M. Anselmino

Role of Cahn and Sivers effects in deep inelastic scattering

The role of intrinsic k {sub perpendicular} in inclusive and semi-inclusive Deep Inelastic Scattering processes (lp{yields}lhX) is studied with exact kinematics within QCD parton model at leading order; the dependence of the unpolarized cross section on the azimuthal angle between the leptonic and the hadron production planes (Cahn effect) is compared with data and used to estimate the average values of k{sub perpendicular} both in quark distribution and fragmentation functions. The resulting picture is applied to the description of the weighted single spin asymmetry A{sub UT}{sup sin({phi}{sub {pi}}{sup -{phi}}{sub S})} recently measured by the HERMES collaboration at DESY; this allows to extract some simple models for the quark Sivers functions. These are compared with the Sivers functions which succeed in describing the data on transverse single spin asymmetries in p{sup {up_arrow}}p{yields}{pi}X processes; the two sets of functions are not inconsistent. The extracted Sivers functions give predictions for the COMPASS measurement of A{sub UT}{sup sin({phi}{sub {pi}}{sup -{phi}}{sub S})} in agreement with recent preliminary data, while their contribution to HERMES A{sub UL}{sup sin{phi}{sub {pi}}} is computed and found to be small. Predictions for A{sub UT}{sup sin({phi}{sub K}{sup -{phi}}{sub S})} for kaon production at HERMES are also given.

Single spin asymmetries in p↑p and inclusive processes

Abstract We consider several single spin asymmetries in inclusive p↑p and p ↑ p processes as higher twist QCD contributions, taking into account spin and intrinsic k ⊥ effects in the quark distribution functions. This approach has been previously applied to the description of the single spin asymmetries observed in p ↑ p→π X reactions and all its parameters fixed: we give here predictions for new processes, which agree with experiments for which data are available, and suggest further possible measurements.We consider several single spin asymmetries in inclusive, transversely polarized proton(antiproton) - proton processes as higher twist QCD contributions, taking into account spin and intrinsic transverse momentum effects in the quark distribution functions. This approach has been previously applied to the description of the single spin asymmetries observed in transversely polarized proton - proton -> pion + X reactions and all its parameters fixed: we give here predictions for new processes, which agree with experiments for which data are available, and suggest further possible measurements.

A Strategy towards the extraction of the Sivers function with TMD evolution

The QCD evolution of the unpolarized Transverse Momentum Dependent (TMD) distribution functions and of the Sivers functions have been discussed in recent papers. Following such results we reconsider previous extractions of the Sivers functions from semi-inclusive deep inelastic scattering data and propose a simple strategy which allows to take into account the Q^2 dependence of the TMDs in comparison with experimental findings. A clear evidence of the phenomenological success of the TMD evolution equations is given, mostly, by the newest COMPASS data off a transversely polarized proton target.

Extracting the sivers function from polarized semi-inclusive deep inelastic scattering data and making predictions

The most recent data on the weighted transverse single spin asymmetry A{sub UT}{sup sin({phi}{sub h}{sup -}{phi}{sub S})} from HERMES and COMPASS Collaborations are analyzed within the LO parton model with unintegrated parton distribution and fragmentation functions; all transverse motions are taken into account, with exact kinematics, in the elementary interactions. The overall quality of the data is such that, for the first time, a rather well constrained extraction of the Sivers function for u and d quarks is possible and is performed. Comparisons with models are made. Based on the extracted Sivers functions, predictions for A{sub UT}{sup sin({phi}{sub h}{sup -}{phi}{sub S})} asymmetries at JLab are given; suggestions for further measurements at COMPASS, with a transversely polarized hydrogen target and selecting favorable kinematical ranges, are discussed. Predictions are also presented for single spin asymmetries in Drell-Yan processes at the Relativistic Heavy Ion Collider and the Facility for Antiproton and Ion Research at GSI-Darmstadt.

General partonic structure for hadronic spin asymmetries

The high energy and large p_T inclusive polarized process, (A, S_A) + (B, S_B) --> C + X, is considered under the assumption of a generalized QCD factorization scheme. For the first time all transverse motions, of partons in hadrons and of hadrons in fragmenting partons, are explicitly taken into account: the elementary interactions are computed at leading order with noncollinear exact kinematics, which introduces many phases in the expressions of their helicity amplitudes. Several new spin and k_T dependent soft functions appear and contribute to the cross sections and to spin asymmetries: we put emphasis on their partonic interpretation, in terms of quark and gluon polarizations inside polarized hadrons. Connections with other notations and further information are given in some Appendices. The formal expressions for single and double spin asymmetries are derived. The transverse single spin asymmetry A_N, for p(transv. polarized) p --> pion + X processes is considered in more detail, and all contributions are evaluated numerically by saturating unknown functions with their upper positivity bounds. It is shown that the integration of the phases arising from the noncollinear kinematics strongly suppresses most contributions to the single spin asymmetry, leaving at work predominantly the Sivers effect and, to a lesser extent, the Collins mechanism.

Sivers effect in Drell-Yan processes

The Sivers distributions recently extracted from semi-inclusive deep inelastic scattering data [M. Anselmino et al., Eur. Phys. J. A 39, 89 (2009)] are used to compute estimates for Sivers asymmetries in Drell-Yan processes which are being planned at several facilities (RHIC, COMPASS, J-PARC, PAX, PANDA, NICA (JINR) and SPASCHARM (IHEP)). Most of these asymmetries turn out to be large and could allow a clear test of the predicted sign change of the Sivers distributions when active in SIDIS and Drell-Yan processes. This is regarded as a fundamental test of our understanding, within QCD and the factorization scheme, of single spin asymmetries.

Constraints on the gluon Sivers distribution via transverse single spin asymmetries at mid-rapidity in p(up) p ---> pi0 X processes at RHIC

We consider the recent RHIC data on the transverse single spin asymmetry (SSA) A_N, measured in p(transv. polarized) p -> pi^0 X processes at mid-rapidity by the PHENIX collaboration. The measurement is consistent with a vanishing SSA. We analyze this experimental information within a hard scattering approach based on a generalized QCD factorization scheme, with unintegrated, transverse momentum dependent (TMD), parton distribution and fragmentation functions. It turns out that, in the kinematical region of the data, only the gluon Sivers effect could give a large contribution to A_N; its vanishing value is thus an indication about the possible size of the gluon Sivers function (GSF). Approximate upper limits on its magnitude are derived. Additional constraints obtained combining available parameterizations of the quark Sivers function and the Burkardt sum rule (BSR) for the Sivers distributions are also discussed.

General Helicity Formalism for Polarized Semi-Inclusive Deep Inelastic Scattering

We study polarized Semi-Inclusive Deep Inelastic Scattering (SIDIS) processes, within the QCD parton model and a factorization scheme, taking into account all transverse motions, of partons inside the initial proton and of hadrons inside the fragmenting partons. We use the helicity formalism. The elementary interactions are computed at LO with non collinear exact kinematics, which introduces phases in the expressions of their helicity amplitudes. Several Transverse Momentum Dependent (TMD) distribution and fragmentation functions appear and contribute to the cross sections and to spin asymmetries. Our results agree with those obtained with different formalisms, showing the consistency of our approach. The full expression for single and double spin asymmetries is derived. Simplified, explicit analytical expressions, convenient for phenomenological studies, are obtained assuming a factorized Gaussian dependence on intrinsic momenta for the TMDs.

Sivers effect and the single spin asymmetry A_N in pp -> hX processes

The single spin asymmetry A_N, for large P_T single inclusive particle production in p(transv. pol.) p collisions, is considered within a generalised parton model and a transverse momentum dependent factorisation scheme. The focus is on the Sivers effect and the study of its potential contribution to A_N, based on a careful analysis of the Sivers functions extracted from azimuthal asymmetries in semi-inclusive deep inelastic scattering processes. It is found that such Sivers functions could explain most features of the A_N data, including some recent STAR results which show the persistence of a non zero A_N up to surprisingly large P_T values.

Higher twist corrections to Bjorken sum rule

Abstract Some higher twist corrections to the Bjorken sum rule are estimated in the framework of a quark-diquark model of the nucleon. The parameters of the model have been previously fixed by fitting the measured higher twist corrections to the unpolarized structure function F2(x, Q2). The resulting corrections to the Bjorken sum rule turn out to be negligible.