Transverse Momentum Dependent Distributions in Hadronic Collisions: p(transv. polarized) p --> D + X and p(transv. polarized) p --> gamma + X

Our understanding of the transverse spin structure of hadrons might definitely get improved by the information we gather on transverse momentum dependent (TMD) distributions. These new functions could also be crucial for a description of the observed transverse single spin asymmetries (SSA). In a hard scattering model for inclusive hadronic reactions, based on a generalized QCD factorization scheme, many mechanisms - namely the Sivers, Collins, Boer-Mulders effects - might contribute to a SSA. We show how the k_T dependent phases arising from the partonic kinematics together with a suitable choice of experimental configurations could help in disentangling the above mentioned effects. We discuss their potential role in two inclusive hadronic processes: heavy meson and photon production in pp and p pbar collisions.Comment: LaTeX 2e, 4 pages, 4 ps figures, uses aipproc.cls. Talk delivered by U. D'Alesio at the "17th International Spin Physics Symposium", SPIN2006, October 2-7, 2006, Kyoto, Japa

Parton intrinsic motion: unpolarized cross sections and the Sivers effect in inclusive particle production

We present a detailed study, performed in the framework of LO perturbative QCD with the inclusion of spin and k_T effects, of unpolarized cross sections for the Drell-Yan process and for inclusive pion and photon production in hadronic collisions, in different kinematical situations. We find a satisfactory agreement between theoretical predictions and experimental data. This supports the study of spin effects and transverse single spin asymmetries (SSA) within the same scheme. We then present results for SSA, generated by the so-called Sivers effect, in inclusive pion production in proton-proton collisions.Comment: LaTeX2e, 4 pages, 4 ps figures, uses ws-procs9x6. Talk delivered by F. Murgia at the ``16th International Spin Physics Symposium'', SPIN2004, October 10-16, 2004, Trieste, Ital

Phenomenology of single transverse spin asymmetries

A unified and consistent phenomenological approach to single transverse spin asymmetries in the framework of perturbative QCD, with the inclusion of a new class of spin and k_T dependent distribution and fragmentation functions, is presented. As an example, results on A_N(p(pol) p --> pion X) and P_Lambda(p p -->Lambda(pol) X) are shown.Comment: 8 pages, 3 figures, uses epsfig.sty. Talk delivered by U. D'Alesio at the ``VIII Convegno su Problemi di Fisica Nucleare Teorica'', October 18-20, 2000, Cortona, Ital

Single spin asymmetries, unpolarized cross sections and the role of partonic transverse momentum

Partonic intrinsic transverse momentum can be essential for the explanation of large single spin asymmetries in hadronic reactions in the framework of perturbative QCD. The status of an ongoing program investigating in a consistent way the role of intrinsic transverse momentum both in unpolarized and polarized processes is discussed. We compute inclusive cross sections for hadron and photon production in hadronic collisions and for Drell-Yan processes; the results are compared with available experimental data in several different kinematical situations.Comment: LaTeX, 6 pages, 4 ps figures, uses graphicx.sty. Talk delivered by U. D'Alesio at the ``15th International Spin Physics Symposium'', SPIN2002, September 9-14, 2002, Brookhaven National Laboratory, Upton (NY), US

Cross sections and single spin asymmetries in hadronic collisions with intrinsic transverse momentum effects

The role of intrinsic transverse momentum both in unpolarized and polarized processes is discussed. We consider inclusive cross sections for pion production in hadronic collisions and for Drell-Yan processes; the results are compared with available experimental data in several different kinematical situations. We reanalyze transverse single spin asymmetries (SSA) observed in inclusive pion production, p(transv. polarized) p --> pion + X, in terms of Sivers effect and show how it can be disentangled in polarized Drell-Yan processes by suitably integrating over some final configurations. Estimates for RHIC experiments are given.Comment: LaTeX, 8 pages, 4 ps figures, uses epsfig.sty. Talk delivered by F. Murgia at the "IX Convegno su Problemi di Fisica Nucleare Teorica", October 9-12, 2002, Cortona, Ital

A general formalism for single and double spin asymmetries in inclusive hadron production

We present a calculation of single and double spin asymmetries for inclusive hadron production in hadronic collisions. Our approach is based on Leading Order (LO) perturbative QCD and generalized factorization theorems, with full account of intrinsic parton momentum, k_T, effects. This leads to a new class of spin and k_T-dependent distribution and fragmentation functions. Limiting ourselves to consider leading twist functions, we show how they could play a relevant role in producing non-vanishing spin asymmetries.Comment: LaTeX, 8 pages, uses ws-procs9x6.cls. Talk delivered by S. Melis at the ``X Convegno su Problemi di Fisica Nucleare Teorica'', October 6-9, 2004, Cortona, Ital

On the importance of Lorentz structure in the parton model: target mass corrections, transverse momentum dependence, positivity bounds

We show that respecting the underlying Lorentz structure in the parton model has very strong consequences. Failure to insist on the correct Lorentz covariance is responsible for the existence of contradictory results in the literature for the polarized structure function g_2(x), whereas with the correct imposition we are able to derive the Wandzura-Wilczek relation for g_2(x) and the target-mass corrections for polarized deep inelastic scattering without recourse to the operator product expansion. We comment briefly on the problem of threshold behaviour in the presence of target-mass corrections. Careful attention to the Lorentz structure has also profound implications for the structure of the transverse momentum dependent parton densities often used in parton model treatments of hadron production, allowing the k_T dependence to be derived explicitly. It also leads to stronger positivity and Soffer-type bounds than usually utilized for the collinear densities.Comment: RevTeX4, 17 pages, 7 eps figure

Single transverse spin asymmetries in inclusive hadron production

A consistent phenomenological approach to the computation of transverse single spin asymmetries in inclusive hadron production is presented, based on the assumed generalization of the QCD factorization theorem to the case in which quark intrinsic motion is taken into account. New k_T and spin dependent quark distribution and fragmentation functions are considered: some of them are fixed by fitting data on p(transv. polarized) + p -> pion + X and predictions are given for single spin asymmetries in l + p(transv. polarized) -> pion + X and gamma^* + p(transv. polarized) -> pion + X processes.Comment: LaTeX, 3 pages, 2 eps figures, uses epsfig.sty, fleqn.sty, npb.sty. Talk delivered by M. Anselmino at the 7th International Workshop on Deep Inelastic Scattering and QCD (DIS99), Zeuthen, April 1999, to appear in Nucl. Phys. B (Proc. Suppl.