Azimuthal and transverse single spin asymmetries in hadronic collisions

We give a short overview of the phenomenology of azimuthal and transverse single spin asymmetries in (un)polarized high-energy hadronic collisions. We briefly summarize a transverse momentum dependent, generalized parton model approach to these polarization phenomena, and discuss some of its applications. Finally, open points and future developments will be outlined.Comment: 8 pages; Talk given at the "5th International Workshop on Quantum Chromodynamics - Theory and Experiment (QCD@Work 2010)", 20-23 June 2010, Martina Franca, Valle d'Itria, Italy; to be published in the AIP Conference Proceedings Serie

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

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

Glueball plus pion production in hadron collisions

Using a non--relativistic gluon bound--state model for glueballs (G), we compute the subprocess $q\,\bar q\, \to\, G\,\pi$, and we therefrom derive the yield of the overall reaction $p\,\bar p\, \to\, G\,\pi X$, assuming the glueball and the pion to be emitted with their transverse momenta large, opposite and approximately equal. Numerical results are presented in the form of $p_T$ spectra for various glueball candidates and their possible quantum states, assuming those particles to be produced, in the type of reactions here considered, at high--energy $p\,\bar p$ colliders such as the CERN Sp\=pS.Comment: 14 pages + 8 figures, REVTeX 3.0, figures appended as uuencoded, compressed postscript file. To appear in Zeit. Phys.

Off-diagonal Helicity Density Matrix Elements for Vector Mesons Produced at LEP

Final state quark-antiquark interactions may give origin to non zero values of the off-diagonal element rho_{1,-1} of the helicity density matrix of vector mesons produced in e+e- annihilations, as confirmed by recent OPAL data on phi and D^*'s. Predictions are given for rho_{1,-1} of several mesons produced at large z and small p_T, i.e. collinear with the parent jet; the values obtained for phi and D^* are in agreement with data.Comment: LaTeX, 12 pages, no figure

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