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.

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

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

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

flag name="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, as opposed to the minimum-bias process above, where all pT scales can be probed. The pT_min cut cannot be put too low, or else unreasonably large jet cross sections will be obtained.

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

flag name="HardQCD:gg2gg" default="off"
Scatterings g g -> g g. Code 111.

flag name="HardQCD:gg2qqbar" default="off"
Scatterings g g -> q qbar. Code 112.

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

flag name="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 Codes 114, 115 and 116.

flag name="HardQCD:qqbar2qqbarNew" default="off"
Scatterings q qbar -> q' qbar', where the outgoing flavours are different from the incoming ones. Code 117.

flag name="HardQCD:qqbar2gg" default="off"
Scatterings q qbar -> g g. Code 118.

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.

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

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

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

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