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, and three-jet topologies as a special subset. Kindly note
that there is a considerable amount of overlap between the soft and
hard process classes, so that you are likely to double-count
if you include both in a run.
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 minimum-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 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:nonDiffractive
(default = off)
The inelastic nondiffrative part of the total cross section, i.e.
what would often be called the "minimum-bias component".
The formalism is based on an
eikonalized description of all the hard QCD processes, so
includes them in combination with low-pT events.
Code 101.
Since the current description is handled by the multiparton-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 multiparton-interactions machinery.
Note: this event class is almost equivalent to the
minimum-bias component of the total cross section. "Minimum-bias"
usually refers to the experimental selection procedure, however,
while "(inelastic) non-diffractive" better relates to the way events
are generated in the program code. (Although also what separates
diffractive from nondiffractive physics can be a matter of definition,
especially once colour reconnection is to be modelled.)
flag SoftQCD:elastic
(default = off)
Elastic scattering A B → A B.
Code 102. It is possible to include
Coulomb corrections, but by default this is off.
flag SoftQCD:singleDiffractive
(default = off)
Single diffractive scattering A B → X B and
A B → A X. See page on
Diffraction for details. Codes 103 and 104.
flag SoftQCD:doubleDiffractive
(default = off)
Double diffractive scattering A B → X_1 X_2.
See page on Diffraction
for details. Code 105.
flag SoftQCD:centralDiffractive
(default = off)
Central diffractive scattering A B → A X B
(a.k.a. double-Pomeron exchange, DPE). See pages on
Total Cross Sections
and on Diffraction for details.
In particular note the SigmaTotal:zeroAXB flag,
which is on in most tunes, meaning no central diffraction, and
that therefore would need to be reset to off after the selection
of a tune (even the default one) to get central diffraction.
Code 106.
flag SoftQCD:inelastic
(default = off)
All of the above processes, except for elastic. Codes 101,
103, 104, 105 and 106.
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.
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 above.
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 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 double-counting
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 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.
Hard QCD 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.