1
<chapter name="QCD Processes">
5
This section is subdivided into soft and hard QCD processes, with
6
open charm and bottom production set aside as a special part of the
7
latter, and three-jet topologies as a special subset. Kindly note
8
that there is a considerable amount of overlap between the soft and
9
hard process classes, so that you are likely to doublecount
10
if you include both in a run.
12
<h3>Soft QCD processes</h3>
14
As a rule, the processes in this class should not be mixed with
15
the simulation of other processes. All by themselves, they are
16
intended to represent the total cross section of hadron collisions,
17
with the exception of the "rare processes" that one wishes to study
18
separately. In particular, jet physics at all scales occurs as part
19
of the minimum-bias description.
22
We here use the "minimum bias" expression as a shorthand for
23
inelastic, nondiffractive events. Strictly speaking, "minimum bias"
24
represents an experimental procedure of accepting "everything", with
25
some non-universal cuts to exclude elastic and diffractive topologies.
26
In practice, the experimental mimimum-bias sample may then contain
27
some contamination of what is in PYTHIA classified as diffractive,
28
especially (high-mass) double diffractive.
31
Some options to modify these cross sections are found on the
32
<aloc href="TotalCrossSections">Total Cross Sections</aloc> page.
34
<flag name="SoftQCD:all" default="off">
35
Common switch for the group of all soft QCD processes,
36
as listed separately in the following.
39
<flag name="SoftQCD:minBias" default="off">
40
Minimum-bias events, based on an <aloc href="MultipartonInteractions">
41
eikonalized description</aloc> of all the hard QCD processes, so
42
includes them in combinationation with low-<ei>pT</ei> events.
44
Since the current description is handled by the multiparton-interactions
45
machinery as part of the parton-level processing, no hard process at
46
all is defined at the process-level part of the event generation.
47
Fortunately, in this case a special
48
<code><aloc href="EventInformation">codeSub()</aloc></code>
49
method provides information on the first, i.e. hardest, subprocess
50
selected by the multiparton-interactions machinery.
54
<flag name="SoftQCD:elastic" default="off">
55
Elastic scattering <ei>A B -> A B</ei>.
56
Code 102. It is possible to include <aloc href="TotalCrossSections">
57
Coulomb corrections</aloc>, but by default this is off.
60
<flag name="SoftQCD:singleDiffractive" default="off">
61
Single diffractive scattering <ei>A B -> X B</ei> and
62
<ei>A B -> A X</ei>. See page on <aloc href="Diffraction">
63
Diffraction</aloc> for details. Codes 103 and 104.
66
<flag name="SoftQCD:doubleDiffractive" default="off">
67
Double diffractive scattering <ei>A B -> X_1 X_2</ei>.
68
See page on <aloc href="Diffraction">Diffraction</aloc>
69
for details. Code 105.
72
<flag name="SoftQCD:centralDiffractive" default="off">
73
Central diffractive scattering <ei>A B -> A X B</ei>
74
(a.k.a. double-Pomeron exchange, DPE).
75
See page on <aloc href="Diffraction">Diffraction</aloc>
76
for details. Code 106.
79
<h3>Hard QCD processes</h3>
81
This group contains the processes for QCD jet production above
82
some minimum <ei>pT</ei> threshold. The <ei>pT_min</ei> cut cannot be put
83
too low, or else unreasonably large jet cross sections will be obtained.
84
This is because the divergent perturbative QCD cross section is used
85
in this process group, without any regularization modifications.
86
An eikonalized description, intended to be valid at all <ei>pT</ei>,
87
is instead included as part of the multiparton-interactions framework,
88
specifically in <code>SoftQCD:minBias</code> above.
89
<note>Warning 1</note>: you <b>must</b> remember to set the
90
<code>PhaseSpace:pTHatMin</code> value if you use any of these
91
processes; there is no sensible default.
92
<note>Warning 2</note>: you <b>must not</b> mix processes from the
93
<code>SoftQCD</code> and <code>HardQCD</code> process groups, since
94
this is likely to lead to doublecounting.
96
<flag name="HardQCD:all" default="off">
97
Common switch for the group of all hard QCD processes,
98
as listed separately in the following.
101
<flag name="HardQCD:gg2gg" default="off">
102
Scatterings <ei>g g -> g g</ei>.
106
<flag name="HardQCD:gg2qqbar" default="off">
107
Scatterings <ei>g g -> q qbar</ei>, where <ei>q</ei> by default
108
is a light quark (<ei>u, d, s</ei>) (see below).
112
<flag name="HardQCD:qg2qg" default="off">
113
Scatterings <ei>q g -> q g</ei> and <ei>qbar g -> qbar g</ei>.
117
<flag name="HardQCD:qq2qq" default="off">
118
Scatterings <ei>q q' -> q q'</ei>, <ei>q qbar' -> q qbar'</ei>,
119
<ei>qbar qbar' -> qbar qbar'</ei>, where <ei>q'</ei> and <ei>q</ei>
120
may agree, but the outgoing flavours equals the incoming ones
124
<flag name="HardQCD:qqbar2gg" default="off">
125
Scatterings <ei>q qbar -> g g</ei>.
129
<flag name="HardQCD:qqbar2qqbarNew" default="off">
130
Scatterings <ei>q qbar -> q' qbar'</ei>, where <ei>q'</ei>
131
by default is a light quark (<ei>u, d, s</ei>) (see below).
135
<modeopen name="HardQCD:nQuarkNew" default="3" min="0" max="5">
136
Number of allowed outgoing new quark flavours in the above
137
<ei>g g -> q qbar</ei> and <ei>q qbar -> q' qbar'</ei> processes,
138
where quarks are treated as massless in the matrix-element expressions
139
(but correctly in the phase space). It is thus assumed that <ei>c cbar</ei>
140
and <ei>b bbar</ei> are added separately with masses taken into account,
141
using the processes below. A change to 4 would also include <ei>c cbar</ei>
142
in the massless approximation, etc. In order to avoid doublecounting
143
the processes below should then not be used simultaneously.
146
<h3>Hard QCD processes: heavy-flavour subset</h3>
148
These processes form a natural part of the above class, but can
149
also be generated separately. Formally the heavy-quark mass makes
150
these matrix elements finite in the <ei>pT -> 0</ei> limit, but at
151
high energies one may still question the validity of the expressions
152
at low <ei>pT</ei> values, like for the other hard-QCD processes.
153
Also as above, an eikonalized description, intended to be valid at all
154
<ei>pT</ei>, is included as part of the multiparton-interactions framework.
155
<br/>Note that the processes below only represent the "tip of the iceberg"
156
of charm and bottom production at high energies, where flavour excitation
157
and shower branchings provide major additional sources. All these sources
158
come together in the descriptions offered by <code>SoftQCD:minBias</code>
159
and <code>HardQCD:all</code>.
161
<flag name="HardQCD:gg2ccbar" default="off">
162
Scatterings <ei>g g -> c cbar</ei>.
166
<flag name="HardQCD:qqbar2ccbar" default="off">
167
Scatterings <ei>q qbar -> c cbar</ei>.
171
<flag name="HardQCD:gg2bbbar" default="off">
172
Scatterings <ei>g g -> b bbar</ei>.
176
<flag name="HardQCD:qqbar2bbbar" default="off">
177
Scatterings <ei>q qbar -> b bbar</ei>.
181
<h3>Hard QCD three-parton processes</h3>
183
Three-parton final states are generated by showers off two-parton
184
processes. Topologies then cannot be specified beforehand, beyond
185
what is provided by the two-parton hard process. For some checks
186
it may be convenient to have access to the dedicated three-parton
187
final states, which is what this set of processes allows.
188
Cross sections have been taken from <ref>Ber81</ref>.
189
<br/>Note that the processes in this section are <it>not</it>
190
affected by the <code>HardQCD:all</code> switch. In fact, it would
191
be doublecounting to include both the <code>HardQCD:all</code> and
192
the <code>HardQCD:3parton</code> processes in a run or study.
193
<br/><b>Warning:</b> this section is still incomplete, e.g. the
194
selection of colour flow is very simple, and so it should only
195
be used with caution.
197
<flag name="HardQCD:3parton" default="off">
198
Common switch for the group of all hard QCD processes with three
199
partons in the final state, as listed separately in the following.
202
<flag name="HardQCD:gg2ggg" default="off">
203
Scatterings <ei>g g -> g g g</ei>.
207
<flag name="HardQCD:qqbar2ggg" default="off">
208
Scatterings <ei>q qbar -> g g g</ei>.
212
<flag name="HardQCD:qg2qgg" default="off">
213
Scatterings <ei>q g -> q g g</ei> and <ei>qbar g -> qbar g g</ei>.
217
<flag name="HardQCD:qq2qqgDiff" default="off">
218
Scatterings <ei>q q' -> q q' g</ei>, <ei>q qbar' -> q qbar' g</ei>,
219
and <ei>qbar qbar' -> qbar qbar' g</ei>.
223
<flag name="HardQCD:qq2qqgSame" default="off">
224
Scatterings <ei>q q -> q q g</ei> and
225
<ei>qbar qbar -> qbar qbar g</ei>
226
(<ei>q qbar -> q qbar g</ei> scatterings are considered separately
227
below, see <code>HardQCD:qqbar2qqbargSame</code>).
231
<flag name="HardQCD:qqbar2qqbargDiff" default="off">
232
Scatterings <ei>q qbar -> q' qbar' g</ei>, where <ei>q'</ei>
233
by default is a light quark (<ei>u, d, s</ei>)
234
(see <code>HardQCD:nQuarkNew</code> above).
238
<flag name="HardQCD:qqbar2qqbargSame" default="off">
239
Scatterings <ei>q qbar -> q qbar g</ei>.
243
<flag name="HardQCD:gg2qqbarg" default="off">
244
Scatterings <ei>g g -> q qbar g</ei>, where <ei>q</ei> by
245
default is a light quark (<ei>u, d, s</ei>)
246
(see <code>HardQCD:nQuarkNew</code> above).
250
<flag name="HardQCD:qg2qqqbarDiff" default="off">
251
Scatterings <ei>q g -> q q' qbar'</ei> and
252
<ei>qbar g -> qbar qbar' q'</ei>, where <ei>q'</ei>
253
by default is a light quark (<ei>u, d, s</ei>)
254
(see <code>HardQCD:nQuarkNew</code> above).
258
<flag name="HardQCD:qg2qqqbarSame" default="off">
259
Scatterings <ei>q g -> q q qbar</ei> and
260
<ei>qbar g -> qbar qbar q</ei>.
267
<!-- Copyright (C) 2012 Torbjorn Sjostrand -->
added
removed
xmldoc/QCDProcesses.xml
