Process Properties
Here is collected some possibilities to affect the generation of
all internally implemented processes in one go, or some coherent
subset thereof. Phase space cuts appear on a special page.
Incoming partons
InFlux
is base class for the combination of allowed incoming
partons in a given process, and keeps track of parton densities and
common weight factors (such as charge) required to give the process
cross section.
There is one useful degree of freedom in this class:
mode name="InFlux:nQuark" default="5" min="0" max="5"
Number of allowed incoming quark flavours in the beams; a change
to 4 would thus exclude b and bbar as incoming
partons, etc.
Generic cross sections
SigmaProcess
is base class for all hard processes
implemented in PYTHIA 8. For 2 -> 1 processes it
should give sigmaHat(sHat), for 2 -> 2 ones
d(sigmaHat(sHat, tHat))/d(tHat).
The size of QCD cross sections is mainly determined by
parameter name="SigmaProcess:alphaSvalue" default="0.1265"
min="0.06" max="0.25"
The alpha_strong value at scale M_Z^2.
The actual value is then regulated by the running to the scale
Q^2, at which alpha_strong is evaluated
mode name="SigmaProcess:alphaSorder" default="1" min="0" max="2"
Order at which alpha_strong runs,
option value="0": zeroth order, i.e. alpha_strong is kept
fixed.
option value="1": first order, which is the normal value.
option value="2": second order. Since other parts of the code do
not go to second order there is no strong reason to use this option,
but there is also nothing wrong with it.
QED radiation is currently regulated by
StandardModel:alphaEMfix
.
Special cross sections
Here settings that affect some special group of processes, but not all.
mode name="SigmaProcess:nQuark" default="3" min="0" max="5"
Number of allowed outgoing new quark flavours in
q qbar -> q' qbar' and g g-> q qbar processes,
where quarks are treated as massless in the matrix-element expressions
(but correctly in the phase space). It is thus assumed that c cbar
and b bbar are added separately with masses taken into account.
A change to 4 would also include c cbar in the massless
approximation, etc.
mode name="SigmaProcess:nQuarkInLoop" default="5" min="3" max="6"
Number of quark flavours included in the box graphs resposible for
g g -> g gamma and g g-> gamma gamma processes.
Owing to the complexity if the massive expressions, quarks are treated
as massless. The default value should be applicable in the range of
transverse momenta above the b mass but below the t one.