626 research outputs found
Quark Counting Rules: Old and New Approaches
I discuss the subject of powerlike asymptotic behavior of hadronic form
factors in pre-QCD analyses of soft (Feynman/Drell-Yan) and hard (West)
mechanisms, and also recent derivation of asymptotics of meson form
factors in AdS/QCD. At the end, I briefly comment on "light-front holography"
ansatz.Comment: Talk at "Shifmania", May 14-17,2009, Minneapolis, M
Quark Pseudo-Distributions at Short Distances
We perform a one-loop study of the small- behavior of the Ioffe-time
distribution (ITD) , the basic function that may be
converted into parton pseudo- and quasi-distributions. We calculate the
corrections at the operator level, so that our results may be later used for
pseudo-distribution amplitudes and generalized parton pseudo-distributions. We
separate two sources of the -dependence at small . One is related
to the ultraviolet (UV) singularities generated by the gauge link, and another
to short-distance logarithms generating perturbative evolution of parton
densities. Our calculation explicitly shows that, for a finite UV cut-off, the
UV-singular terms vanish when . The UV divergences are absent in the
ratio ("reduced" ITD). Still, it
has a non-trivial short-distance behavior due to terms
generating perturbative evolution of the parton densities. We give an explicit
expression, up to constant terms, for the reduced ITD at one loop. It may be
used in extraction of PDFs from the lattice QCD simulations. We also use our
results to get new insights concerning the structure of parton
quasi-distributions at one-loop level.Comment: 10 pages, 4 figures, typos fixed, references added, some changes in
tex
Symmetries and structure of skewed and double distributions
Extending the concept of parton densities onto nonforward matrix elements
of quark and gluon light-cone operators, one can use two types of
nonperturbative functions: double distributions (DDs) f(x,\alpha;t), F(x,y;t)
and skewed (off&nonforward) parton distributions (SPDs) H(x,\xi;t),
F_\zeta(X,t). We treat DDs as primary objects producing SPDs after integration.
We emphasize the role of DDs in understanding interplay between X (x) and \zeta
(\xi) dependences of SPDs.In particular, the use of DDs is crucial to secure
the polynomiality condition: Nth moments of SPDs are Nth degree polynomials in
the relevant skewedness parameter \zeta or \xi. We propose simple ansaetze for
DDs having correct spectral and symmetry properties and derive model
expressions for SPDs satisfying all known constraints. Finally, we argue that
for small skewedness, one can obtain SPDs from the usual parton densities by
averaging the latter with an appropriate weight over the region [X-\zeta,X] (or
[x - \xi, x + \xi]).Comment: 10 pages, Latex, 3 figure
Compton scattering and nonforward parton distributions
The hard exclusive electroproduction processes provide new information about
hadronic structure accumulated in nonforward parton distributions. The NFPD's
are universal hybrid functions having the properties of parton densities,
hadronic form factors and distribution amplitudes. They give a unified
description of various hard exclusive and inclusive reactions. The basic
supplier of information about nonforward parton distributions is deeply virtual
Compton scattering which offers a remarkable example of Bjorken scaling
phenomena in exclusive processes. Wide-angle real Compton scattering is an
ideal tool to test angle-dependent scaling laws characteristic for soft overlap
mechanism. Hard meson electroproduction is the best candidate to see pQCD hard
gluon exchange in exclusive reactions.Comment: 11 pages, Latex, 6 figures; Contribution to proceedings of the
workshop ``Physics and instrumentation with 6-12 GeV beams'', Jefferson Lab,
June 15-18, 199
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