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<title>QCD Processes</title>
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<h2>QCD Processes</h2>
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.
<h3>Soft QCD processes</h3>
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.
<p/>
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.
<p/>
Some options to modify these cross sections are found on the
<a href="TotalCrossSections.html" target="page">Total Cross Sections</a> page.
<p/><code>flag </code><strong> SoftQCD:all </strong>
(<code>default = <strong>off</strong></code>)<br/>
Common switch for the group of all soft QCD processes,
as listed separately in the following.
<p/><code>flag </code><strong> SoftQCD:nonDiffractive </strong>
(<code>default = <strong>off</strong></code>)<br/>
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 <a href="MultipartonInteractions.html" target="page">
eikonalized description</a> of all the hard QCD processes, so
includes them in combination with low-<i>pT</i> events.
Code 101.<br/>
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
<code><a href="EventInformation.html" target="page">codeSub()</a></code>
method provides information on the first, i.e. hardest, subprocess
selected by the multiparton-interactions machinery.
<br/><b>Note</b>: this flag was precviously named
<code>SoftQCD:minBias</code>, since it is almost equivalent to
the minimum-bias component of the total cross section. It has invited
some confusion, however, since "minimum-bias" usually refers to the
experimental procedure, 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.) For backwards compatibility the
<code>SoftQCD:minBias</code> name can be used as an alternative to
<code>SoftQCD:nonDiffractive</code> for some time yet.
<p/><code>flag </code><strong> SoftQCD:elastic </strong>
(<code>default = <strong>off</strong></code>)<br/>
Elastic scattering <i>A B -> A B</i>.
Code 102. It is possible to include <a href="TotalCrossSections.html" target="page">
Coulomb corrections</a>, but by default this is off.
<p/><code>flag </code><strong> SoftQCD:singleDiffractive </strong>
(<code>default = <strong>off</strong></code>)<br/>
Single diffractive scattering <i>A B -> X B</i> and
<i>A B -> A X</i>. See page on <a href="Diffraction.html" target="page">
Diffraction</a> for details. Codes 103 and 104.
<p/><code>flag </code><strong> SoftQCD:doubleDiffractive </strong>
(<code>default = <strong>off</strong></code>)<br/>
Double diffractive scattering <i>A B -> X_1 X_2</i>.
See page on <a href="Diffraction.html" target="page">Diffraction</a>
for details. Code 105.
<p/><code>flag </code><strong> SoftQCD:centralDiffractive </strong>
(<code>default = <strong>off</strong></code>)<br/>
Central diffractive scattering <i>A B -> A X B</i>
(a.k.a. double-Pomeron exchange, DPE). See pages on
<a href="TotalCrossSections.html" target="page">Total Cross Sections</a>
and on <a href="Diffraction.html" target="page">Diffraction</a> for details.
In particular note the <code>SigmaTotal:zeroAXB</code> flag,
which is off in most tunes, and that therefore would need to
be reset to on after the selection of a tune (even the default one).
Code 106.
<p/><code>flag </code><strong> SoftQCD:inelastic </strong>
(<code>default = <strong>off</strong></code>)<br/>
All of the above processes, except for elastic. Codes 101,
103, 104, 105 and 106.
<h3>Hard QCD processes</h3>
This group contains the processes for QCD jet production above
some minimum <i>pT</i> threshold. The <i>pT_min</i> 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 <i>pT</i>,
is instead included as part of the multiparton-interactions framework,
specifically in <code>SoftQCD:nonDiffractive</code> above.
<br/><b>Warning 1</b>: you <b>must</b> remember to set the
<code>PhaseSpace:pTHatMin</code> value if you use any of these
processes; there is no sensible default.
<br/><b>Warning 2</b>: you <b>must not</b> mix processes from the
<code>SoftQCD</code> and <code>HardQCD</code> process groups, since
this is likely to lead to double-counting.
<p/><code>flag </code><strong> HardQCD:all </strong>
(<code>default = <strong>off</strong></code>)<br/>
Common switch for the group of all hard QCD processes,
as listed separately in the following.
<p/><code>flag </code><strong> HardQCD:gg2gg </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scatterings <i>g g -> g g</i>.
Code 111.
<p/><code>flag </code><strong> HardQCD:gg2qqbar </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scatterings <i>g g -> q qbar</i>, where <i>q</i> by default
is a light quark (<i>u, d, s</i>) (see below).
Code 112.
<p/><code>flag </code><strong> HardQCD:qg2qg </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scatterings <i>q g -> q g</i> and <i>qbar g -> qbar g</i>.
Code 113.
<p/><code>flag </code><strong> HardQCD:qq2qq </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scatterings <i>q q' -> q q'</i>, <i>q qbar' -> q qbar'</i>,
<i>qbar qbar' -> qbar qbar'</i>, where <i>q'</i> and <i>q</i>
may agree, but the outgoing flavours equals the incoming ones
Code 114.
<p/><code>flag </code><strong> HardQCD:qqbar2gg </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scatterings <i>q qbar -> g g</i>.
Code 115.
<p/><code>flag </code><strong> HardQCD:qqbar2qqbarNew </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scatterings <i>q qbar -> q' qbar'</i>, where <i>q'</i>
by default is a light quark (<i>u, d, s</i>) (see below).
Code 116.
<p/><code>mode </code><strong> HardQCD:nQuarkNew </strong>
(<code>default = <strong>3</strong></code>; <code>minimum = 0</code>; <code>maximum = 5</code>)<br/>
Number of allowed outgoing new quark flavours in the above
<i>g g -> q qbar</i> and <i>q qbar -> q' qbar'</i> processes,
where quarks are treated as massless in the matrix-element expressions
(but correctly in the phase space). It is thus assumed that <i>c cbar</i>
and <i>b bbar</i> are added separately with masses taken into account,
using the processes below. A change to 4 would also include <i>c cbar</i>
in the massless approximation, etc. In order to avoid double-counting
the processes below should then not be used simultaneously.
<h3>Hard QCD processes: heavy-flavour subset</h3>
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 <i>pT -> 0</i> limit, but at
high energies one may still question the validity of the expressions
at low <i>pT</i> values, like for the other hard-QCD processes.
Also as above, an eikonalized description, intended to be valid at all
<i>pT</i>, is included as part of the multiparton-interactions framework.
<br/>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
<code>SoftQCD:nonDiffractive</code> and <code>HardQCD:all</code>.
<p/><code>flag </code><strong> HardQCD:gg2ccbar </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scatterings <i>g g -> c cbar</i>.
Code 121.
<p/><code>flag </code><strong> HardQCD:qqbar2ccbar </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scatterings <i>q qbar -> c cbar</i>.
Code 122.
<p/><code>flag </code><strong> HardQCD:hardccbar </strong>
(<code>default = <strong>off</strong></code>)<br/>
Sum of the previous two event types.
Codes 121 and 122.
<p/><code>flag </code><strong> HardQCD:gg2bbbar </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scatterings <i>g g -> b bbar</i>.
Code 123.
<p/><code>flag </code><strong> HardQCD:qqbar2bbbar </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scatterings <i>q qbar -> b bbar</i>.
Code 124.
<p/><code>flag </code><strong> HardQCD:hardbbbar </strong>
(<code>default = <strong>off</strong></code>)<br/>
Sum of the previous two event types.
Codes 123 and 124.
<h3>Hard QCD three-parton processes</h3>
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 [<a href="Bibliography.html" target="page">Ber81</a>].
<br/>Note that the processes in this section are <it>not</it>
affected by the <code>HardQCD:all</code> switch. In fact, it would
be double-counting to include both the <code>HardQCD:all</code> and
the <code>HardQCD:3parton</code> processes in a run or study.
<br/><b>Warning:</b> this section is still incomplete, e.g. the
selection of colour flow is very simple, and so it should only
be used with caution.
<p/><code>flag </code><strong> HardQCD:3parton </strong>
(<code>default = <strong>off</strong></code>)<br/>
Common switch for the group of all hard QCD processes with three
partons in the final state, as listed separately in the following.
<p/><code>flag </code><strong> HardQCD:gg2ggg </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scatterings <i>g g -> g g g</i>.
Code 131.
<p/><code>flag </code><strong> HardQCD:qqbar2ggg </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scatterings <i>q qbar -> g g g</i>.
Code 132.
<p/><code>flag </code><strong> HardQCD:qg2qgg </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scatterings <i>q g -> q g g</i> and <i>qbar g -> qbar g g</i>.
Code 133.
<p/><code>flag </code><strong> HardQCD:qq2qqgDiff </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scatterings <i>q q' -> q q' g</i>, <i>q qbar' -> q qbar' g</i>,
and <i>qbar qbar' -> qbar qbar' g</i>.
Code 134.
<p/><code>flag </code><strong> HardQCD:qq2qqgSame </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scatterings <i>q q -> q q g</i> and
<i>qbar qbar -> qbar qbar g</i>
(<i>q qbar -> q qbar g</i> scatterings are considered separately
below, see <code>HardQCD:qqbar2qqbargSame</code>).
Code 135.
<p/><code>flag </code><strong> HardQCD:qqbar2qqbargDiff </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scatterings <i>q qbar -> q' qbar' g</i>, where <i>q'</i>
by default is a light quark (<i>u, d, s</i>)
(see <code>HardQCD:nQuarkNew</code> above).
Code 136.
<p/><code>flag </code><strong> HardQCD:qqbar2qqbargSame </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scatterings <i>q qbar -> q qbar g</i>.
Code 137.
<p/><code>flag </code><strong> HardQCD:gg2qqbarg </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scatterings <i>g g -> q qbar g</i>, where <i>q</i> by
default is a light quark (<i>u, d, s</i>)
(see <code>HardQCD:nQuarkNew</code> above).
Code 138.
<p/><code>flag </code><strong> HardQCD:qg2qqqbarDiff </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scatterings <i>q g -> q q' qbar'</i> and
<i>qbar g -> qbar qbar' q'</i>, where <i>q'</i>
by default is a light quark (<i>u, d, s</i>)
(see <code>HardQCD:nQuarkNew</code> above).
Code 139.
<p/><code>flag </code><strong> HardQCD:qg2qqqbarSame </strong>
(<code>default = <strong>off</strong></code>)<br/>
Scatterings <i>q g -> q q qbar</i> and
<i>qbar g -> qbar qbar q</i>.
Code 140.
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