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Welcome to "Single-Spin Asymmetries"
Single-Transverse Spin Asymmetries (SSA)
Single-transverse spin asymmetries (SSA) in strong interactions have a long history, starting from the 1970s and 1980s when surprisingly large single-transverse spin asymmetries were observed in and , where really none were expected. They have again attracted much interest in recent years from both experimental and theoretical sides. In particular, first measurements by the STAR, PHENIX, and BRAHMS collaborations at RHIC have now become available which again reveal large single transverse spin asymmetries for hadron production in polarized proton proton scattering. This extends the SSA observations from the fixed target energy range to the collider regime.
Meanwhile, experimental studies in Deep Inelastic Scattering by the HERMES collaboration at DESY, SMC at CERN, and CLAS at JLab also show a remarkably large SSA in semi-inclusive hadron production, , when the proton is transversely polarized. On the theoretical side, there are several approaches to understanding SSA within Quantum Chromodynamics (QCD). For example, to explain the large SSAs for hadron production in hadron collisions, a mechanism that takes into account the contribution from quark-gluon-quark correlations (twist-3) in the nucleon was proposed. On the other hand, possible origins of SSA in DIS and hadronic scattering were also found in leading-twist transverse momentum dependent parton distributions. Current theoretical efforts aim at a better conceptual understanding of these two types of mechanisms, and of their connections.
The spin program now well underway at RHIC provides unique opportunities to study single spin asymmetries in polarized proton proton scattering. We believe that this is a good time to bring together the theorists and experimentalists of this field to review and discuss the current theoretical status and the latest experimental results. The comparison of the results for SSAs from RHIC measurements with those from Deep Inelastic Scattering will be extremely helpful for understanding the underlying physics. The interactions between theorists and experimentalists should be important for identifying and discussing key observables at RHIC, and so to shape the further program with transverse polarization.
Last Modified: June 20, 2005