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SLAC-R-371
Beamstrahlung and QED Backgrounds at Future Linear Colliders

Abstract

Future electron-positron colliders, with center-of-mass energies above 1/2 TeV, must be of the linear, single-pass type, since the energy loss to synchrotron radiation at a storage ring would be unacceptably high. The single-pass configuration requires extremely dense particle bunches, which will have very strong collective electromagnetic fields. As the bunches cross, the field of each disrupts the other, and the electrons and positrons radiate photons under this transverse acceleration. This radiation is called beamstrahlung. Beamstrahlung can take away a large fraction of the available collision energy at such machines, but it also makes it possible to study electron-photon and photon-photon interactions.

This dissertation is a detailed study of several aspects of beamstrahlung and related phenomena. The problem is formulated as the relativistic scattering of an electron from a strong but slowly varying potential. The solution is readily interpreted in terms of a classical electron trajectory, and differs from the solution of the corresponding classical problem mainly in the effect of quantum recoil due to the emission of hard photons. When the general solution is expanded for the case of an almost-uniform field, the leading term is identical to the well-known formula for quantum synchrotron radiation. The first non-leading term is negligible in all cases of interest where the expansion is valid.

In applying the standard synchrotron radiation formula to the beamstrahlung problem, the effects of radiation reaction on the emission of multiple photons can be significant for some machine designs. Another interesting feature is the helicity dependence of the radiation process, which is relevant to the case where the electron beam is polarized.

The inverse process of coherent electron-positron pair production by a beamstrahlung photon is a potentially serious background source at future colliders, since low-energy pairs can exit the bunch at a large angle. Pairs can also be produced incoherently by the collision of two photons, either real (from beamstrahlung) or virtual (emitted by a passing electron or positron). The rates, spectra, and angular distributions for both the coherent and incoherent processes are estimated here. At a 1/2 TeV machine the incoherent process will be more common, resulting in roughly l06 pairs per bunch crossing. One member of each pair is always pushed outward, at an angle determined by its energy, by the field of the oncoming bunch. In addition, a small number of pairs are initially produced with a comparable or larger angle.

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