QCD Processes

This section is subdivided into soft and hard QCD processes, with open charm and bottom production set aside as a special part of the latter.

Soft QCD processes

As a rule, the processes in this class should not be mixed with the simulation of other processes. All by themselves, they are intended to represent the total cross section of hadron collisions, with the exception of the "rare processes" that one wishes to study separately. In particular, jet physics at all scales occurs as part of the minimum-bias description.

We here use the "minimum bias" expression as a shorthand for inelastic, nondiffractive events. Strictly speaking, "minimum bias" represents an experimental procedure of accepting "everything", with some non-universal cuts to exclude elastic and diffractive topologies. In practice, the experimental mimimum-bias sample may then contain some contamination of what is in PYTHIA classified as diffractive, especially (high-mass) double diffractive.

Some options to modify these cross sections, and especially to include Coulomb corrections to the elastic cross section, are found on the Total Cross Sections page.

flag  SoftQCD:all   (default = off)
Common switch for the group of all soft QCD processes, as listed separately in the following.

flag  SoftQCD:minBias   (default = off)
Minimum-bias events, based on an eikonalized description of all the hard QCD processes, so includes them in combinationation with low-pT events. Code 101.
Since the current description is handled by the multiple-interactions machinery as part of the parton-level processing, no hard process at all is defined at the process-level part of the event generation. Fortunately, in this case a special codeSub() method provides information on the first, i.e. hardest, subprocess selected by the multiple-interactions machinery.

flag  SoftQCD:elastic   (default = off)
Elastic scattering A B -> A B. Code 102.

flag  SoftQCD:singleDiffractive   (default = off)
Single diffractive scattering A B -> X B and A B -> A X. Codes 103 and 104.

flag  SoftQCD:doubleDiffractive   (default = off)
Double diffractive scattering A B -> X_1 X_2. Code 105.

Hard QCD processes

This group contains the processes for QCD jet production above some minimum pT threshold. The pT_min cut cannot be put too low, or else unreasonably large jet cross sections will be obtained. An eikonalized description, intended to be valid at all pT, is included as part of the multiple-interactions framework, e.g. in SoftQCD:minBias above.

flag  HardQCD:all   (default = off)
Common switch for the group of all hard QCD processes, as listed separately in the following.

flag  HardQCD:gg2gg   (default = off)
Scatterings g g -> g g. Code 111.

flag  HardQCD:gg2qqbar   (default = off)
Scatterings g g -> q qbar, where q by default is a light quark (u, d, s) (see below). Code 112.

flag  HardQCD:qg2qg   (default = off)
Scatterings q g -> q g and qbar g -> qbar g. Code 113.

flag  HardQCD:qq2qq   (default = off)
Scatterings q q' -> q q', q qbar' -> q qbar', qbar qbar' -> qbar qbar', where q' and q may agree, but the outgoing flavours equals the incoming ones Code 114.

flag  HardQCD:qqbar2gg   (default = off)
Scatterings q qbar -> g g. Code 115.

flag  HardQCD:qqbar2qqbarNew   (default = off)
Scatterings q qbar -> q' qbar', where q' by default is a light quark (u, d, s) (see below). Code 116.

mode  HardQCD:nQuarkNew   (default = 3; minimum = 0; maximum = 5)
Number of allowed outgoing new quark flavours in the above g g -> q qbar and q qbar -> 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, using the processes below. A change to 4 would also include c cbar in the massless approximation, etc. In order to avoid doublecounting the processes below should then not be used simultaneously.

Hard QCD processes: heavy-flavour subset

These processes form a natural part of the above class, but can also be generated separately. Formally the heavy-quark mass makes these matrix elements finite in the pT -> 0 limit, but at high energies one may still question the validity of the expressions at low pT values, like for the other hard-QCD processes. Also as above, an eikonalized description, intended to be valid at all pT, is included as part of the multiple-interactions framework.
Note that the processes below only represent the "tip of the iceberg" of charm and bottom production at high energies, where flavour excitation and shower branchings provide major additional sources. All these sources come together in the descriptions offered by SoftQCD:minBias and HardQCD:all.

flag  HardQCD:gg2ccbar   (default = off)
Scatterings g g -> c cbar. Code 121.

flag  HardQCD:qqbar2ccbar   (default = off)
Scatterings q qbar -> c cbar. Code 122.

flag  HardQCD:gg2bbbar   (default = off)
Scatterings g g -> b bbar. Code 123.

flag  HardQCD:qqbar2bbbar   (default = off)
Scatterings q qbar -> b bbar. Code 124.