Transverse single-spin asymmetries (SSAs) were first explored in the mid-1970s at Argonne National Lab and FermiLab. Both of these measurements led to strikingly large effects that were unexplainable in the naive parton model. The origin of the SSA remains a unsolved problem over 40 years.Recently it is was found that the twist-3 framework in perturbative QCD calculation can give a reasonable description of the large SSA. In this talk, I will introduce the reason why the twist-3 framework has a capability to solve the problem and discuss recent theoretical developments in the study of spin asymmeties.                                        

Host: Prof. Xiangdong Ji