<chapter name="QCD Processes"> 
 
<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 
<aloc href="TotalCrossSections">Total Cross Sections</aloc> page. 
 
<flag name="SoftQCD:all" default="off"> 
Common switch for the group of all soft QCD processes, 
as listed separately in the following. 
</flag> 
 
<flag name="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 <aloc href="MultipartonInteractions"> 
eikonalized description</aloc> of all the hard QCD processes, so 
includes them in combination with low-<ei>pT</ei> 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><aloc href="EventInformation">codeSub()</aloc></code> 
method provides information on the first, i.e. hardest, subprocess 
selected by the multiparton-interactions machinery. 
<note>Note</note>: 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. 
</flag> 
 
<flag name="SoftQCD:elastic" default="off"> 
Elastic scattering <ei>A B &rarr; A B</ei>. 
Code 102. It is possible to include <aloc href="TotalCrossSections"> 
Coulomb corrections</aloc>, but by default this is off. 
</flag> 
 
<flag name="SoftQCD:singleDiffractive" default="off"> 
Single diffractive scattering <ei>A B &rarr; X B</ei> and 
<ei>A B &rarr; A X</ei>. See page on <aloc href="Diffraction"> 
Diffraction</aloc> for details. Codes 103 and 104. 
</flag> 
 
<flag name="SoftQCD:doubleDiffractive" default="off"> 
Double diffractive scattering <ei>A B &rarr; X_1 X_2</ei>. 
See page on <aloc href="Diffraction">Diffraction</aloc> 
for details. Code 105. 
</flag> 
 
<flag name="SoftQCD:centralDiffractive" default="off"> 
Central diffractive scattering <ei>A B &rarr; A X B</ei> 
(a.k.a. double-Pomeron exchange, DPE). See pages on 
<aloc href="TotalCrossSections">Total Cross Sections</aloc> 
and on <aloc href="Diffraction">Diffraction</aloc> 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. 
</flag> 
 
<flag name="SoftQCD:inelastic" default="off"> 
All of the above processes, except for elastic. Codes 101, 
103, 104, 105 and 106. 
</flag> 
 
<h3>Hard QCD processes</h3> 
 
This group contains the processes for QCD jet production above 
some minimum <ei>pT</ei> threshold. The <ei>pT_min</ei> 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 <ei>pT</ei>, 
is instead included as part of the multiparton-interactions framework, 
specifically in <code>SoftQCD:nonDiffractive</code> above. 
<note>Warning 1</note>: 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. 
<note>Warning 2</note>: 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. 
   
<flag name="HardQCD:all" default="off"> 
Common switch for the group of all hard QCD processes, 
as listed separately in the following. 
</flag> 
 
<flag name="HardQCD:gg2gg" default="off"> 
Scatterings <ei>g g &rarr; g g</ei>. 
Code 111. 
</flag> 
 
<flag name="HardQCD:gg2qqbar" default="off"> 
Scatterings <ei>g g &rarr; q qbar</ei>, where <ei>q</ei> by default 
is a light quark (<ei>u, d, s</ei>) (see below). 
Code 112. 
</flag> 
 
<flag name="HardQCD:qg2qg" default="off"> 
Scatterings <ei>q g &rarr; q g</ei> and <ei>qbar g &rarr; qbar g</ei>. 
Code 113. 
</flag> 
 
<flag name="HardQCD:qq2qq" default="off"> 
Scatterings <ei>q q' &rarr; q q'</ei>, <ei>q qbar' &rarr; q qbar'</ei>, 
<ei>qbar qbar' &rarr; qbar qbar'</ei>, where <ei>q'</ei> and <ei>q</ei> 
may agree, but the outgoing flavours equals the incoming ones 
Code 114. 
</flag> 
 
<flag name="HardQCD:qqbar2gg" default="off"> 
Scatterings <ei>q qbar &rarr; g g</ei>. 
Code 115. 
</flag> 
 
<flag name="HardQCD:qqbar2qqbarNew" default="off"> 
Scatterings <ei>q qbar &rarr; q' qbar'</ei>, where <ei>q'</ei> 
by default is a light quark (<ei>u, d, s</ei>) (see below). 
Code 116. 
</flag> 
 
<modeopen name="HardQCD:nQuarkNew" default="3" min="0" max="5"> 
Number of allowed outgoing new quark flavours in the above 
<ei>g g &rarr; q qbar</ei> and <ei>q qbar &rarr; q' qbar'</ei> processes, 
where quarks are treated as massless in the matrix-element expressions 
(but correctly in the phase space). It is thus assumed that <ei>c cbar</ei> 
and <ei>b bbar</ei> are added separately with masses taken into account, 
using the processes below. A change to 4 would also include <ei>c cbar</ei> 
in the massless approximation, etc. In order to avoid double-counting 
the processes below should then not be used simultaneously. 
</modeopen> 
 
<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 <ei>pT &rarr; 0</ei> limit, but at 
high energies one may still question the validity of the expressions 
at low <ei>pT</ei> values, like for the other hard-QCD processes. 
Also as above, an eikonalized description, intended to be valid at all 
<ei>pT</ei>, 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>. 
 
<flag name="HardQCD:gg2ccbar" default="off"> 
Scatterings <ei>g g &rarr; c cbar</ei>. 
Code 121. 
</flag> 
 
<flag name="HardQCD:qqbar2ccbar" default="off"> 
Scatterings <ei>q qbar &rarr; c cbar</ei>. 
Code 122. 
</flag> 
 
<flag name="HardQCD:hardccbar" default="off"> 
Sum of the previous two event types. 
Codes 121 and 122. 
</flag> 
 
<flag name="HardQCD:gg2bbbar" default="off"> 
Scatterings <ei>g g &rarr; b bbar</ei>. 
Code 123. 
</flag> 
 
<flag name="HardQCD:qqbar2bbbar" default="off"> 
Scatterings <ei>q qbar &rarr; b bbar</ei>. 
Code 124. 
</flag> 
 
<flag name="HardQCD:hardbbbar" default="off"> 
Sum of the previous two event types. 
Codes 123 and 124. 
</flag> 
 
<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 <ref>Ber81</ref>. 
<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. 
   
<flag name="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> 
 
<flag name="HardQCD:gg2ggg" default="off"> 
Scatterings <ei>g g &rarr; g g g</ei>. 
Code 131. 
</flag> 
 
<flag name="HardQCD:qqbar2ggg" default="off"> 
Scatterings <ei>q qbar &rarr; g g g</ei>. 
Code 132. 
</flag> 
 
<flag name="HardQCD:qg2qgg" default="off"> 
Scatterings <ei>q g &rarr; q g g</ei> and <ei>qbar g &rarr; qbar g g</ei>. 
Code 133. 
</flag> 
 
<flag name="HardQCD:qq2qqgDiff" default="off"> 
Scatterings <ei>q q' &rarr; q q' g</ei>, <ei>q qbar' &rarr; q qbar' g</ei>, 
and <ei>qbar qbar' &rarr; qbar qbar' g</ei>. 
Code 134. 
</flag> 
 
<flag name="HardQCD:qq2qqgSame" default="off"> 
Scatterings <ei>q q &rarr; q q g</ei> and 
<ei>qbar qbar &rarr; qbar qbar g</ei> 
(<ei>q qbar &rarr; q qbar g</ei> scatterings are considered separately 
below, see <code>HardQCD:qqbar2qqbargSame</code>). 
Code 135. 
</flag> 
 
<flag name="HardQCD:qqbar2qqbargDiff" default="off"> 
Scatterings <ei>q qbar &rarr; q' qbar' g</ei>, where <ei>q'</ei> 
by default is a light quark (<ei>u, d, s</ei>) 
(see <code>HardQCD:nQuarkNew</code> above). 
Code 136. 
</flag> 
 
<flag name="HardQCD:qqbar2qqbargSame" default="off"> 
Scatterings <ei>q qbar &rarr; q qbar g</ei>. 
Code 137. 
</flag> 
 
<flag name="HardQCD:gg2qqbarg" default="off"> 
Scatterings <ei>g g &rarr; q qbar g</ei>, where <ei>q</ei> by 
default is a light quark (<ei>u, d, s</ei>) 
(see <code>HardQCD:nQuarkNew</code> above). 
Code 138. 
</flag> 
 
<flag name="HardQCD:qg2qqqbarDiff" default="off"> 
Scatterings <ei>q g &rarr; q q' qbar'</ei> and 
<ei>qbar g &rarr; qbar qbar' q'</ei>, where <ei>q'</ei> 
by default is a light quark (<ei>u, d, s</ei>) 
(see <code>HardQCD:nQuarkNew</code> above). 
Code 139. 
</flag> 
 
<flag name="HardQCD:qg2qqqbarSame" default="off"> 
Scatterings <ei>q g &rarr; q q qbar</ei> and 
<ei>qbar g &rarr; qbar qbar q</ei>. 
Code 140. 
</flag> 
 
 
</chapter> 
 
<!-- Copyright (C) 2014 Torbjorn Sjostrand --> 
 
