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  InFlux:nQuark   (default = 5; minimum = 0; maximum = 5)
Number of allowed incoming quark flavours in the beams; a change to 4 would thus exclude b and bbar as incoming partons, etc.

There is also the possibility to obtain some documentation.

flag  InFlux:showChannels   (default = off)
At initialization show which incoming flavours and flavour combinations have beeen set up, process by process, along with initial weight (e.g. charges or CKM factors) assigned to each of them.

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

parm  SigmaProcess:alphaSvalue   (default = 0.1265; minimum = 0.06; maximum = 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  SigmaProcess:alphaSorder   (default = 1; minimum = 0; maximum = 2)
Order at which alpha_strong runs,
option 0 : zeroth order, i.e. alpha_strong is kept fixed.
option 1 : first order, which is the normal value.
option 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 interactions are regulated by the alpha_electromagnetic value at the pT^2 scale of an interaction.

mode  SigmaProcess:alphaEMorder   (default = 1; minimum = -1; maximum = 1)
The running of alpha_em used in hard processes.
option 1 : first-order running, constrained to agree with StandardModel:alphaEMmZ at the Z^0 mass.
option 0 : zeroth order, i.e. alpha_em is kept fixed at its value at vanishing momentum transfer.
option -1 : zeroth order, i.e. alpha_em is kept fixed, but at StandardModel:alphaEMmZ, i.e. its value at the Z^0 mass.

Special cross sections

Here settings that affect some special group of processes, but not all.

mode  SigmaProcess:nQuark   (default = 3; minimum = 0; maximum = 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  SigmaProcess:nQuarkInLoop   (default = 5; minimum = 3; maximum = 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.

mode  SigmaProcess:gmZmode   (default = 0; minimum = 0; maximum = 2)
Choice of full gamma^*/Z^0 structure or not in relevant processes.
option 0 : full gamma^*/Z^0 structure, with interference included.
option 1 : only pure gamma^* contribution.
option 2 : only pure Z^0 contribution.