Azimuthal asymmetries for hadron distributions inside jets in hadronic collisions

Using a generalized parton model approach including spin and intrinsic parton motion effects, and assuming the validity of factorization for large p_T jet production in hadronic collisions, we study the azimuthal distribution around the jet axis of leading pions, produced in the jet fragmentation process. We identify the observable leading-twist azimuthal asymmetries for the unpolarized and single-polarized case related to both quark and gluon-originated jets. We account for all physically allowed combinations of the transverse momentum dependent (TMD) parton distribution and fragmentation functions, with special attention to the Sivers, Boer-Mulders, and transversity quark distributions, and to the Collins fragmentation function for quarks (and to the analogous functions for gluon partons).Comment: 8 pages, 5 figures, to appear in the proceedings of 19th International Spin Physics Symposium (SPIN2010), Sep 27-Oct 2, 2010, Juelich (Germany

TMDs, universality and factorization

We present a short overview on transverse momentum dependent parton distribution and fragmentation functions, giving their partonic interpretation and ways to access them. We then discuss the issue of their universality and its connection to factorization in perturbative QCD.Comment: 8 pages, 1 ps figure, uses cimento.cls. Talk delivered at the "Third International Workshop on Transverse Polarization Phenomena in Hard Scattering" (Transversity 2011), 29 Aug.-2 Sep. 2011, Veli Losinj (Croatia). Submitted to Nuovo Cimento

Transversity: theory and phenomenology

The distribution of transversely polarized quarks inside a transversely polarized nucleon, known as transversity, encodes a basic piece of information on the nucleon structure, sharing the same status with the more familiar unpolarized and helicity distributions. I will review its properties and discuss different ways to access it, with highlights and limitations. Recent phenomenological extractions and perspectives are also presented.Comment: 5 pages, 3 figures. Invited talk delivered at the "International Workshop on Diffraction in High-Energy Physics" (DIFFRACTION 2012), Sept. 10-15, 2012, Puerto del Carmen, Lanzarote, Canary Islands (Spain); to appear in "AIP Conference Proceedings

Azimuthal distributions of pions inside jets at RHIC

We evaluate the azimuthal asymmetries for the distributions of leading pions inside a jet, produced in high-energy proton-proton collisions, in kinematic configurations currently under active investigation at RHIC. Adopting a transverse momentum dependent approach, which assumes the validity of a perturbative QCD factorization scheme and takes into account all the spin and intrinsic parton motion effects, we show how the main mechanisms underlying these asymmetries, namely the Sivers and the Collins effects, can be disentangled. Furthermore, we consider the impact of color-gauge invariant initial and final state interactions and suggest a method for testing the universality properties of the Sivers function for quarks.Comment: 4 pages, 3 figures, to appear in the proceedings of the 3rd Workshop on the QCD structure of the nucleon (QCD-N'12), Bilbao (Spain), 22-26 October 201

Azimuthal distributions of pions inside a jet in hadronic collisions

Using a generalized parton model approach including spin and intrinsic parton motion effects, and assuming the validity of factorization for large pT jet production in hadronic collisions, we study the azimuthal distribution around the jet axis of leading pions, produced in the jet fragmentation process. We identify the observable leading-twist azimuthal asymmetries, which are generated by all the physically allowed combinations of transverse momentum dependent (TMD) parton distribution and fragmentation functions. In particular, we show how one can isolate the Collins and Sivers contributions, and suggest a test of the process dependence of the Sivers function by considering the effect of color-gauge invariant initial and final state interactions.Comment: 6 pages, 3 figures, to appear in the proceedings of Third International Workshop on Transverse Polarization Phenomena in Hard Scattering (Transversity 2011), Veli Lo\v{s}inj (Croatia), 29 August-2 September 201

Testing the universality of the Collins function in pion-jet production at RHIC

By adopting a generalised parton model approach at leading order in QCD, including spin and intrinsic parton motion effects, we study the Collins azimuthal asymmetries for pions within a large-$p_T$ jet produced at mid-rapidity in polarised hadronic collisions. Using available information on the quark transversity distributions and the pion Collins functions, as extracted from semi-inclusive deeply inelastic scattering and $e^+e^-\to h_1 h_2\, X$ processes, we compute estimates for the Collins asymmetries in kinematical configurations presently investigated at RHIC by the STAR Collaboration. Collins-like asymmetries, involving linearly polarised gluons, are also considered. Our predictions, compared against available preliminary data, show a very good agreement, even if some discrepancies, to be further scrutinized both theoretically and experimentally, appear in the transverse momentum dependence of the Collins asymmetry. These results are in favour of the predicted universality of the Collins function and of a mild, if any, evolution with the hard scale of the asymmetries.Comment: 10 pages, 5 figures (10 plots); v2: discussion on jet kT dependence of the SSAs extended and one plot on the corresponding unpolarized cross section added (Fig. 3, right panel); few comments on factorization breaking effects and on the universality issue of the Collins function added; three references added; results unchanged; matches the published versio

Probing the gluon Sivers function in p↑p→J/ψ Xp^\uparrow p\to J/\psi\,X and p↑p→D Xp^\uparrow p \to D\,X

We present a study of transverse single-spin asymmetries (SSAs) in $p^\uparrow p\to J/\psi\,X$ and $p^\uparrow p\to D X$ within the framework of the generalized parton model (GPM), which includes both spin and transverse momentum effects, and show how they can provide useful information on the still almost unknown gluon Sivers function. Moreover, by adopting a modified version of this model, named color gauge invariant (CGI) GPM, we analyze the impact of the initial- and final-state interactions on our predictions. As a consequence, we find that these two processes are sensitive to different gluon Sivers functions, which can be expressed as linear combinations of two distinct, universal gluon distributions. We therefore define proper observables which could allow for a separate extraction of these two independent Sivers functions. At the same time, we show how it would be possible to discriminate between the GPM and the CGI-GPM approaches by comparing the corresponding estimates of SSAs with present and future experimental results at RHIC.Comment: 19 pages, 8 figures, 1 table; v2: minor changes, matches the published versio

Single-spin asymmetries: the Trento conventions

During the workshop "Transversity: New Developments in Nucleon Spin Structure" (ECT*, Trento, Italy, 14-18 June 2004), a series of recommendations was put forward by the participants concerning definitions and notations for describing effects of intrinsic transverse momentum of partons in semi-inclusive deep inelastic scattering.Comment: 6 pages, 1 figure. Outcome of discussions at the workshop "Transversity: New Developments in Nucleon Spin Structure" (ECT*, Trento, Italy, 14-18 June 2004). Footnote 2 adde