Theoretical aspects of transversity observables

Theoretical aspects of transversity observables are reviewed. The main focus is on two leading twist transversity single spin asymmetries, one arising from the Collins effect and one from the interference fragmentation functions. Electron-positron annihilation experiments which are required to obtain these fragmentation functions are discussed, as well as the issues of factorization, evolution and Sudakov factors for the relevant observables. These theoretical considerations pinpoint the most realistic scenarios towards measurements of transversity.Comment: 4 pages, uses espcrc2.sty, Talk presented at the "International Workshop on the Spin Structure of the Proton and Polarized Collider Physics", ECT*, Trento, Italy, July 23-28, 200

Theoretical aspects of spin physics

A summary is given of how spin enters in collinearly factorizing processes. Next, theoretical aspects of polarization in processes beyond collinear factorization are discussed in more detail, with special focus on recent developments concerning the color gauge invariant definitions of transverse momentum dependent distribution and fragmentation functions, such as the Sivers and Collins effect functions. This has particular relevance for azimuthal single spin asymmetries, which currently receive much theoretical and experimental attention.Comment: 10 pages, 11 eps figures; Talk presented at the Ringberg Workshop `New Trends in HERA Physics 2003', Ringberg Castle, Tegernsee, Germany, September 28 - October 3, 200

Gluon TMD studies at EIC

A high-energy Electron-Ion Collider (EIC) would offer a most promising tool to study in detail the transverse momentum distributions of gluons inside hadrons. This applies to unpolarized as well as linearly polarized gluons inside unpolarized protons, and to left-right asymmetric distributions of gluons inside transversely polarized protons, the so-called gluon Sivers effect. The inherent process dependence of these distributions can be studied by comparing to similar, but often complementary observables at LHC.Comment: 6 pages, no figures, contribution to the proceedings of the 6th International conference on Physics Opportunities at an ElecTron-Ion Collider (POETIC VI), \'Ecole Polytechnique, Palaiseau, France, September 7-11, 201

Single Hadronic-Spin Asymmetries in Weak Interaction Processes

We show that measurements of single-spin asymmetries (SSAs) in charged current weak interaction processes such as deep inelastic neutrino scattering on a polarized target and inclusive $W$ production in polarized hadron-hadron collisions discriminate between the two fundamental QCD mechanisms (the Sivers and Collins effects) which have been proposed to explain such time-reversal-odd asymmetries. It has recently been shown that QCD final-state interactions due to gluon exchange between the struck quark and the proton spectators in semi-inclusive deep inelastic lepton scattering will produce non-zero Sivers-type single-spin asymmetries which survive in the Bjorken limit. We show that this QCD final-state interaction produces identical SSAs in charged and neutral current reactions. Furthermore, the contribution of each quark to the SSA from this mechanism is proportional to the contribution of that quark to the polarized baryon's anomalous magnetic moment. In contrast, the Collins effect contribution to SSAs depends on the transversity distribution of quarks in the polarized target. Since the charged current only couples to quarks of one chirality, it cannot sense the transversity distribution of the target, and thus it gives no Collins-type contribution to single-spin correlations.Comment: 12 pages, 1 figur

Overview of TMD evolution

Transverse momentum dependent parton distributions (TMDs) appear in many scattering processes at high energy, from the semi-inclusive DIS experiments at a few GeV to the Higgs transverse momentum distribution at the LHC. Predictions for TMD observables crucially depend on TMD factorization, which in turn determines the TMD evolution of the observables with energy. In this contribution to SPIN2014 TMD factorization is outlined, including a discussion of the treatment of the nonperturbative region, followed by a summary of results on TMD evolution, mostly applied to azimuthal asymmetries.Comment: 6 pages, no figures, contribution to the proceedings of the 21st International Symposium on Spin Physics (Spin2014), Beijing, China, October 20-24, 201

Gluon saturation effects on single spin asymmetries

We consider forward pion production in p p collisions at RHIC energies,which probes the so-called Extended Geometric Scaling region. Upon inclusion of small-x effects via an anomalous dimension within the Color Glass Condensate formalism at leading order in alpha_s, a good description of the cross section as a function of the transverse momentum of the produced pion is obtained. The latter is essential for extractions of the Sivers effect from polarized p p collisions, since it is a sensitive probe of the slope of the cross section. Hence, the presented approach is well suited to extract the Sivers effect from single spin asymmetries in forward pion production at high energies.Comment: 4 pages, 3 figures, talk presented at the 17th International Spin Physics Symposium, SPIN2006, Kyoto, Japan, October 2-7, 200

Gluon TMDs in quarkonium production

Quarkonium production offers good possibilities to study gluon TMDs. In this proceedings contribution this topic is explored for the linearly polarized gluons inside unpolarized hadrons and unpolarized gluons inside transversely polarized hadrons. It is argued that $\chi_{b0/2}$ and $\eta_b$ production at LHC are best to study the effects of linearly polarized gluons in hadronic collisions, by means of angular independent ratios of ratios of cross sections. This can be directly compared to $\cos 2\phi$ asymmetries in heavy quark pair and dijet production in DIS at a future high-energy Electron-Ion Collider (EIC), which probe the same TMDs. In the small-$x$ limit this corresponds to the Weizs\"acker-Williams (WW) gluon distributions, which should show a change in behavior for transverse momenta around the saturation scale. Together with investigations of the dipole (DP) gluon distributions, this can provide valuable information about the polarization of the Color Glass Condensate if sufficiently small $x$ are reached. Quarkonia can also be useful in the study of single transverse spin asymmetries. For transversely polarized hadrons the gluon distribution can be asymmetric, which is referred to as the Sivers effect. It leads to single spin asymmetries in for instance $J/\psi$ (pair) production at AFTER@LHC, which probe the WW or $f$-type gluon Sivers TMD. It allows for a test of a sign-change relation w.r.t. the gluon Sivers TMD probed at an EIC in open heavy quark pair production. Single spin asymmetries in backward inclusive $C$-odd quarkonium production, such as $J/\psi$ production, may offer probes of the DP or $d$-type gluon Sivers TMD at small $x$-values in the polarized proton, which in that limit corresponds to a correlator of a single Wilson loop, describing the spin-dependent odderon.Comment: 9 pages, no figures, contribution to the proceedings of the ECT* Workshop: New Observables in Quarkonium Production (Quarkonium2016), Trento, Italy, 28 February - 4 March 201