Prompt photon production

MSEL = 10
ISUB =

14	$\mathrm{q}_i \overline{\mathrm{q}}_i \to \mathrm{g}\gamma$
18	$\mathrm{f}_i \overline{\mathrm{f}}_i \to \gamma \gamma$
29	$\mathrm{q}_i \mathrm{g}\to \mathrm{q}_i \gamma$
114	$\mathrm{g}\mathrm{g}\to \gamma \gamma$
115	$\mathrm{g}\mathrm{g}\to \mathrm{g}\gamma$

In hadron colliders, processes ISUB = 14 and 29 give the main source of single-$\gamma$ production, with ISUB = 115 giving an additional contribution which, in some kinematics regions, may become important. For $\gamma$-pair production, the process ISUB = 18 is often overshadowed in importance by ISUB = 114.

Another source of photons is bremsstrahlung off incoming or outgoing quarks. This has to be treated on an equal footing with QCD parton showering. For time-like parton-shower evolution, i.e. in the final-state showering and in the side branches of the initial-state showering, photon emission may be switched on or off with MSTJ(41). Photon radiation off the space-like incoming quark or lepton legs is similarly regulated by MSTP(61).

Warning: the cross sections for the box graphs 114 and 115 become very complicated, numerically unstable and slow when the full quark mass dependence is included. For quark masses much below the $\hat{s}$ scale, the simplified massless expressions are therefore used -- a fairly accurate approximation. However, there is another set of subtle numerical cancellations between different terms in the massive matrix elements in the region of small-angle scattering. The associated problems have not been sorted out yet. There are therefore two possible solutions. One is to use the massless formulae throughout. The program then becomes faster and numerically stable, but does not give, for example, the characteristic dip (due to destructive interference) at top threshold. This is the current default procedure, with five flavours assumed, but this number can be changed in MSTP(38). The other possibility is to impose cuts on the scattering angle of the hard process, see CKIN(27) and CKIN(28), since the numerically unstable regions are when $\vert\cos\hat{\theta}\vert$ is close to unity. It is then also necessary to change MSTP(38) to 0.