<chapter name="QCD Processes"> 
 
  <h2>Hard QCD Processes</h2> 
 
This group contains the processes for QCD jet production above 
some minimum <ei>pT</ei> threshold, with 
open charm and bottom production set aside as a special part, 
and three-jet topologies as a special subset. 
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>, see the manual section 
on <aidx href="QCDSoftProcesses">Soft QCD Processes</aidx>. 
<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 2&rarr;2 processes, 
as listed separately in the following. 
</flag> 
 
 
<h3>Light Quarks and Gluons</h3> 
 
<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>Heavy Flavours</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>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> 
 
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