Collisions of electron and positron bunches at the interaction point of the linear collider at SLAC have led to the first detected emission of beamstrahlung. 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. PACS numbers: 41.70.+t, 29.15.Dt, 29.90.+r With the advent of the linear collider 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 veryhigh-energy storage rings. 2 There is a considerable body of theoretical work on beamstrahlung in the literature, covering various energy regimes and beam parameters, 3 and the topic continues to develop at a lively pace. It has not been possible to observe the radiation, however, until the Stanford Linear Collider (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 7=9xl0 4 ), with bunches of about 10 10 electrons and 6xl0 9 positrons. At collision, the bunches were approximately Gaussian along all three axes, with rms length about 750 jum, and transverse rms sizes typically below 5 /im.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, as in this Letter, be treated classically.The charge-density distributions of each beam have Gaussian lengths A., and, in the simplified case of round cross sections, Gaussian radius o. N is the bunch population, and subscripts 1 and 2 refer, respectively, to the beam whose radiation is being calculated and to the target beam. The impact parameter between beam centers is d. The energy radiated is jj 8 N ] NJr e 3 mc 2 y 2 3V/T
