HOWTO use hdgeant4 as a beam Monte Carlo generator

Internal beam generator in hdgeant

In HOWTO to generate electromagnetic background the use of the BEAM card in control.in to activate the built-in coherent photon beam generator was described. One use case for this internal beam generator is to model beam-related scattering background that overlays the signals from hadronic reactions produced by the beam interacting in the GlueX target. Now that we have large samples of random and PS triggers to provide a more accurate representation of these backgrounds, that functionality is no longer widely used for physics simulations. However, there are other use cases for the built-in coherent beam generator that remain relevant, including the following.

1. simulation of the PS acceptance
2. simulation of the beam transverse intensity and polarization profile
3. simulation of the TPOL trigger, acceptance, and analyzing power
4. simulation of Compton and Bethe Heitler reactions in the GlueX target
5. any other process whose acceptance might be sensitive to the transverse beam position and momentum profile