471 research outputs found
Two-Photon Effects in Lepton-AntiLepton Pair Photoproduction from a Nucleon Target using Real Photons
We consider the production of a lepton-antilepton pair by real photons off a
hadronic target. The interference of one and two photon exchange amplitudes
leads to a charge asymmetry term that may be calculated explicitly in the
large-t limit in terms of hadronic distribution amplitudes. A rather compact
expression emerges for the leading order asymmetry at fixed angle in the
centre-of-mass of the lepton pair. The magnitude appears sizeable and is
approximately independent of the pair mass in the asymptotic limit.Comment: 14 pages, 7 figures. Title changed, references adde
The Angular Momentum and g_1^p Sum Rules for the Proton
The gauge invariant operator formulation of the angular momentum sum rule
for the proton is presented and contrasted with the sum
rule for the first moment of the polarised structure function . The
decoupling of the axial charge from the angular momentum sum rule is
highlighted and the possible QCD field-theoretic basis for an angular momentum
sum rule of the form {1\over2} = {1\over2}\D q + \D g + L_q + L_g is
critically discussed.Comment: 5 pages, LaTeX, uses espcrc2.sty; invited talk at QCD00, Montpellier,
July 200
Counting Form Factors of Twist-Two Operators
We present a simple method to count the number of hadronic form factors based
on the partial wave formalism and crossing symmetry. In particular, we show
that the number of independent nucleon form factors of spin-n, twist-2
operators (the vector current and energy-momentum tensor being special
examples) is n+1. These generalized form factors define the generalized
(off-forward) parton distributions that have been studied extensively in the
recent literature. In proving this result, we also show how the J^{PC} rules
for onium states arise in the helicity formalism.Comment: 7 pages, LaTeX (revtex
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