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

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

Average transverse momentum quantities approaching the lightfront

In this contribution to Light Cone 2014, three average transverse momentum quantities are discussed: the Sivers shift, the dijet imbalance, and the $p_T$ broadening. The definitions of these quantities involve integrals over all transverse momenta that are overly sensitive to the region of large transverse momenta, which conveys little information about the transverse momentum distributions of quarks and gluons inside hadrons. TMD factorization naturally suggests alternative definitions of such integrated quantities, using Bessel-weighting and rapidity cut-offs, with the conventional definitions as limiting cases. The regularized quantities are given in terms of integrals over the TMDs of interest that are well-defined and moreover have the advantage of being amenable to lattice evaluations.Comment: 6 pages, no figures, contribution to the proceedings of Light Cone 2014, Raleigh, North Carolina, May 26-30, 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

Aspects of TMD evolution of azimuthal asymmetries

In this contribution TMD evolution of azimuthal asymmetries, in particular of the Sivers and double Collins asymmetries, is addressed. A comparison of the scale dependence is made between asymmetries described with TMD factorization at low transverse momentum and those described with collinear factorization at high transverse momentum. Finally, the advantages of Bessel weighting are discussed: convergence of transverse momentum integrals, suppression of large transverse momentum contributions, and well-defined lattice QCD evaluations of Bessel-weighted TMDs including proper gauge links.Comment: 9 pages, 2 figures, to appear in the proceedings of the QCD Evolution Workshop (QCD2013), Thomas Jefferson National Accelerator Facility, Newport News, VA, May 6-10, 201

Anomalous Drell-Yan asymmetry from hadronic or QCD vacuum effects

The anomalously large cos(2 phi) asymmetry measured in the Drell-Yan process is discussed. Possible origins of this large deviation from the Lam-Tung relation are considered with emphasis on the comparison of two particular proposals: one that suggests it arises from a QCD vacuum effect and one that suggests it is a hadronic effect. Experimental signatures distinguishing these effects are discussed.Comment: 8 pages, 4 eps figures; Talk presented at the International Workshop on Transverse Polarization Phenomena in Hard Processes (Transversity 2005), Villa Olmo, Como, Italy, September 7-10, 200

Mapping the Transverse Nucleon Spin

The transverse nucleon spin can be transferred to the quarks and gluons in several ways. In the factorizing, hard scattering processes to be considered, these are parameterized at leading twist by the transversity distribution function and at next-to-leading twist by quark-gluon correlation functions. The latter enter the description of the structure function $g_2$ and possibly of single spin asymmetries. It is discussed what is known about these functions and what are the remaining open issues.Comment: 9 pages, uses espcrc1.sty, Invited talk at the European Workshop on the QCD Structure of the Nucleon (QCD-N'02), Ferrara, Italy, April 3-6, 2002, Some minor changes on page