Hard QCD Processes
 
  - Light Quarks and Gluons
- Heavy Flavours
- Three-parton processes
This group contains the processes for QCD jet production above 
some minimum pT threshold, with 
open charm and bottom production set aside as a special part, 
and three-jet topologies as a special subset. 
The pT_min cut cannot be put 
too low, or else unreasonably large jet cross sections will be obtained. 
This is because the divergent perturbative QCD cross section is used 
in this process group, without any regularization modifications. 
An eikonalized description, intended to be valid at all pT, 
is instead included as part of the multiparton-interactions framework, 
specifically inSoftQCD:nonDiffractive, see the manual section 
on Soft QCD Processes. 
Warning 1: you must remember to set the 
PhaseSpace:pTHatMin value if you use any of these 
processes; there is no sensible default. 
Warning 2: you must not mix processes from the 
SoftQCD and HardQCD process groups, since 
this is likely to lead to double-counting. 
 
 
flag   HardQCD:all   
 (default = off)
Common switch for the group of all hard QCD 2→2 processes, 
as listed separately in the following. 
   
 
 
 
Light Quarks and Gluons
 
 
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 double-counting 
the processes below should then not be used simultaneously. 
   
 
 
Heavy Flavours
 
 
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 multiparton-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:nonDiffractive 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:hardccbar   
 (default = off)
Sum of the previous two event types. 
Codes 121 and 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. 
   
 
flag   HardQCD:hardbbbar   
 (default = off)
Sum of the previous two event types. 
Codes 123 and 124. 
   
 
 
Three-parton processes
 
 
Three-parton final states are generated by showers off two-parton 
processes. Topologies then cannot be specified beforehand, beyond 
what is provided by the two-parton hard process. For some checks 
it may be convenient to have access to the dedicated three-parton 
final states, which is what this set of processes allows. 
Cross sections have been taken from [Ber81]. 
Note that the processes in this section are  not 
affected by the HardQCD:all switch. In fact, it would 
be double-counting to include both the HardQCD:all and 
the HardQCD:3parton processes in a run or study. 
Warning: this section is still incomplete, e.g. the 
selection of colour flow is very simple, and so it should only 
be used with caution. 
 
flag   HardQCD:3parton   
 (default = off)
Common switch for the group of all hard QCD processes with three 
partons in the final state, as listed separately in the following. 
   
 
flag   HardQCD:gg2ggg   
 (default = off)
Scatterings g g → g g g. 
Code 131. 
   
 
flag   HardQCD:qqbar2ggg   
 (default = off)
Scatterings q qbar → g g g. 
Code 132. 
   
 
flag   HardQCD:qg2qgg   
 (default = off)
Scatterings q g → q g g and qbar g → qbar g g. 
Code 133. 
   
 
flag   HardQCD:qq2qqgDiff   
 (default = off)
Scatterings q q' → q q' g, q qbar' → q qbar' g, 
and qbar qbar' → qbar qbar' g. 
Code 134. 
   
 
flag   HardQCD:qq2qqgSame   
 (default = off)
Scatterings q q → q q g and 
qbar qbar → qbar qbar g 
(q qbar → q qbar g scatterings are considered separately 
below, see HardQCD:qqbar2qqbargSame). 
Code 135. 
   
 
flag   HardQCD:qqbar2qqbargDiff   
 (default = off)
Scatterings q qbar → q' qbar' g, where q' 
by default is a light quark (u, d, s) 
(see HardQCD:nQuarkNew above). 
Code 136. 
   
 
flag   HardQCD:qqbar2qqbargSame   
 (default = off)
Scatterings q qbar → q qbar g. 
Code 137. 
   
 
flag   HardQCD:gg2qqbarg   
 (default = off)
Scatterings g g → q qbar g, where q by 
default is a light quark (u, d, s) 
(see HardQCD:nQuarkNew above). 
Code 138. 
   
 
flag   HardQCD:qg2qqqbarDiff   
 (default = off)
Scatterings q g → q q' qbar' and 
qbar g → qbar qbar' q', where q' 
by default is a light quark (u, d, s) 
(see HardQCD:nQuarkNew above). 
Code 139. 
   
 
flag   HardQCD:qg2qqqbarSame   
 (default = off)
Scatterings q g → q q qbar and 
qbar g → qbar qbar q. 
Code 140.