F. Murgia

Update on transversity and Collins functions from SIDIS and e+e− data

We present an update of a previous global analysis of the experimental data on azimuthal asymmetries in semi-inclusive deep inelastic scattering (SIDIS), from the HERMES and COMPASS Collaborations, and in e + e − → h 1 h 2 X processes, from the Belle Collaboration. Compared to the first extraction, a more precise determination of the Collins fragmentation function and the transversity distribution function for u and d quarks is obtained.

Single spin asymmetry for p↑p → πX in perturbative QCD

Abstract Within the QCD-improved parton model and assuming the factorization theorem to hold in the helicity basis and for higher twist contributions, we show how nonzero single spin asymmetries in hadron-hadron high energy and moderately large p T inclusive processes can be obtained, even in massless perturbative QCD, provided the quark intrinsic motion is taken into account. A simple model is constructed which reproduces the main features of the data on the single spin asymmetry observed in inclusive pion production in p p collisions.

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.

Extracting the Sivers function from polarized SIDIS 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.

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.

Accessing Sivers gluon distribution via transverse single spin asymmetries in p(transv. polarized) p ---> D X processes at RHIC

The production of

Sivers Single-Spin Asymmetry in Photon-Jet Production

D

Parton intrinsic motion: Suppression of the Collins mechanism for transverse single spin asymmetries in p(up) p ---> pi X

mesons in the scattering of transversely polarized protons off unpolarized protons at BNL RHIC offers a clear opportunity to gain information on the Sivers gluon distribution function.