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.