Single spin asymmetries in high energy reactions and nonperturbative QCD effects

Abstract

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.

Spin physics through QCD instantons

“…Throughout, the spin dependent asymmetries will be evaluated at the partonic level. Their conversion to the hadronic level will follow the qualitative arguments presented in [24][25][26].…”

“…The divergence in (25) stems from the exchange of soft gluons in the box diagram. In [31] it was regulated using a constituent gluon mass m g .…”

“…Perturbative QCD does not support the T -odd contributions, which are usually parametrized in the initial state (Sivers effect) [20,21] or the final state (Collins effect) [22,23]. Non-perturbative QCD with instantons allow for large spin asymmetries as discussed by Kochelev and others [24][25][26][27]. In [24] a particularly large Pauli form factor was noted, with an important contribution to the Single Spin Asymmetry (SSA) in polarized proton on proton scattering.…”

Anomalous quark chromomagnetic moment and single-spin asymmetries

“…Furthermore, instantons lead to the anomalous quark-gluon chromomagnetic vertex with a large quark helicity-flip [14,10]. Therefore, they can give the important contribution to SSA [14,15,16,17,18,10,19].…”

Double spin asymmetries through QCD instantons

“…Perturbative QCD does not support the T-odd contributions, which are usually parametrized in the initial state (Sivers effect) [19,20] or the final state (Collins effect) [21,22]. Non-perturbative QCD with instantons allow for large spin asymmetries as discussed by Kochelev and others [23][24][25][26]. In [23] a particularly large Pauli form factor was noted, with an important contribution to the Single Spin Asymmetry (SSA) in polarized proton on proton scattering.…”