11,896 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
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
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
Model Independent Bound on the Unitarity Triangle from CP Violation in B-> pi+ pi- and B-> psi K_S
We derive model independent lower bounds on the CKM parameters (1-rhobar) and
etabar as functions of the mixing-induced CP asymmetry S in B-> pi+ pi- and
sin(2 beta) from B->psi K_S. The bounds do not depend on specific results of
theoretical calculations for the penguin contribution to B-> pi+ pi-. They
require only the very conservative condition that a hadronic phase, which
vanishes in the heavy-quark limit, does not exceed 90 degrees in magnitude. The
bounds are effective if -sin(2 beta) < S < 1. Dynamical calculations indicate
that the limits on rhobar and etabar are close to their actual values.Comment: 5 pages, 2 figure
Microscopic basis for pattern formation and anomalous transport in two-dimensional active gels
Active gels are a class of biologically-relevant material containing embedded
agents that spontaneously generate forces acting on a sparse filament network.
In vitro experiments of protein filaments and molecular motors have revealed a
range of non- equilibrium pattern formation resulting from motor motion along
filament tracks, and there are a number of hydrodynamic models purporting to
describe such systems. Here we present results of extensive simulations
designed to elucidate the microscopic basis underpinning macroscopic flow in
active gels. Our numerical scheme includes thermal fluctuations in filament
positions, excluded volume interactions, and filament elasticity in the form of
bending and stretching modes. Motors are represented individually as bipolar
springs governed by rate-based rules for attachment, detachment and
unidirectional motion of motor heads along the filament contour. We
systematically vary motor density and speed, and uncover parameter regions
corresponding to unusual statics and dynamics which overlap but do not
coincide. The anomalous statics arise at high motor densities and take the form
of end-bound localized filament bundles for rapid motors, and extended clusters
exhibiting enhanced small-wavenumber density fluctuations and power-law
cluster-size distributions for slow, processive motors. Anomalous dynamics
arise for slow, processive motors over a range of motor densities, and are most
evident as superdiffusive mass transport, which we argue is the consequence of
a form of effective self-propulsion resulting from the polar coupling between
motors and filaments.Comment: 14 pages, 17 figures. Minor clarifications and updated/additional
references. To appear in Soft Matte
- …