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 in SoftQCD: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.