First observation of beamstrahlung.

Collisions of electron and positron bunches at the interaction point of the SLC (the linear collider at SLAC) h ave led to the first detected emission of beam-strahlung. This radiation, caused by the collective electromagnetic fields of one beam deflecting particles of the other, is a potential tool for optimizing collisions in linear colliders. With the advent of the linear collider1 as a tool for the study of high energy elementary particle physics, there has developed a strong interest in the physics of the beams in such machines. Of particular interest is the interaction point (IP), where the two beams must be brought to superimposed foci, with transverse sizes in the micron range or smaller. New methods are needed for the measurement and monitoring of these beams in collision. We describe here the first observation of beamstrahlung-an electromagnetic radiation from the collision of the beams. The phenomenon promises to be a valuable operating tool for linear colliders and very high energy storage rings.2 There is a considerable body of theoretical work on beamstrahlung in the literature, covering various energy regimes and beam parameters: and the topic continues to develop at a lively pace. It has not been possible to observe the radiation, however, until the SLC at SLAC began to collide high energy electron and positron beams with exceptionally intense focal spots. For the data reported here, typical beam energies were 46 GeV (Lorentz factor y = 9 x lo*), with bunches of about lOlo electrons and 6 x 10' positrons. At collision, the bunches were approximately Gaussian along all three axes, with RMS length about 750 microns, and transverse RMS sizes typically below 5 microns. The magnetic fields around one of these dense bunches can approach 10 T. Consequently each particle trajectory is deflected (equally by the magnetic and electric fields), and emits synchrotron radiation. It is this radiation which is termed beamstrahlung. Until conditions are such that its energy is comparable with the energy of the beam, it may be treated classically. The charge density distributions of each beam have Gaussian lengths X, and, in the simplified case of round cross sections, Gaussian radius 0. N is the bunch 2 3. The proton must have a distance of closest approach to the interaction region of greater than 0.6 mm in the x-y plane. 4. At their x-y vertex, the two tracks must have a z difference of less …