Dynamical next-to-next-to-leading order parton distributions and the perturbative stability of F_L(x,Q^2)

It is shown that the previously noted extreme perturbative instability of the longitudinal structure function F_L(x,Q^2) in the small Bjorken-$x$ region, is a mere artefact of the commonly utilized `standard' gluon distributions. In particular it is demonstrated that using the appropriate -- dynamically generated -- parton distributions at NLO and NNLO, F_L(x,Q^2) turns out to be perturbatively rather stable already for Q^2 $\geq$ O(2-3 GeV^2).Comment: 4 pages, 3 figures, 1 table, to appear in the proceedings of 14th International QCD Conference (QCD08), Montpellier, France, 7-12 July 200

Linear polarization of gluons and Higgs plus jet production at the LHC

We show that the production of a Higgs boson in association with a jet in proton-proton collisions is sensitive to the linear polarization of gluons inside unpolarized protons. We present the analytical expressions, at leading order in perturbative QCD, for various transverse momentum dependent observables, which can be measured at the LHC. Since there are no experimental constraints on the linearly polarized gluon distribution, its effects are studied by adopting two different models. In particular, we find that the $\cos2\phi$ azimuthal asymmetry is a very promising observable because it could give access to the sign of this new distribution.Comment: 7 pages, 3 figures, to appear in the Proceedings of XXIII International Workshop on Deep-Inelastic Scattering (DIS 2015), Dallas, Texas (USA), 27 April - May 1 201

The Curvature of F_2^p(x, Q^2) as a Probe of Perturbative QCD Evolutions in the small-x Region

Perturbative NLO and NNLO QCD evolutions of parton distributions are studied, in particular in the (very) small-x region, where they are in very good agreement with all recent precision measurements of F_2^p(x,Q^2). These predictions turn out to be also rather insensitive to the specific choice of the factorization scheme (MS or DIS). A characteristic feature of perturbative QCD evolutions is a positive curvature of F_2^p which increases as x decreases. This perturbatively stable prediction provides a sensitive test of the range of validity of perturbative QCD.Comment: 4 pages, 3 figures, to appear in the Proceedings of the XVth International Workshop on Deep Inelastic Scattering (DIS07), Munich, Germany, April 16-20 200

Probing the transverse dynamics and polarization of gluons inside the proton at the LHC

Transverse momentum dependent gluon distributions encode fundamental information on the structure of the proton. Here we show how they can be accessed in heavy quarkonium production in proton-proton collisions at the LHC. In particular, their first determination could come from the study of an isolated J/psi or Upsilon particle, produced back to back with a photon.Comment: 4 pages, 2 figures, to appear in the proceedings of the 49th Rencontres de Moriond, QCD and High Energy Interactions, La Thuile (Italy), 22 - 29 March 201

Transverse momentum dependent gluon distributions at the LHC

Linearly polarized gluons inside an unpolarized proton contribute to the transverse momentum distributions of (pseudo)scalar particles produced in hadronic collisions, such as Higgs bosons and quarkonia with even charge conjugation ($\eta_c$, $\eta_b$, $\chi_{c0}$, $\chi_{b0}$). Moreover, they can produce azimuthal asymmetries in the associated production of a photon and a $J/\psi$ or a $\Upsilon$ particle, in a kinematic configuration in which they are almost back to back. These observables, which can be measured in the running experiments at the LHC, could lead to a first extraction of both the polarized and the unpolarized gluon distributions and allow for a study of their process and energy scale dependences.Comment: 8 pages, 5 figures, to appear in the Proceedings of the QCD Evolution Workshop, Santa Fe, New Mexico, USA, May 12-16 201

Impact of gluon polarization on Higgs plus jet production at the LHC

In this paper we consider Higgs plus jet production as a process that is sensitive to the linear polarization of gluons inside the unpolarized protons of the LHC. The leading order expressions for the transverse momentum distribution of the Higgs plus jet pair are provided in terms of transverse momentum dependent quark and gluon distributions. This includes both angular independent and azimuthal angular dependent contributions, presented directly in the laboratory frame. Lacking experimental constraints on the linearly polarized gluon distribution, we study its effects on Higgs plus jet production using two different models to illustrate the generic features and maximal effects. It is found that the $\cos 2 \phi$ distribution may be the most promising observable, as it is driven by only one initial linearly polarized gluon. The potential advantages of the Higgs plus jet process compared to other processes sensitive to the linear polarization of gluons are discussed.Comment: 12 pages, 6 figures; final version to appear in Physical Review

Polarized gluon studies with charmonium and bottomonium at LHCb and AFTER

Recently it has been put forward that linearly polarized gluons inside unpolarized protons affect the transverse momentum distribution of final state particles in hadronic collisions. They lead to a characteristic modulation of the differential cross section in Higgs production and to azimuthal asymmetries in, for instance, heavy quark pair production. Here we study the effect on charmonium and bottomonium production in hadronic collisions, such as at LHCb and at the proposed fixed target experiment AFTER at LHC. We focus mainly on the scalar and pseudoscalar quarkonia, eta_c, chi_{c0}, eta_b, chi_{b0}, which allow for an angular independent investigation. Within the framework of transverse momentum dependent factorization in combination with the nonrelativistic QCD based color-singlet quarkonium model, we show for small transverse momentum (q_T^2 \ll 4 M_Q^2) that the scalar and pseudoscalar quarkonium production cross sections are modified in different ways by linearly polarized gluons, while their effects on the production of higher angular momentum quarkonium states are strongly suppressed. Comparisons to chi_{c2}, chi_{b2} production can help to cancel out uncertainties. Together with the analogous study in Higgs production at LHC, quarkonium production can moreover be used to test the scale dependence of the linearly polarized gluon distribution over a large energy range.Comment: 12 pages, 5 figures; v2: revised version to appear in PR

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

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