Single spin asymmetries in high energy reactions and nonperturbative QCD effects

We discuss some experimental and theoretical results on single spin asymmetries (SSA) in high energy lepton-hadron and hadron-hadron reactions. In particular, recent results on meson SSA obtained by HERMES are considered in detail. We also discuss the SSA results obtained recently by COMPASS, as well as those from BRAHMS, PHENIX and STAR. Special attention is paid to a possible nonperturbative QCD mechanism that might be responsible for the observed meson SSA. This mechanism originates from the spin-flip quark-gluon chromomagnetic interaction induced by the complex topological structure of the QCD vacuum. We argue that in semi-inclusive deep-inelastic scattering a large SSA is expected not only for mesons but also for baryons due to strong nonperturbative final state interactions between ud-diquark and u-quark in the fragmenting proton.     

QCD resummation for single spin asymmetries

We study the transverse momentum dependent factorization for single spin asymmetries in Drell-Yan and semi-inclusive deep inelastic scattering processes at one-loop order. The next-to-leading order hard factors are calculated in the Ji-Ma-Yuan factorization scheme. We further derive the QCD resummation formalisms for these observables following the Collins-Soper-Sterman method. The results are expressed in terms of the collinear correlation functions from initial and/or final state hadrons coupled with the Sudakov form factor containing all order soft-gluon resummation effects. The scheme-independent coefficients are calculated up to one-loop order.     

Azimuthal and spin asymmetries in DIS

In these lectures I want to discuss how the structure functions in deep inelastic scattering relate to quark and gluon correlation functions. In particular we will consider the issue of intrinsic transverse momenta of quarks, which becomes important in processes like 1-particle inclusive leptoproduction. Some examples of cross sections and asymmetries, in particular in polarized scattering processes are discussed. We also discuss the operator structure for azimuthal asymmetries and their evolution.     

Perturbative QCD and weak asymmetries in lepton pair production

The interference of electromagnetic and weak production mechanisms for lepton pair production may give rise to several effects which violate parity and charge symmetry. These effects are generally of the order of 1% for dilepton masses of 10 GeV. The theoretical calculations presented here show that experimental studies of these asymmetries may be useful. In particular, measurements of these asymmetries in collisions of pions with polarized protons may lead to a greatly enhanced understanding of the polarization distribution of quarks in a polarized proton. The polarization structure of the d quark is shown to be of special interest. Measurement of the parity-violating asymmetries in proton-polarized proton collisions may prove to be a sensitive probe of the flavor symmetry of the proton antiquark sea. Analysis of the parity conserving charge asymmetry (which is predicted to occur in collisions of unpolarized hadrons) allows a unique further test of the Drell-Yan model for lepton pair production, as well as of our understanding of weak interactions.The above asymmetries mentioned are computed within the framework of the parton model. The focus of this work is on the asymmetries calculated with the inclusion of first-order perturbative QCD effects. The asymmetries are calculated in a differential form for values of the dilepton transverse momentum large compared with the typical (～1 GeV) scale of nonperturbative effects, and also in a form in which this transverse momentum has been integrated over. Corrections to the parton model results, which can be large, show rather different structure for the various asymmetries.A parity-violating asymmetry which may occur in collisions of unpolarized hadrons is also discussed. This asymmetry is very sensitive to the nonperturbative structure of hadrons, and is estimated to be approximately 0.01% for dilepton masses of 10 GeV.     

Sivers and Collins single spin asymmetries

The Sivers and Collins asymmetries are the most prominent Single Spin Asymmetries (SSA) in Semi-Inclusive Deeply Inelastic Scattering (SIDIS) with transverse target polarization. In this talk we present our understanding of these phenomena.     

Beam normal spin asymmetries: Theory

The beam normal spin asymmetry in elastic electron-nucleon scattering is discussed. This beam normal spin asymmetry depends on the imaginary part of two-photon exchange processes between electron and nucleon, and measures the non-forward structure functions of the nucleon. After briefly reviewing the theoretical formalism, we discuss calculations in the threshold region, in the resonance region, as well as in the diffractive region, corresponding with high energy and forward angles.     

On spin dependent potentials in supersymmetric QCD

Starting from the supersymmetric extension of the nonabelian path dependent phase factor we try to get an understanding of the physical role of its components. We extend to SUSY-QCD (broken or unbroken) the technique of expanding the quark propagator in gauge field background in terms of inverse powers of quark mass which has been used earlier by Eichten and Feinberg to study spin dependent quark potentials in QCD. The contribution to the \(Q\bar Q\) potential induced by gluino exchange is expressed in terms of gluino insertions into the Wilson loop. A rough quantitative estimate is given on the basis of perturbation theory. Finally the extension to the \(S\bar Q\) and 3Q system is discussed.     

Target-normal single spin asymmetries measured with positrons

The European Physical Journal A - The dipole response of the proton-magic nucleus $${}^{124}\hbox {Sn}$$ was previously investigated with electromagnetic and hadronic probes. Different responses...     

Reaction asymmetries and the spin orbit interaction

The use of reaction asymmetries to deduce spin orbit interactions in heavy ion reactions is discussed. It is suggested that in most cases they are too sensitive to many other phenomena to be useful tools and reanalysis of some existing data demonstrates that large spin orbit interactions are not required to reproduce the observed asymmetries.     

Azimuthal and single spin asymmetries in hard scattering processes

In this article we review the present understanding of azimuthal and single spin asymmetries for inclusive and semi-inclusive particle production in unpolarized and polarized hadronic collisions at high energy and moderately large transverse momentum. After summarizing the experimental information available, we discuss and compare the main theoretical approaches formulated in the framework of perturbative QCD. We then present in some detail a generalization of the parton model with inclusion of spin and intrinsic transverse momentum effects. In this context, we extensively discuss the phenomenology of azimuthal and single spin asymmetries for several processes in different kinematical configurations. A comparison with the predictions of other approaches, when available, is also given. We finally emphasize some relevant open points and challenges for future theoretical and experimental investigation.     

Azimuthal and single spin asymmetries in deep inelastic scattering

Azimuthal and single spin asymmetries play a crucial role in the study of the spin structure of hadrons in terms of their elementary constituents. Exploiting them to uncover information over subtle distribution and fragmentation functions, which cannot easily by accessed in other ways, is what this talk is about.     

Radiative corrections to the helicity asymmetries for the Drell-Yan process in QCD

The next-to-leading-logarthmic corrections to the helicity cross sections for the Drell-Yan process are calculated. While it is well-known that these corrections are large, it is shown here that when considering the helicity asymmetry (initial-initial) these large radiative effects cancel. Thus the leading-order calculation for the asymmetry is a good approximation.