15,130 research outputs found
Improving the equivalent-photon approximation in electron--positron collisions
The validity of the equivalent-photon approximation for two-photon processes
in electron--positron collisions is critically examined. Commonly used forms to
describe hadronic two-photon production are shown to lead to sizeable errors.
An improved two-photon luminosity function is presented, which includes
beyond-leading-logarithmic effects and scalar-photon contributions. Comparisons
of various approximate expressions with the exact calculation in the case of
the total hadronic cross section are given. Furthermore, effects of the poorly
known low-Q2 behaviour of the virtual hadronic cross sections are discussed.Comment: 10 pages, Latex, uses 12pt.sty, no figur
Quarkonium production: velocity-scaling rules and long-distance matrix elements
The hierarchy of long-distance matrix elements (MEs) for quarkonium
production depends on their scaling with the velocity of the heavy quark in
the bound state. Ranges for the velocities in various bound states and
uncertainties of colour-singlet MEs are estimated in a quark-potential model.
Different possibilities for the scaling with of the MEs are discussed; they
depend on the actual values of and the QCD scale. As an application, J/psi
polarization in e^+e^- annihilation is discussed. The first non-perturbative
estimates of colour-octet MEs are presented and compared with phenomenological
determinations. Finally, various predictions of prompt quarkonium production at
LEP are compared.Comment: 17 pages, Latex, uses 12pt.sty and epsfig.sty, 2 figure
On the non-perturbative part of the photon structure function
We discuss a dispersion relation in the photon mass and show how (in
principle) model-independent constraints on the parton distribution functions
of the photon, notably a momentum sumrule, can be obtained. We present two sets
of parametrizations, SaS~1 and~2, corresponding to two rather extreme
realizations of the non-perturbative part. Inclusive electron scattering off a
real photon is found to be insufficient to constrain the non-perturbative
components. The additional sensitivity provided by the photon virtuality is
outlined. Previous approaches to model the non-perturbative input distributions
are commented upon.Comment: Latex, 7 page
Three Mechanisms for Bar Thickening
We present simulations of bar-unstable stellar discs in which the bars
thicken into box/peanut shapes. Detailed analysis of the evolution of each
model revealed three different mechanisms for thickening the bars. The first
mechanism is the well-known buckling instability, the second is the vertical
excitation of bar orbits by passage through the 2:1 vertical resonance, and the
third is a gradually increasing fraction of bar orbits trapped into this
resonance. Since bars in many galaxies may have formed and thickened long ago,
we have examined the models for fossil evidence in the velocity distribution of
stars in the bar, finding a diagnostic to discriminate between a bar that had
buckled from the other two mechanisms.Comment: 18 pages, 17 figures, accepted to appear in MNRAS, comments welcom
A scenario for high-energy gamma-gamma interactions
A real photon has a complicated nature, whereby it may remain unresolved or
fluctuate into a vector meson or a perturbative q-qbar pair. In gamma-gamma
events, this gives three by three combinations of the nature of the two
incoming photons, and thus six distinct event classes. The properties of these
classes are partly constrained by the choices already made in our related
gamma-p model. It is therefore possible to predict the energy-dependence of the
cross section for each of the six components separately. The total cross
section gives support to the idea that a simple factorized ansatz with a
pomeron and a reggeon term can be a good approximation. Event properties
undergo a stepwise evolution from p-p to gamma-p to gamma-gamma events, with
larger charged multiplicity, more transverse energy flow and a higher jet rate
in the latter process.Comment: 1+27 pages, LaTeX2e, 20 ps/eps figures included in file using
filecontents environment
On association in regression: the coefficient of determination revisited
Universal coefficients of determination are investigated which quantify the strength of the relation between a vector of dependent variables Y and a vector of independent covariates X. They are defined as measures of dependence between Y and X through theta(x), with theta(x) parameterizing the conditional distribution of Y given X=x. If theta(x) involves unknown coefficients gamma the definition is conditional on gamma, and in practice gamma, respectively the coefficient of determination has to be estimated. The estimates of quantities we propose generalize R^2 in classical linear regression and are also related to other definitions previously suggested. Our definitions apply to generalized regression models with arbitrary link functions as well as multivariate and nonparametric regression. The definition and use of the proposed coefficients of determination is illustrated for several regression problems with simulated and real data sets
Comparison of two model frameworks for fiber dispersion in the elasticity of soft biological tissues
This study compares two models that are used to describe the elastic properties of fiber-reinforced materials with dispersed fibers, in particular some soft biological tissues such as arterial walls and cartilages. The two model approaches involve different constitutive frameworks, one being based on a generalized structure tensor (GST) and the other on the method of angular integration (AI). By using two representative examples, with the same number of parameters for each model, it is shown that the predictions of the two models are virtually identical for a significant range of large deformations, which contradicts conclusions contained in several papers that are based on faulty analysis. Additionally, each of the models is fitted to sets of uniaxial data from the circumferential and axial directions of the adventitia of a human aorta, both models providing excellent agreement with the data. While the predictions of the two models are comparable and exclusion of compressed fibers can be accommodated by either model, it is well known that the AI model requires more computational time than the GST model when used within a finite element environment, in particular if compressed fibers are excluded
Vector-Meson Electroproduction from Generalized Vector Dominance
Including destructively interfering off-diagonal transitions of
diffraction-dissociation type, we arrive at a formulation of GVD for exclusive
vector-meson production in terms of a continuous spectral representation of
dipole form. The transverse cross-section sigma_T for gamma* p -> V p behaves
asymptotically as 1/Q^4, while R_V = sigma_L/sigma_T becomes asymptotically
constant. Contributions violating s-channel helicity conservation stay at the
10-15% level established in low-energy photoproduction and diffractive
hadron--hadron interactions. The data for phi- and rho-meson production for 0
\lsim Q^2 \lsim 20 GeV^2 from HERA are found to be in agreement with these
predictions.Comment: 16 pages, latex, 3 figure
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