220 research outputs found
On AdS/QCD correspondence and the partonic picture of deep inelastic scattering
We critically examine the question of scaling of the Deep Inelastic
Scattering process in the medium Bjorken x region on a scalar boson in the
framework of the AdS/QCD correspondence. To get the right polarization
structure of the forward electroproduction amplitude, we show that one needs to
add (at least) the scalar to scalar and scalar to vector hadronic amplitudes.
This illustrates how the partonic picture may emerge from a simple scenario
based on the AdS/QCD correspondence, provided one allows the conformal
dimension of the hadronic field to equal 1 and use the concept of "hadron -
parton duality" .Comment: Second version with a few added clarifications and references. Third
version with a misprint corrected in Eq. 35 and following unnumbered
equation; to appear in Physics Letters
Hadron Helicity Violation in Exclusive Processes: Quantitative Calculations in Leading Order QCD
We study a new mechanism for hadronic helicity flip in high energy hard
exclusive reactions. The mechanism proceeds in the limit of perfect chiral
symmetry, namely without any need to flip a quark helicity. The fundamental
feature of the new mechanism is the breaking of rotational symmetry of the hard
collision by a scattering plane in processes involving independent quark
scattering. We show that in the impulse approximation there is no evidence for
of the helicity violating process as the energy or momentum transfer is
increased over the region 1 GeV^2 < Q^2 < 100 GeV^2. In the asymptotic region
Q^2> 1000 GeV^2, a saddle point approximation with doubly logarithmic accuracy
yields suppression by a fraction of power of Q^2. ``Chirally--odd" exclusive
wave functions which carry non--zero orbital angular momentum and yet are
leading order in the high energy limit, play an important role.Comment: uuencoded LaTeX file (21 pages) and PostScript figure
Disentangling positivity constraints for generalized parton distributions
Positivity constraints are derived for the generalized parton distributions
(GPDs) of spin-1/2 hadrons. The analysis covers the full set of eight twist-2
GPDs. Several new inequalities are obtained which constrain GPDs by various
combinations of usual (forward) unpolarized and polarized parton distributions
including the transversity distribution.Comment: 9 pages (REVTEX), typos correcte
On timelike Compton scattering at medium and high energies
We emphasize the complementarity of timelike and spacelike studies of deep
exclusive processes, taking as an example the case of timelike Compton
Scattering (TCS) i.e. the exclusive photoproduction of a lepton pair with large
invariant mass, vs deeply virtual Compton scattering (DVCS) i.e. the exclusive
leptoproduction of a real photon. Both amplitudes factorize with the same
generalized parton distributions (GPDs) as their soft parts and coefficient
functions which differ significantly at next to leading order in alpha_s. We
also stress that data on TCS at very high energy should be available soon
thanks to the study of ultraperipheral collisions at the LHC, opening a window
on quark and gluon GPDs at very small skewness.Comment: 8 pages, Presented at the workshop "30 years of strong interactions",
Spa, Belgium, 6-8 April 201
The Proton Electromagnetic Form Factor and Quark Orbital Angular Momentum
We analyze the proton electromagnetic form factor ratio
as a function of momentum transfer
within perturbative QCD. We find that the prediction for at large
momentum transfer depends on the exclusive quark wave functions, which are
unknown. For a wide range of wave functions we find that $ QF_2/F_1 \sim\
const$ at large momentum transfer, in agreement with recent JLAB data.Comment: 8 pages, 2 figures. To appear in Proceedings of the Workshop QCD
2002, IIT Kanpur, 18-22 November (2002
Hard Exclusive Pion Electroproduction at Backward Angles With CLAS
We report on the first measurement of cross sections for exclusive deeply virtual pion electroproduction off the proton, ep → e\u27nπ+, above the resonance region at backward pion center-of-mass angles. The ϕ∗ π-dependent cross sections were measured, from which we extracted three combinations of structure functions of the proton. Our results are compatible with calculations based on nucleon-to-pion transition distribution amplitudes (TDAs). These non-perturbative objects are defined as matrix elements of threequark-light-cone-operators and characterize partonic correlations with a particular emphasis on baryon charge distribution inside a nucleon
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