One of the experiments performed in particle physics consist in colliding two counter-streaming beams, one composed of electrons and the other of positrons. One relativistic beam, composed by charged particles of the same kind, is stable and the coulomb repulsive force is counteracted as 1/² by the confining magnetic field generated by the charges motion. However when two beams collide the oncoming beam field acts on the bunch as a thin lens focusing the particles on the axis of motion. This focusing produces what is called pinching of the beam: the beam density increases in the focusing spot. After pinching the beam defocuses.


During the interaction, the beam may suffer none, one or multiple pinching according to the strength of the oncoming beam field and to the particles energy. The presence of these pinching spots alters the beam density profile; at the end of the interaction, the density profile results disrupted.


The number of pinching spots at the interaction point distinguishes three regimes: low disruption regime (no pinching), transition regime (one pinching) and confinement regime (multiple pinching). The presence of these pinching spots increases the luminosity respect to a collision without disruption. What looks as a pro brings also cons: while bending the particles loose energy through beamstrahlung radiation and are deflected at angles that are not suitable for the collider, hence disruption is generally avoided.

The movie illustrates the evolution of the two beams density profile during the interaction in a configuration that leads to the confinement regime. Many pinching spots appear and the original density shape is strongly modified.

To reference this movie, use Del Gaudio, F. et al., 42^nd EPS Conference on Plasma Physics P5.224 (2015).