<chapter name="QCD Processes"> 
 
<h2>Soft QCD Processes</h2> 
 
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. Note, therefore, 
that there is a considerable amount of overlap between the soft and 
hard QCD process classes, so that you are likely to double-count 
if you include both in a run. 
 
<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 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> 
 
<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:singleDiffractiveXB" default="off"> 
Single diffractive scattering <ei>A B &rarr; X B</ei>. 
See pages on <aloc href="TotalCrossSections">Total Cross Sections</aloc> 
and <aloc href="Diffraction">Diffraction</aloc> for details. Code 103. 
</flag> 
 
<flag name="SoftQCD:singleDiffractiveAX" default="off"> 
Single diffractive scattering <ei>A B &rarr; A X</ei>. 
See pages on <aloc href="TotalCrossSections">Total Cross Sections</aloc> 
and <aloc href="Diffraction">Diffraction</aloc> for details. Code 104. 
</flag> 
 
<flag name="SoftQCD:singleDiffractive" default="off"> 
Both of the above single diffractive processes. Codes 103 and 104. 
</flag> 
 
<flag name="SoftQCD:doubleDiffractive" default="off"> 
Double diffractive scattering <ei>A B &rarr; X_1 X_2</ei>. 
See pages on <aloc href="TotalCrossSections">Total Cross Sections</aloc> 
and <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 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> 
 
<flag name="SoftQCD:inelastic" default="off"> 
All of the above processes, except for elastic. Codes 101, 
103, 104, 105 and 106. 
</flag> 
 
<p/> 
<note>Note</note>: The repertoire of 
<aloc href="QCDLowEnergyProcesses">Low-energy QCD processes</aloc> 
largely overlaps with the one here, but in a simplified form 
without any perturbative activity at all. It is thus mainly suited 
for low energies, where also some special processes occur, such as 
annihilation or scattering through a resonance. The 
<aloc href="BeamParameters">variable-energy beams</aloc> 
framework allow a smooth transition between the two, assuming you have 
enabled matching sets of processes for the two scenarios. A normal 
choice would be to enable all the processes, but one could e.g. decide 
to use the nonDiffractive subprocess only. 
 
</chapter> 
 
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