39,625 research outputs found
RQM description of PS meson form factors, constraints from space-time translations, and underlying dynamics
The role of Poincar\'e covariant space-time translations is investigated in
the case of the pseudoscalar-meson charge form factors. It is shown that this
role extends beyond the standard energy-momentum conservation, which is
accounted for in all relativistic quantum mechanics calculations. It implies
constraints that have been largely ignored until now but should be fulfilled to
ensure the full Poincar\'e covariance. The violation of these constraints,
which is more or less important depending on the form of relativistic quantum
mechanics that is employed, points to the validity of using a single-particle
current, which is generally assumed in calculations of form factors. In short,
these constraints concern the relation of the momentum transferred to the
constituents to the one transferred to the system. How to account for the
related constraints, as well as restoring the equivalence of different
relativistic quantum mechanics approaches in estimating form factors, is
discussed. Some conclusions relative to the underlying dynamics are given in
the pion case.Comment: 37 pages, 13 figures; figures completed for notations, revised text
with better emphasis on differences with previous works; accepted for
publication in EPJ
Form factors in relativistic quantum mechanics: constraints from space-time translations
The comparison of form factors calculated from a single-particle current in
different relativistic quantum mechanic approaches evidences tremendous
discrepancies. The role of constraints coming from space-time translations is
considered here with this respect. It is known that invariance under these
translations implies the energy-momentum conservation relation that is usually
assumed to hold globally. Transformations of the current under these
translations, which lead to this result, also imply constraints that have been
ignored so far in relativistic quantum mechanic approaches. An implementation
of these constraints is discussed in the case of a model with two scalar
constituents. It amounts to incorporate selected two-body currents to all
orders in the interaction. Discrepancies for form factors in different
approaches can thus be removed, contributing to restore the equivalence of
different approaches. Results for the standard front-form approach ()
are found to fulfill the constraints and are therefore unchanged. The relation
with results from a dispersion-relation approach is also made.Comment: 8 pages, 5 figures; to be published in the proceedings of LC2008;
Light Cone 2008. Relativistic Nuclear and Particle Physics, Mulhouse : France
(2008
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LBM, a useful tool for mesoscale modelling of single phase and multiphase flow – the variety of applications and approaches at Nottingham
This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.Giving an overview of Nottingham group’s recent progress on numerical modelling and
approaches in developing and applying the lattice Boltzmann method (LBM), the paper tries to demonstrate that the LBM is a useful tool for mesoscale modelling of single phase and multiphase flow. The variety of applications of the LBM modelling is reported, which include single phase fluid flow and heat transfer around or across rotational cylinder of curved boundary, two-phase flow in mixing layer, electroosmotically driven flow in thin liquid layer, bubbles/drops flow and coalescence in conventional channels and in
microchannels with confined boundary, liquid droplets in gas with relative large density ratio; viscous fingering phenomena of immiscible fluids displacement, and flow in porous media
Performance analysis with network-enhanced complexities: On fading measurements, event-triggered mechanisms, and cyber attacks
Copyright © 2014 Derui Ding et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.Nowadays, the real-world systems are usually subject to various complexities such as parameter uncertainties, time-delays, and nonlinear disturbances. For networked systems, especially large-scale systems such as multiagent systems and systems over sensor networks, the complexities are inevitably enhanced in terms of their degrees or intensities because of the usage of the communication networks. Therefore, it would be interesting to (1) examine how this kind of network-enhanced complexities affects the control or filtering performance; and (2) develop some suitable approaches for controller/filter design problems. In this paper, we aim to survey some recent advances on the performance analysis and synthesis with three sorts of fashionable network-enhanced complexities, namely, fading measurements, event-triggered mechanisms, and attack behaviors of adversaries. First, these three kinds of complexities are introduced in detail according to their engineering backgrounds, dynamical characteristic, and modelling techniques. Then, the developments of the performance analysis and synthesis issues for various networked systems are systematically reviewed. Furthermore, some challenges are illustrated by using a thorough literature review and some possible future research directions are highlighted.This work was supported in part by the National Natural Science Foundation of China under Grants 61134009, 61329301, 61203139, 61374127, and 61374010, the Royal Society of the UK, and the Alexander von Humboldt Foundation of Germany
Relationship between the gamma-ray burst pulse width and energy due to the Doppler effect of fireballs
We study in details how the pulse width of gamma-ray bursts is related with
energy under the assumption that the sources concerned are in the stage of
fireballs. Due to the Doppler effect of fireballs, there exists a power law
relationship between the two quantities within a limited range of frequency.
The power law range and the power law index depend strongly on the observed
peak energy as well as the rest frame radiation form, and the upper and
lower limits of the power law range can be determined by . It is found
that, within the same power law range, the ratio of the of the rising
portion to that of the decaying phase of the pulses is also related with energy
in the form of power laws. A platform-power-law-platform feature could be
observed in the two relationships. In the case of an obvious softening of the
rest frame spectrum, the two power law relationships also exist, but the
feature would evolve to a peaked one. Predictions on the relationships in the
energy range covering both the BATSE and Swift bands for a typical hard burst
and a typical soft one are made. A sample of FRED (fast rise and exponential
decay) pulse bursts shows that 27 out of the 28 sources belong to either the
platform-power-law-platform feature class or the peaked feature group,
suggesting that the effect concerned is indeed important for most of the
sources of the sample. Among these bursts, many might undergo an obvious
softening evolution of the rest frame spectrum.Comment: Accepted for publication in The Astrophysical Journa
Theoretical Constraints and Systematic Effects in the Determination of the Proton Form Factors
We calculate the two-photon exchange corrections to electron-proton
scattering with nucleon and intermediate states. The results show a
dependence on the elastic nucleon and nucleon--transition form factors
used as input which leads to significant changes compared to previous
calculations. We discuss the relevance of these corrections and apply them to
the most recent and precise data set and world data from electron-proton
scattering. Using this, we show how the form factor extraction from these data
is influenced by the subsequent inclusion of physical constraints. The
determination of the proton charge radius from scattering data is shown to be
dominated by the enforcement of a realistic spectral function. Additionally,
the third Zemach moment from the resulting form factors is calculated. The
obtained radius and Zemach moment are shown to be consistent with Lamb shift
measurements in muonic hydrogen.Comment: minor changes, added references, version to appear in PR
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