In order to increase the rate of useful physics events in charged particle colliders it is necessary to improve their luminosity, i.e. to increase the intensity of colliding bunches and to focus them on a very small spot size at the collision point. However, in collision, charged particle bunches experience a strong mutual nonlinear electromagnetic interaction. In the standard collision schemes the beam-beam interaction prevents reaching the desired bunch intensities and/or the required small transverse beam sizes, thus limiting the luminosity of a collider.
The novel collision scheme proposed at Frascati makes it possible to focus high intensity bunches on a very tiny collision spot and, at the same time, allows overcoming the problem of the electromagnetic interaction by suppressing nonlinear resonances excited in beam-beam collisions.
One of the distinct features of the new scheme is the use of nonlinear electromagnetic lenses, the sextupole magnets. The strength of such lenses, and respectively the focal position along the beam orbit (“waist”) depends on a particle transverse deviation from the orbit. Namely, because of such unusual particle’s waist “crabbed” positions with respect to the beam motion direction the scheme is called “Crab Waist Collision Scheme”.
This innovative concept of nonlinear focusing of colliding bunches has been successfully tested at DAFNE in operational conditions providing luminosity for two different experimental detectors, SIDDHARTA and KLOE-2. Considering high potential efficiency of the scheme for increasing collision luminosity and its relative simplicity for implementation several new collider projects have been proposed and are under development at present. These are the SuperKEKB B-factory already under commissioning in Japan, the SuperC-Tau factory proposed in Novosibirsk and entered in the short list of Russian mega-science projects, the new 100-km electron-positron Future Circular Collider (FCC-ee) under design study at CERN, the Higgs Factory CEPC in China and some others.