²C on Au with the 4π FASA setup. The mean multiplicities of IMF saturate at a value of around 2 for incident energies above 6 GeV. An attempt at describing the observed IMF multiplicities in the two-stage scenario, a fast cascade followed by a statistical multifragmentation, fails. Agreement with the measured IMF multiplicities is obtained by introducing an intermediate expansion phase and modifying empirically the excitation energies and masses of remnants. The angular distributions and energy spectra from p-induced collisions are in agreement with the scenario of “thermal” multifragmentation of a hot and expanded target spectator. In the case of ¹²C + Au(22.4 GeV) and ⁴He (14.6 GeV) + Au collisions, deviations from a pure thermal breakup are seen in the fragment energy spectra, which are harder than those both from model calculations and from the measured ones for p-induced collisions. This difference is attributed to a collective flow with the expansion velocity at the surface of about 0.1c (for ¹²C + Au collisions).     

Scattering of protons by <Superscript>9</Superscript>B and <Superscript>10</Superscript>B isotopes at intermediate energies in the diffraction theory

Differential cross sections of the elastic scattering of protons of intermediate energies by ⁹B and ¹⁰B nuclei are calculated within the Glauber theory. The matrix elements of elastic scattering are derived with a wave function in the three-particle 2αp-model (for ⁹B) and with an oscillatory wave function (for ¹⁰B). The differential cross-sections are calculated with allowance for the triple collisions on ⁹B nuclei and in the approximation of double collisions on ¹⁰B nuclei. The sensitivity of the differential cross-section to the ⁹B nucleus structure is analyzed. The differential cross section of ¹⁰B at E = 197 MeV is compared to the results from calculations using the distorted wave method.     

Production of Kr2+ in collisions of rare gas ions with Kr

Total and differential cross sections for the production of fast Kr²⁺ ions in collisions of He⁺, Ne⁺ and Ar⁺ with Kr were measured at primary energies below 500 eV. In the system Ar⁺+Kr most of these reactions occur in close collisions and are accompanied by a large momentum transfer. For Ne⁺+ Kr collisions the angular distribution in the centre of mass system is approximately isotropic over a wide angular range. The cross section values for the Kr²⁺ production amount to 1% of the total charge transfer cross section in the investigated energy range.     

Quasielastic transfer reactions in 238U + 197Au and 197Au + 197Au collisions near the Coulomb barrier

Differential cross sections as a function of cm angle were measured for 1n- and 2n-transfer reactions in ²³⁸U + ¹⁹⁷ Au and¹⁹⁷ Au + ¹⁹⁷ Au collisions in the energy range from 0.881 V_c to 1.093 V_c and 0.825 V_c to 0.964 V_c, respectively. For ¹⁹⁸Au and ¹⁹⁹Au from the ²³⁸U + ¹⁹⁷Au collisions, for reduced distances of closest approach d _o 1.55 fm, the angular distributions at all bombarding energies are well described by the semiclassical theory. Equivalently, the transfer probabilities show the expected exponential decrease with increasing d _o over many orders of magnitude. For all other transfer products from ²³⁸U + ¹⁹⁷Au collisions, and for all transfer products from ¹⁹⁷Au + ¹⁹⁷Au collisions, markedly reduced cross sections relative to the semiclassical theory are observed for central collisions at all bombarding energies, even for values of d _o that are well outside the region where absorption is known to set in. Only for the more peripheral collisions, one observes agreement of the angular distributions (transfer probabilities) with the semiclassical expectations. The deviations for central collisions are absent for reactions with positive Q _gg values and scale roughly with increasingly negative values of Q _gg, i.e. with increasing Q-value mismatch. Channel coupling is proposed as the relevant mechanism.     

On the observability of direct photons in ultrarelativistic nucleus-nucleus collisions by means of inclusive photon and hadron spectra

A method is presented to search for a direct photon signal in collisions between nuclei at ultrarelativistic energies. In the proposed method the analysis is performed in the center of momentum system of the radiating system, being the cms of the colliding nucleons in case of collisions of symmetric systems. This results in a maximal strength of a possible direct photon signal in the observed energy spectra, since the smearing of the signal due to the Lorentz transformation between the cms and the laboratory frame in case of fixed target experiments is absent. The feasibility of detecting a direct photon signal is examined by comparison of calculated photon and hadron spectra for central and peripheral ³²S on S collisions at a nucleon-nucleon cms energy of about 20 GeV, being the energy domain of the CERN-SPS fixed target experiments.     

指数变分方法应用于电荷交换反应

本文考虑如下的电荷交换反应:H⁺+H(1s)→H(1s)+H⁺。用指数变分方法得到类H⁺₂的波函数,并将这样的波函数用于上述的电荷交换反应。对入射H⁺的能量为60和125keV的两种情况,计算了电荷交换反应的角分布。结果表明,指数变分波函数带来明显的改进,理论结果与实验数据符合得比较好。 关键词：     

The theoretical calculation of collision-induced predissociation

A theoretical treatment of the collision-induced predissociation of a diatomic molecule caused by collisions with an atom is given in terms of the impact parameter method. Numerical calculations of the collision-induced predissociation of the D ¹Π _u state of the hydrogen molecule due to collisions with helium atoms have been performed using an ab initio calculation of the electronic interaction of the helium atom and the hydrogen molecule. The results show good agreement with the available experimental evidence. The continuum energy of the predissociation fragments is shown to have an energy spread determined by the range of the interaction potential.     

Subthreshold K+ production in proton-nucleus collisions

Double differential K⁺ cross sections have been measured in p+C collisions at 1.2, 1.5 and 2.5 GeV beam energy and in p+Pb collisions at 1.2 and 1.5 GeV. The K⁺ spectrum taken at 2.5 GeV can be reproduced quantitatively by a model calculation which takes into account first chance proton-nucleon collisions and internal momentum with energy distribution of nucleons according to the spectral function. At 1.2 and 1.5 GeV beam energy the K⁺ data excess significantly the model predictions for first chance collisions. When taking secondary processes into account the results of the calculations are in much better agreement with the data.     

Quantum and semiclassical dynamics of differential optical collisions

We apply quantum and semiclassical theories to differential optical collisions Na(3²S_1/2) + Kr + Na(3²P_1/2,3/2) + Kr. Our results provide a basis to analyze recent experiments in which for the first time optical collisions were investigated with angular resolution under crossed-beam conditions. A characteristic feature of the differential cross sections is the pronounced oscillatory structure due to interferences of different Condon paths. These Stueckelberg oscillations form an extremely sensitive probe of the collisional dynamics and of the molecular interactions. We demonstrate perspectives to determine geometric properties of the collision complex by excitation with polarized light. By final state analysis nonadiabatic (spin-orbit, rotational) interactions can be studied with complete control of the path. In summary it is shown that the method of differential detection of optical collisions opens a variety of new accesses to atomic and molecular subcollisions. Received: 30 July 1997 / Received in final form: 5 November 1997 / Accepted: 8 January 1998     

Subthreshold K+ production in proton-nucleus collisions

Double differential K⁺cross sections have been measured in p+C collisions at 1.2, 1.5 and 2.5 GeV beam energy and in p+Pb collisions at 1.2 and 1.5 GeV. The K⁺ spectrum taken at 2.5 GeV can be reproduced quantitatively by a model calculation which takes into account first chance proton-nucleon collisions and internal momentum with energy distribution of nucleons according to the spectral function. At 1.2 and 1.5 GeV beam energy the K⁺ data excess significantly the model predictions for first chance collisions. When taking secondary processes into account the results of the calculations are in much better agreement with the data.     

Polarization of Λ0 hyperons as a signature for the quark-gluon plasma

The momentum distribution of Λ⁰ hyperons produced from the quark-gluon plasma (QGP) in ultra-relativistic heavy-ion collisions is calculated in dependence on their polarization. The momentum distribution of Λ⁰ hyperons is defined by matrix elements of relativistic quark Wigner operators, which are calculated within the effective quark model with chiral U(3)×U(3) symmetry and the quark-gluon transport theory. We show that the polarization of the Λ⁰ hyperon depends on the spin of the strange quark that agrees well with the DeGrand-Miettinen model. We show that Λ⁰ hyperons, produced from the QGP, are fully unpolarized. This means that a detection of unpolarized Λ⁰ hyperons, produced in ultra-relativistic heavy-ion collisions, should serve as one of the signatures for the existence of the QGP in intermediate states of ultra-relativistic heavy-ion collisions.     

Two-dimensional multiplicity fluctuation analysis of target residues in nuclear collisions

Multiplicity fluctuation of the target residues emitted in the interactions in a wide range of projectile energies from 500 A MeV to 60 A GeV is investigated in the framework of two-dimensional scaled factorial moment methodology.The evidence of non-statistical multiplicity fluctuation is found in 16 O-AgBr collisions at 60 A GeV,but not in 56 Fe-AgBr collisions at 500 A MeV,84 Kr-AgBr collisions at 1.7 A GeV,16 O-AgBr collisions at 3.7 A GeV and 197 Au-AgBr collisions at 10.7 A GeV.     

Electron detachment and fragmentation in collisions between 1.25 keV/carbon and clusters and

The destruction cross-section for 22.5 and 50 keV C^1- , for 10 and 50 keV C₈ ^1- and for 50 and 75 keV C₆₀ ^1- clusters in collisions with H₂ has been measured by an attenuation method. The destruction of the cluster anions is dominated by electron detachment rather than fragmentation and is of the order of the geometric cross-section. The cross-sections vary little with bombarding energy. Received: 16 September 1998 / Received in final form: 23 February 1999     

Phenomenology of photon-e ± collisions

The physical interest ofγe ^± collisions is examined. A basic formalism is established. Crosssections are computed with general couplings and polarization states. Illustrations are given for QED tests, Z⁰ andW ^± production, various electro-weak processes including γγ collisions and the search for new currents and particles.     

Deuterons from 300 GeV proton collisions calculated from the Lund model

We present an alternative approach to describe deuteron production in high energy particle collisions. The phenomenological Lund-model has been used to give the number and momentum distribution of secondary nucleons produced in 300 GeV proton-proton collisions. Deuterons are assumed to be produced in a final state interaction between the nucleons inNN d reactions inside a volume of 1 F³. The results are compared to experimental data.Data became available for antideuteron production in e⁺e^– annihilation. * A Lund Monte Carlo calculation for the production of secondary nucleons in e⁺e^– collisions exist. The results from a final state interaction were too small by a factor 10³. A calculation for a coalescent model which uses the overlap of the proton-neutron momentum distribution with a distribution from a Hulthen wave function is in good agreement with the data. The coalescent model works well in this case. It is because the relative momentum of the coalescent nucleons produced for 10 GeV cm energy in e⁺e^– collisions is very much smaller than the relative momentum of the nucleons produced in 300 GeV fixed target energy ofpp collisions.     

Constraint Molecular Dynamics and results on isotopic distributions produced in heavy ion collision — The Sn + Ni systems

The Constraint Molecular Dynamics approach is illustrated together with calculation on different heavy ion collisions. In particular the charge-mass distribution produced in the collision ¹²⁴Sn + ⁶⁴Ni and ¹¹²Sn + ⁵⁸Ni at 35 MeV/nucleon is discussed. By comparing the Y = (N ? Z)/A distributions for fragments produced in central collisions it results that this observable can be sensitive to the dynamic effects induced on the two systems by the different charge/mass ratio.     

高能重离子碰撞中的参与者数和核子-核子碰撞数

采用核-核碰撞的Glauber模型, 给出了高能重离子碰撞中的参与者数和核子-核子碰撞数随碰撞参数的分布方程, 并用其讨论了\{s_NN\}^1/2=200GeV的Au+Au碰撞中的参与者数与核子-核子碰撞数随对心度的变化关系, 所得结果与PHENIX合作组所给出的实验结果符合得很好. 关键词： Glauber 模型 参与者数 核子-核子碰撞数