Single-spin physics: experimental trends and their origin

Abstract

The review of experimental trends in the physics of single-spin processes and their origin in the framework of an effective color field model is presented. A global analysis of polarization data for a large set of different reactions in h-h, h-A, A-A and lepton-A collisions is performed. In particular, the single-spin asymmetry, the polarization of hyperons and antihyperons are analyzed. The analysis revealed many common trends in the behavior of the experimental data, depending on the kinematic variables, the quark structure of hadrons and the mass of the colliding ions. The origin of large single-spin effects can be attributed to the influence of force such as the Stern-Gerlach-like one in a chromomagnetic field and to the Thomas precession in chromoelectric component of the Effective Color Field (ECF). Precession of the spin of the quarks in a chromomagnetic field is a major cause of the oscillation of the polarization as a function of kinematic variables. Focusing of quarks in the ECF leads to the resonant energy dependence of the single-spin observables. The predictions of large single-spin effects for the different reactions are presented, in particular, the expected oscillations of SSA and polarization, as well as change their sign at RHIC energies due to the growth of ECF with increasing reaction energy or the atomic weight of the ions. The Quark Counting Rules determine the sign and magnitude of the ECF.