Summary

This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Physics. Please check back later for the full article.

The conditions for the Sokolov-Ternov effect to occur are approximately satisfied by electrons (or positrons) circulating on the design orbit of a planar storage ring. Indeed, self-polarization was first observed in the electron/positron colliders Anneau de Collisions d’Orsay (ACO) and VEPP-2. Beam polarization offers an additional tool for understanding the physics events. The possibility of having polarized electron/positron beams for free is therefore appealing. However, the Sokolov-Ternov polarization time constant, proportional to 1/γ⁵ and to the third power of the bending radius, restricts the region of interest for self-polarization. For the about 100 km Future Circular Collider (FCC) under study at CERN, the polarization constant is about 10 days at 45 GeV beam energy. At high energy the randomization of the particle trajectory due to photon emission in a storage ring with finite alignment precision of the magnets introduces spin diffusion and limits the attainable polarization.

In addition, in a collider the force exerted by the counter-rotating particles impact the beam polarization. This force increases with beam intensity and experiments are reluctant to pass up luminosity for polarization. To this day the electron(positron)/proton collider HERA has been the only high energy collider where electron (and positron) self-polarization was an integral part of the physics program.