Andreas Metz

Semi-inclusive deep inelastic scattering at small transverse momentum

We study the cross section for one-particle inclusive deep inelastic scattering off the nucleon for low transverse momentum of the detected hadron. We decompose the cross section in terms of structure functions and calculate them at tree level in terms of transverse-momentum-dependent parton distribution and fragmentation functions. Our results are complete in the one-photon exchange approximation at leading and first subleading twist accuracy, with both beam and target polarization.

Universality of Soft and Collinear Factors in Hard-Scattering Factorization

Universality in QCD factorization of parton densities, fragmentation functions, and soft factors is endangered by the process dependence of the directions of Wilson lines in their definitions. We find a choice of directions that is consistent with factorization and that gives universality between e(+)e(-) annihilation, semi-inclusive deep-inelastic scattering, and the Drell-Yan process. Universality is only modified by a time-reversal transformation of the soft function and parton densities between Drell-Yan and the other processes, whose only effect is the known reversal of sign for T-odd parton densities such as the Sivers function. The modifications of the definitions needed to remove rapidity divergences with lightlike Wilson lines do not affect the results.

Relations between generalized and transverse momentum dependent parton distributions

Recent work suggests nontrivial relations between generalized parton distributions on the one hand and (naive time-reversal odd) transverse momentum dependent distributions on the other. Here we review the present knowledge on such type of relations. Moreover, as far as spectator model calculations are concerned, the existing results are considerably extended. While various relations between the two types of parton distributions can be found in the framework of spectator models, so far no nontrivial model-independent relations have been established.

Generalized parton correlation functions for a spin-0 hadron

The fully unintegrated, off-diagonal quark-quark correlator for a spin-1/2 hadron is parameterized in terms of so-called generalized parton correlation functions. Such objects, in particular, can be considered as mother distributions of generalized parton distributions on the one hand and transverse momentum dependent parton distributions on the other. Therefore, our study provides new, model-independent insights into the recently proposed nontrivial relations between generalized and transverse momentum dependent parton distributions. We find that none of these relations can be promoted to a model-independent status. As a by-product we obtain the first complete classification of generalized parton distributions beyond leading twist. The present paper is a natural extension of our previous corresponding analysis for spin-0 hadrons.

Intrinsic transverse parton momenta in deeply inelastic reactions

Intrinsic transverse parton momenta pT play an important role in the understanding of azimuthal/spin asymmetries in semi-inclusive deep-inelastic scattering (SIDIS) and the Drell-Yan process (DY). We review and update what is presently known about pT from these processes. In particular, we address the question to which extent data support the popular Gauss model for the pT -distributions. We find that the Gauss model works very well, and observe that the intrinsic transverse momenta in SIDIS and DY are compatible, which is a support for the factorization approach. As a byproduct we recover a simple but practical way of taking into account the energy dependence of pT -distributions.

Gluon-exchange in spin-dependent fragmentation

Abstract The fragmentation of an unpolarized quark into a transversely polarized spin- 1 2 particle is studied in the framework of a simple model. Special attention is payed to the gluon exchange which is incorporated in the gauge link of the fragmentation function, and which we model by an Abelian gauge field. The transverse single spin asymmetries in e + e − annihilation and semi-inclusive deep-inelastic scattering are calculated in the one-loop approximation. For e + e − annihilation one finds a cancellation between contributions from two on-shell intermediate states, which have no counterpart in deep-inelastic scattering. As a consequence of this cancellation, the model predicts the same spin asymmetry for both processes implying that, in the one-loop approximation, the corresponding fragmentation function is universal.

Dilepton production from polarized hadron hadron collisions

In this paper we present a comprehensive formalism for dilepton production from the collision of two polarized spin-

Parameterization of the quark-quark correlator of a spin- 1/2 hadron

\frac{1}{2}

Sivers effect in semiinclusive deeply inelastic scattering

hadrons by identifying the general angular distribution of the cross section in combination with a complete set of structure functions. The various structure functions are computed in the parton model approximation where we mainly consider the case when the transverse momentum of the dilepton pair is much smaller than its invariant mass. In this kinematical region dilepton production can be described in terms of transverse momentum dependent parton distributions.

Sivers effect in semi-inclusive DIS and in the Drell-Yan process

Abstract The general parameterization of the quark–quark correlation function for a spin- 1 2 hadron is considered. The presence of the Wilson line ensuring color gauge invariance of the correlator induces structures that were not given explicitly in the existing literature. In particular, the general form of the transverse momentum dependent correlator entering various hard scattering processes is derived. In this case two new time-reversal odd parton distributions appear at the twist-3 level.