Simple model for photonium production

We present a simplified model for the production of photonium in which an incident positron picks up an off-shell electron. The vertex form factor and coupling constant for one state are fixed by experimental measurements. Predictions for the production of additional states and for two-photon decay experiments are then made assuming the production vertex is the same for all states of photonium.     

Radiative corrections to very high energy electron-proton scattering

A “leading log” analysis of radiative corrections to inclusive ep scattering at HERA energies is presented. Neutral current weak interactions are considered at the level of the Born cross section as well as of the modifications of vertex and vacuum polarization parts. The leading log approximation has been also improved in the single hard and multiple soft photon emission by appropriate kinematical considerations. Some numerical results are presented for polarized electron and positron beams.     

Evaluation of structural vacancies for 1/1-Al–Re–Si approximant crystals by positron annihilation spectroscopy

The size of structural vacancies and structural vacancy density of 1/1-Al–Re–Si approximant crystals with different Re compositions were evaluated by positron annihilation lifetime and Doppler broadening measurements. Incident positrons were found to be trapped at the monovacancy-size open space surrounded by Al atoms. From a previous analysis using the maximum entropy method and Rietveld method, such an open space is shown to correspond to the centre of Al icosahedral clusters, which locates at the vertex and body centre. The structural vacancy density of non-metallic Al₇₃Re₁₇Si₁₀ was larger than that of metallic Al₇₃Re₁₅Si₁₂. The observed difference in the structural vacancy density reflects that in bonding nature and may explain that in the physical properties of the two samples.     

Parameter vertex method and its parallel solution for evaluating the dynamic response bounds of structures with interval parameters

In this article, we propose a parameter vertex method to determine the upper and lower bounds of the dynamic response of structures with interval parameters, which can be regarded as an extension of the matrix vertex method proposed by Qiu and Wang. The matrix vertex method requires considerable computation time and encounters the dependency problem in practice, thereby limiting its application in engineering. The proposed parameter vertex method can avoid the dependency problem, and the number of possible vertex combinations in the proposed method is significantly less than that in the matrix vertex method. The parameter vertex method requires that each matrix element in the dynamic differential equation is monotonic with respect to the uncertain parameter, and that the dynamic response reaches its extreme value when the uncertain parameter is at its endpoint. To further reduce the runtime, both vertical and transversal parallel algorithms are introduced and integrated into the parameter vertex method to improve its computational efficiency. Two numerical examples are presented to demonstrate the proposed method combined with both parallel algorithms. The performances of the two parallel algorithms are thoroughly studied. The parameter vertex method combined with parallel algorithm can be used for large-scale computing.     

Full fermion-boson vertex function derived in terms of symmetry relations in Abelian gauge theory

Nonperturbative studies such as confinement and dynamical chiral symmetry breaking need the nonperturbative interacting vertex functions. In this paper, an approach to determining the full fermion-boson vertex function in four-dimensional Abelian gauge theory is presented: this full vertex function is derived in terms of a set of normal (longitudinal) and transverse Ward-Takahashi relations for the fermion-boson (vector) and axial-vector vertices in the momentum space in the case of massless fermion. Such a derived fermion-boson vertex function should be satisfied both perturbatively and nonperturbatively. The fact that such a derived full fermion-boson vertex function to one-loop order holds indeed is proven and the nonperturbative form of this vertex is also under discussion.     

测量接触镜近轴后顶焦距可行性分析

主要论述了由焦度仪测量的接触镜实际后顶焦距转化为近轴后顶焦距的可行性问题以及近轴后顶焦距与实际应用相适应性的问题。通过分析和比对一些厂家生产的接触镜光学结构参量 r,n,d后认为 ,由于接触镜球差较大 ,而球差由光学结构参量确定 ,因而同一近轴后顶焦距不同 ,光学结构参量的接触镜实测后顶焦距也不同 ,并且偏差超出了允许范围。所以实际后顶焦距转化为近轴后顶焦距是不恰当的。同时近轴后顶焦距的倒数表征的光焦度与临床验光给出的实际光焦度意义不同 ,因而定义与实际应用不相适应。作者最后提出了接触镜后顶光焦度定义的修正建议     

An effective two reggeon two reggeon + particle vertex in Lipatov’s effective action and regge kinematics

The vertex of gluon production from the interaction between two reggeons associated with projectiles and two reggeons associated with targets is studied. The vertex can be used to calculate the collision cross section of two pairs of nucleons. The transversality of the vertex is verified along with its good behavior for large longitudinal momenta, which is essential for the vertex’s applicability. Poles are discussed for the zero values of longitudinal momenta. It is found that the poles remain, in contrast to the case of one projectile.     

An efficient approach for feature-preserving mesh denoising

With the growing availability of various optical and laser scanners, it is easy to capture different kinds of mesh models which are inevitably corrupted with noise. Although many mesh denoising methods proposed in recent years can produce encouraging results, most of them still suffer from their computational efficiencies. In this paper, we propose a highly efficient approach for mesh denoising while preserving geometric features. Specifically, our method consists of three steps: initial vertex filtering, normal estimation, and vertex update. At the initial vertex filtering step, we introduce a fast iterative vertex filter to substantially reduce noise interference. With the initially filtered mesh from the above step, we then estimate face and vertex normals: an unstandardized bilateral filter to efficiently smooth face normals, and an efficient scheme to estimate vertex normals with the filtered face normals. Finally, at the vertex update step, by utilizing both the filtered face normals and estimated vertex normals obtained from the previous step, we propose a novel iterative vertex update algorithm to efficiently update vertex positions. The qualitative and quantitative comparisons show that our method can outperform the selected state of the art methods, in particular, its computational efficiency (up to about 32 times faster).     

Bosonization and BRST invariant three-superstring Caneschi-Schwimmer-Veneziano vertex

Requiring the equivalence to the non-bosonized BRST invariant Caneschi-Schwimmer-Veneziano vertex (CSV vertex) for the Neveu-Schwarz sector of the superstring, we first construct the bosonized representation of the three-string CSV vertex for all fermionic fields and bosonic ghosts in the superstring. Taking the product of all these vertices and requiring the constraint equations for the bosonized operators as well as for the spin operator, we construst the bosonic representation of the three-superstring CSV vertex. Imposing the GSO projection, the resulting vertex is applicable to the coupling of any type of three-string states with arbitrary Bose sea level, including also the Ramond sector. The geometrical meaning of the constraint equation and the characterization of the CSV vertex as a transition operator are discussed.     

N-Superstring vertex and the vertex operator for the emission of the superstring

The BRST invariant, supersymmetric N-string vertex which applies to both, the Neveu-Schwarz and Ramond sector of the superstring is formulated using the vertex operator for the emission of a superstring. It is shown that the N-superstring vertex thus obtained is cyclic symmetric when GSO projected on-shell states and operators for the external strings are applied. The constraint equations of this vertex and their singularity structure are examined and we show that this vertex also has the required property of a transition operator. We also give a proof of its BRST invariance and supersymmetry.     

Comparison of cascading failures in small-world and scale-free networks subject to vertex and edge attacks

Complex networks may undergo a global cascade of overload failures when a single highly loaded vertex or edge is intentionally attacked. Here we use the recent load model of cascading failures to investigate the performance of the small-world (SW) and scale-free (SF) networks subject to deliberate attacks on vertex and edge. Simulation results suggest that compared with the SW network, the SF network is more vulnerable to deliberate vertex attacks and more robust to deliberate edge attacks. In the SF network, deliberate vertex attacks can result in larger cascading failures than deliberate edge attacks; however, in the SW network the situation is opposite. Furthermore, with the increase of the rewiring probability the SW network becomes more and more robust to deliberate vertex and edge attacks.     

Measurement of diffractive photoproduction of vector mesons at large momentum transfer at HERA

Elastic and proton–dissociative photoproduction of , and vector mesons (, , respectively) have been measured in interactions at HERA up to GeV, where t is the four-momentum transfer squared at the photon–vector–meson vertex. The analysis is based on a data sample in which photoproduction reactions were tagged by detection of the scattered positron in a special-purpose calorimeter. This limits the photon virtuality, , to values less than 0.01 GeV, and selects a average center-of-mass energy of = 94 GeV. Results for the differential cross sections, , for , and mesons are presented and compared to the results of recent QCD calculations. Results are also presented for the t-dependence of the pion-pair invariant-mass distribution in the mass region and of the spin-density matrix elements determined from the decay-angle distributions. The Pomeron trajectory has been derived from measurements of the W dependence of the elastic differential cross sections for both and mesons. Received: 16 October 1999 / Revised version: 17 January 2000 / Published online: 6 April 2000     

An intense pulsed positron beam

A positron pulsing system for an intense positron beam generated by an electron linac is reported. The pulsing system generates an intense pulsed positron beam of variable energy and variable pulse period. The pulsed positron beam is used as a non-destructive probe for various material research. In this paper, we also discuss applications of the pulsed positron beam: positron lifetime spectroscopy, age-momentum correlation spectroscopy, positronium time-of-flight measurement, and positron annihilation-induced Auger electron spectroscopy with a time-of-flight technique.     

Positron Characteristics in Cadmium and Zinc Chalcogenides

Electron energy levels and positron states have been calculated for cadmium and zinc chalcogenide compounds within the pseudo-potential approach and the independent particle model.Furthermore,the present contribution deals with the electron and positron chemical potentials allowing the calculation of the positron affinity to different materials of interest and hetero-structures formed by these materials.Besides,we here determine the positron diffusion constant by means of the positron deformation potential.An attempt has been made to scale positron affinity and diffusion constant with the lattice constant and the band gap energy,respectively.Such scaling is found to be not possible.The information gathered by the present study is of prime importance for a better understanding of positron trapping at interfaces and precipitates and should be useful in slow positron beam experiments.     

元素周期表中元素单晶基于局域密度理论的正电子寿命计算

在局域密度近似理论（LDA）的基础上用中性原子叠加模型和有限插分方法（SNA-FD）计算了元素周期表中各种元素单晶的正电子体寿命和单空位寿命.分析了不同结构的单晶中自由正电子的分布信息和湮没信息.元素单晶的正电子寿命计算值与文献中的实验测量值相符合,表明LDA基础上的SNA-FD方法可以作为单晶中正电子湮没理论计算的有效研究手段. 关键词： 局域密度近似理论 正电子寿命     

Symmetric vertex models on planar random graphs

We solve a 4-(bond)-vertex model on an ensemble of 3-regular (Φ³) planar random graphs, which has the effect of coupling the vertex model to 2D quantum gravity. The method of solution, by mapping onto an Ising model in field, is inspired by the solution by Wu et.al. of the regular lattice equivalent – a symmetric 8-vertex model on the honeycomb lattice, and also applies to higher valency bond vertex models on random graphs when the vertex weights depend only on bond numbers and not cyclic ordering (the so-called symmetric vertex models).The relations between the vertex weights and Ising model parameters in the 4-vertex model on Φ³ graphs turn out to be identical to those of the honeycomb lattice model, as is the form of the equation of the Ising critical locus for the vertex weights. A symmetry of the partition function under transformations of the vertex weights, which is fundamental to the solution in both cases, can be understood in the random graph case as a change of integration variable in the matrix integral used to define the model.Finally, we note that vertex models, such as that discussed in this paper, may have a role to play in the discretisation of Lorentzian metric quantum gravity in two dimensions.     

Conceptual design of an intense positron source based on an LIA

Accelerator based positron sources are widely used due to their high intensity. Most of these accelerators are RF accelerators. An LIA (linear induction accelerator) is a kind of high current pulsed accelerator used for radiography. A conceptual design of an intense pulsed positron source based on an LIA is presented in the paper. One advantage of an LIA is its pulsed power being higher than conventional accelerators, which means a higher amount of primary electrons for positron generations per pulse. Another advantage of an LIA is that it is very suitable to decelerate the positron bunch generated by bremsstrahlung pair process due to its ability to adjustably shape the voltage pulse. By implementing LIA cavities to decelerate the positron bunch before it is moderated, the positron yield could be greatly increased. These features may make the LIA based positron source become a high intensity pulsed positron source.     

Effects of Vertex Activity and Self-organized Criticality Behavior on a Weighted Evolving Network

Effects of vertex activity have been analyzed on a weighted evolving network. The network is characterized by the probability distribution of vertex strength, each edge weight and evolution of the strength of vertices with different vertex activities. The model exhibits self-organized criticality behavior. The probability distribution of avalanche size for different network sizes is also shown. In addition, there is a power law relation between the size and the duration of an avalanche and the average of avalanche size has been studied for different vertex activities.     

Boundary correlation functions of the six and nineteen vertex models with domain wall boundary conditions

Boundary correlation functions of the six and nineteen vertex models on an N×N lattice with domain wall boundary conditions are studied. The general expression of the boundary correlation functions is obtained for the six vertex model by the use of the quantum inverse scattering method. For the nineteen vertex model, the boundary correlation functions are shown to be expressed in terms of those for the six vertex model.     

Discrete symmetry groups of vertex models in statistical mechanics

We analyze discrete symmetry groups of vertex models in lattice statistical mechanics represented as groups of birational transformations. They can be seen as generated by involutions corresponding respectively to two kinds of transformations onq×q matrices: the inversion of theq×q matrix and an (involutive) permutation of the entries of the matrix. We show that the analysis of the factorizations of the iterations of these transformations is a precious tool in the study of lattice models in statistical mechanics. This approach enables one to analyze two-dimensionalq ⁴-state vertex models as simply as three-dimensional vertex models, or higher-dimensional vertex models. Various examples of birational symmetries of vertex models are analyzed. A particular emphasis is devoted to a three-dimensional vertex model, the 64-state cubic vertex model, which exhibits a polynomial growth of the complexity of the calculations. A subcase of this general model is seen to yield integrable recursion relations. We also concentrate on a specific two-dimensional vertex model to see how the generic exponential growth of the calculations reduces to a polynomial growth when the model becomes Yang-Baxter integrable. It is also underlined that a polynomial growth of the complexity of these iterations can occur even for transformations yielding algebraic surfaces, or higher-dimensional algebraic varieties.