50,981 research outputs found
Twist-3 effects in deeply virtual Compton scattering made simple
We show that electromagnetic gauge invariance requires a 'spin rotation' of
the quarks in the usual twist-2 contribution to the amplitude for deeply
virtual Compton scattering. This rotation is equivalent to the inclusion of
certain kinematical twist-3 (Wandzura-Wilczek type) terms, which have been
derived previously using other methods. The new representation of the twist-3
terms is very compact and allows for a simple physical interpretation.Comment: 4 pages, 3 figures, uses ws-p8-50x6-00.cls . Proceedings of the
Baryons 2002 Conference, Jefferson Lab, Newport News, VA, March 3-8, 200
Chiral dynamics and peripheral transverse densities
In the partonic (or light-front) description of relativistic systems the
electromagnetic form factors are expressed in terms of frame-independent charge
and magnetization densities in transverse space. This formulation allows one to
identify the chiral components of nucleon structure as the peripheral densities
at transverse distances b = O(M_pi^{-1}) and compute them in a parametrically
controlled manner. A dispersion relation connects the large-distance behavior
of the transverse charge and magnetization densities to the spectral functions
of the Dirac and Pauli form factors near the two-pion threshold at timelike t =
4 M_pi^2. Using relativistic chiral effective field theory in the leading-order
approximation, we (a) derive the asymptotic behavior (Yukawa tail) of the
isovector transverse densities in the "chiral" region b = O(M_pi^{-1}) and the
"molecular" region b = O(M_N^2/M_pi^3); (b) perform the heavy-baryon expansion;
(c) explain the relative magnitude of the peripheral charge and magnetization
densities in a simple mechanical picture; (d) include Delta intermediate states
and study the densities in the large-N_c limit of QCD; (e) quantify the spatial
region where the chiral components are numerically dominant; (f) calculate the
chiral divergences of the b^2-weighted moments of the transverse densities
(charge and magnetic radii) and determine their spatial support. Our approach
provides a concise formulation of the spatial structure of the nucleon's chiral
component and offers new insights into basic properties of the chiral
expansion. It relates the information extracted from low-t elastic form factors
to the generalized parton distributions probed in peripheral high-energy
scattering processes.Comment: 52 pages, 13 figure
Light-front representation of chiral dynamics in peripheral transverse densities
The nucleon's electromagnetic form factors are expressed in terms of the
transverse densities of charge and magnetization at fixed light-front time. At
peripheral transverse distances the densities are governed
by chiral dynamics and can be calculated model-independently using chiral
effective field theory (EFT). We represent the leading-order chiral EFT results
for the peripheral transverse densities as overlap integrals of chiral
light-front wave functions, describing the transition of the initial nucleon to
soft pion-nucleon intermediate states and back. The new representation (a)
explains the parametric order of the peripheral transverse densities; (b)
establishes an inequality between the spin-independent and -dependent
densities; (c) exposes the role of pion orbital angular momentum in chiral
dynamics; (d) reveals a large left-right asymmetry of the current in a
transversely polarized nucleon and suggests a simple interpretation. The
light-front representation enables a first-quantized, quantum-mechanical view
of chiral dynamics that is fully relativistic and exactly equivalent to the
second-quantized, field-theoretical formulation. It relates the charge and
magnetization densities measured in low-energy elastic scattering to the
generalized parton distributions probed in peripheral high-energy scattering
processes. The method can be applied to nucleon form factors of other
operators, e.g. the energy-momentum tensor.Comment: 28 pages, 9 figure
Low-complexity high-performance GFSK receiver with carrier frequency offset correction
This paper presents an implementation of a GFSK receiver based on matched filtering of a sequence of K successive bits. This enables improved detection and superior BER performance but requires 2K matched filters of considerable complexity. Exploiting redundancy by performing phase propagation of successive single-bit stages, we propose an efficient receiver implementation. Results presented highlight the benefits of the proposed methd in terms of computational cost and performance compared to standard methods. We also address carrier frequency offset, and suggest a blind algorithm for its elimination. Performance results are exemplarily shown for a Bluetooth system
Return migration: theory and empirical evidence
In this paper we discuss forms of migration that are non-permanent. We focus on
temporary migrations where the decision to return is taken by the immigrant. These
migrations are likely to be frequent, and we provide some evidence for the UK. We
then develop a simple model which rationalizes the decision of a migrant to return to
his home country, despite a persistently higher wage in the host country. We
consider three motives for a temporary migration: Differences in relative prices in
host- and home country, complementarities between consumption and the location
where consumption takes place, and the possibility of accumulating human capital
abroad which enhances the immigrant's earnings potential back home. For the last
return motive, we discuss extensions which allow for immigrant heterogeneity, and
develop implications for selective in- and out- migration
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