** Next:** Single production
** Up:** Electroweak Gauge Bosons
** Previous:** Electroweak Gauge Bosons
** Contents**

###

Prompt photon production

`MSEL` = 10

`ISUB` =

In hadron colliders, processes `ISUB` = 14 and 29 give the main source
of single- production, with `ISUB` = 115 giving an additional
contribution which, in some kinematics regions, may become important.
For -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 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
is close to unity. It is then
also necessary to change `MSTP(38)` to 0.

** Next:** Single production
** Up:** Electroweak Gauge Bosons
** Previous:** Electroweak Gauge Bosons
** Contents**
Stephen Mrenna
2007-10-30