Single spin asymmetry leading to a new development in hadron physics

The E704 experiment conducted at the Fermi National Accelerator Laboratory found a large single spin asymmetry (SSA), which has lead to a great extention of the framework for high energy processes based on Quantum Chromodynamics (QCD). The SSA reflects the chiral symmetry breaking in the strong interaction. Clarification of its origin requires formulating the intrinsic transverse momentum of quarks and the quark-gluon correlation in the hadrons, which has become a hot and challenging subject in QCD. Experimentally, RHIC at the Brookhaven National Laboratory is measuring SSA at higher energy, and the measurement of SSA in lepton-nucleon scattering is also ongoing. This article is devoted to survey the recent development in the understanding of SSA. (author)