Title: Simple derivation of the Fong-Wandzura pulse sequence

We analyze the breaking of the Wandzura--Wilczek relation for the g_2 structure function, emphasizing its connection with transverse momentum dependent parton distribution functions. We find that the relation is broken by two distinct twist-3 terms, and clarify how these can be separated in measurements of double-spin asymmetries in semi-inclusive deep inelastic scattering. Through a quantitative analysis of available g_2 data we also show that the breaking of the Wandzura--Wilczek relation can be as large as 15--30% of the size of g_2.

Power corrections for virtual Compton scattering at leading twist are determined at operator level. From the complete off-cone representation of the twist-2 Compton operator integral representations for the trace, antisymmetric and symmetric part of that operator are derived. The operator valued invariant functions are written in terms of iterated operators and may lead to interrelations. For matrix elements they go over into relations for generalized parton distributions. Reducing to the s-channel relevant part one gets operator preforms of the Wandzura-Wilczek and the (target mass corrected) Callan-Gross relations whose structure is exactly the same as known from the case of deepmore » inelastic scattering; taking nonforward matrix elements one reproduces earlier results [B. Geyer, D. Robaschik and J. Eilers, Nucl. Phys. B704, 279 (2005).] for the absorptive part of the virtual Compton amplitude. All these relations, obtained without any approximation or using equations of motion, are determined solely by the twist-2 structure of the underlying operator and, therefore, are purely of geometric origin.« less

The longitudinal-transverse spin asymmetry A{sub LT} in the polarized Drell-Yan process depends on twist-3 quark distributions of nucleon. In addition to the contributions associated with the twist-3 operators, these distributions contain the 'Wandzura-Wilczek (WW)' part, which is determined by a certain integral of twist-2 distributions. The recently obtained empirical information on the transversity allows a realistic estimate of the WW contribution to A{sub LT} for the polarized pp-bar collisions. Our results indicate that rather large A{sub LT}({approx}10%) can be observed in the proposed spin experiments at GSI, with novel pattern as a function of dilepton mass compared with A{sub TT}more » and A{sub LL}.« less

We review the study the Wandzura--Wilczek relation for the structure function g{sub 2}, with a particular attention on the connection with the framework of Transverse Momentum Dependent factorization. We emphasize that the relation is broken by two distinct twist-3 terms. In the light of these findings, we clarify what can be deduced from the available experimental data on g{sub 2}, which indicate a breaking of the order 20-40%, and how to individually measure the twist-3 terms.