5,449 research outputs found
Two-Quark Correlations in the Hard Electromagnetic Nucleon Form Factors
The, so called, "hard-scattering approach" represents a suitable framework
for the perturbative treatment of exclusive hadronic processes at large
energies and (transverse) momentum transfers. In this context, diquarks can
serve as a useful phenomenological concept to model non-perturbative effects
which are still observable in the kinematic range accessible by present-day
experiments. We outline how a description of baryons as quark-diquark systems
can be understood as an effective theory in the sense that the pure quark
hard-scattering approach is recovered in the limit of asymptotically large
momentum transfers. Our arguments are based on a reformulation of the
hard-scattering formalism in terms of quark-diquark degrees of freedom. This
reformulation provides the exact form of photon- and gluon-diquark vertices and
corresponding vertex functions (diquark form factors) in the limit of
asymptotically large momentum transfers -- and thus also asymptotic constraints
which should be fulfilled by phenomenological quark-diquark models for hard
scattering. As an application of this reformulation we present an analysis of
the hard electromagnetic nucleon form factors with respect to their
quark-diquark content.Comment: 4 pages, uses aipproc.cl
Mass Effects in Hard Exclusive Photoproduction of Mesons
We report on an attempt to describe hard exclusive photoproduction of
mesons, i.e. the reaction , by means of a
modified version of the hard-scattering approach, in which the proton is
treated as a quark-diquark rather than a three-quark system. In order to
improve the applicability of the model at momentum transfers of only a few GeV
we take into account constituent-mass effects in the calculation of the
perturbative scattering amplitude. With a standard -meson distribution
amplitude and diquark-model parameters adopted from preceding investigations of
other photon-induced reactions our predictions for differential cross sections
overestimate the naive extrapolation of the low-momentum transfer ZEUS data.
Our results, however, reveal the importance of taking into account the
charm-quark mass.Comment: 4 pages, 4 eps-figures, uses espcrc1.st
Double handbag description of proton-antiproton annihilation into a heavy meson pair
We propose to describe the process in a
perturbative QCD motivated framework where a double-handbag hard process factorizes from transition distribution
amplitudes, which are quasi forward hadronic matrix elements of operators, where q denotes light quarks and c denotes the heavy quark.
We advocate that the charm-quark mass acts as the large scale allowing this
factorization. We calculate this process in the simplified framework of the
scalar diquark model and present the expected cross sections for the PANDA
experiment at GSI-FAIR.Comment: 25 page
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