Incoherent Pair Production

    In addition to the interactions between a particle and a macro-scopic field such as beamstrahlung and laser-Compton, there are particle-particle interactions between e, e and that have to be simulated. The present version of CAIN include the following processes:

Breit-Wheeler

Bethe-Heitler

Landau-Lifshitz

Bremsstrahlung

These QED processes have relatively large event rates even at high energies.

The treatment of these incoherent processes in CAIN is slightly different from other (coherent) processes since the event rates of the former are usually much smaller than the latter.

• The parent macro particles are not eliminated nor changed after interaction. The polarization change described in Sec.5.2.2 is not taken into account.
• More than one event may be generated in one time step because the change of parent partciles after the first event need not be taken into account.

In contrast to ABEL, CAIN does not assume the particles are ultra-relativistic. Therefore, unless the beam-beam field created by the pair particles becomes significant, their trajectories are correctly calculated. (Note, however, that only the beam field due to the on-coming beam is taken into account in the present version. The beam field in the same beam may have significant effects on low energy particles.)

Among the four QED processes above, the Breit-Wheeler process is treated as a fundamental process. Others are reduced to the former (or to the Compton process, in the case of Bremsstrahlung) by using the virtual photon approximation. As for the polarization, only the circular polarization of initial photons in the direct (non-virtual) Breit-Wheeler process is taken into account.

 
• Breit-Wheeler Process
• Virtual (almost real) photon approximation