Appendix D: Event Generation.

Let's start with some comments lifted from the subroutine generate:

! Generated quantities: (phase_space NOT YET IMPLEMENTED).
!
! phase_space: Generate electron E,yptar,xptar and hadron yptar,xptar??
! doing_hyd_elast: fixed Em, generate electron angles
! doing_deuterium: fixed Em, generate electron fully and proton angles, calc Ep
! doing_eep, A>2: generate electron and hadron energy and angles (calc Em/Pm).
! doing_pion: fixed Em, generate electron energy/angles, p_fermi,
!       hadron angles
! doing_kaon: as doing_pion.
!
! The above is summarized in the following table:
!
!                    ELECTRON                  HADRON
!               ------------------      ------------------
!               E       yptar   xptar   E       yptar   xptar   p_fermi
!
!H(e,e'p)               X       X
!D(e,e'p)       X       X       X               X       X
!A(e,e'p)       X       X       X       X       X       X
!----------------------------------------------------------------------
!H(e,e'pi/K)    X       X       X               X       X
!A(e,e'pi/K)    X       X       X               X       X       X
!----------------------------------------------------------------------
!phase_space    X       X       X       ?       X       X
!
! So our procedure is the following:
! 1) Always generate electron yptar and xptar
! 2) generate hadron yptar and xptar for all cases except H(e,e'p), D(e,e'p)
! 3) generate p_fermi for D(e,e'p), and D/He pion production
! 4) generate electron E for all but hydrogen elastic and deuterium.
! 5) Generate hadron E for A(e,e'p)
! 6) Set missing energy for cases where it is hardwired
!
! After we generate xptar/yptar/energy, we calculate physics angles (theta/phi),
!  momenta, unit vectors, etc... here and/or in complete_ev.
!
! Note that there are also jacobians associated with some and/or all of
! the above.
! 1: We generate uniformly in xptar/yptar, not theta/phi.  We define the
! phase space volume (genvol contribution) as the product of the xptar/yptar
! range, and have a jacobian for each event taking into account the mapping
! between the solid angle on the unit sphere, and the dxptar/dyptar volume
! (the jacobian is 1/cos**3(dtheta), where dtheta is the angle between the
! event and the central spectrometer vector
! 2: For the D(e,e'p), we take Em as fixed in order to calculate the proton
! energy.  There is a jacobian ( |dEp'/dEm| ).  It comes from integrating
! over the energy conservation delta function: delta(E_D - E_p - E_n - Em).

OK, now that that's all clear, the following sections will go over the assumptions and calculations for each process. The following describe the steps necessary to generate the electron and hadron four-vectors at the vertex. Using these values, the physics scattering angles are calculated (see geometry.ps in the simc/documents directory), along with $\nu, \vec{q}, Q^2, E_m, T_{rec}$, and any other desired physics quantities for the specified physics process.