3 <title>QCD Processes</title>
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11 This section is subdivided into soft and hard QCD processes, with
12 open charm and bottom production set aside as a special part of the
13 latter, and three-jet topologies as a special subset. Kindly note
14 that there is a considerable amount of overlap between the soft and
15 hard process classes, so that you are likely to doublecount
16 if you include both in a run.
18 <h3>Soft QCD processes</h3>
20 As a rule, the processes in this class should not be mixed with
21 the simulation of other processes. All by themselves, they are
22 intended to represent the total cross section of hadron collisions,
23 with the exception of the "rare processes" that one wishes to study
24 separately. In particular, jet physics at all scales occurs as part
25 of the minimum-bias description.
28 We here use the "minimum bias" expression as a shorthand for
29 inelastic, nondiffractive events. Strictly speaking, "minimum bias"
30 represents an experimental procedure of accepting "everything", with
31 some non-universal cuts to exclude elastic and diffractive topologies.
32 In practice, the experimental mimimum-bias sample may then contain
33 some contamination of what is in PYTHIA classified as diffractive,
34 especially (high-mass) double diffractive.
37 Some options to modify these cross sections are found on the
38 <a href="TotalCrossSections.html" target="page">Total Cross Sections</a> page.
40 <p/><code>flag </code><strong> SoftQCD:all </strong>
41 (<code>default = <strong>off</strong></code>)<br/>
42 Common switch for the group of all soft QCD processes,
43 as listed separately in the following.
46 <p/><code>flag </code><strong> SoftQCD:minBias </strong>
47 (<code>default = <strong>off</strong></code>)<br/>
48 Minimum-bias events, based on an <a href="MultipleInteractions.html" target="page">
49 eikonalized description</a> of all the hard QCD processes, so
50 includes them in combinationation with low-<i>pT</i> events.
52 Since the current description is handled by the multiple-interactions
53 machinery as part of the parton-level processing, no hard process at
54 all is defined at the process-level part of the event generation.
55 Fortunately, in this case a special
56 <code><a href="EventInformation.html" target="page">codeSub()</a></code>
57 method provides information on the first, i.e. hardest, subprocess
58 selected by the multiple-interactions machinery.
62 <p/><code>flag </code><strong> SoftQCD:elastic </strong>
63 (<code>default = <strong>off</strong></code>)<br/>
64 Elastic scattering <i>A B -> A B</i>.
65 Code 102. It is possible to include <a href="TotalCrossSections.html" target="page">
66 Coulomb corrections</a>, but by default this is off.
69 <p/><code>flag </code><strong> SoftQCD:singleDiffractive </strong>
70 (<code>default = <strong>off</strong></code>)<br/>
71 Single diffractive scattering <i>A B -> X B</i> and
72 <i>A B -> A X</i>. See page on <a href="Diffraction.html" target="page">
73 Diffraction</a> for details. Codes 103 and 104.
76 <p/><code>flag </code><strong> SoftQCD:doubleDiffractive </strong>
77 (<code>default = <strong>off</strong></code>)<br/>
78 Double diffractive scattering <i>A B -> X_1 X_2</i>.
79 See page on <a href="Diffraction.html" target="page">Diffraction</a>
80 for details. Code 105.
83 <h3>Hard QCD processes</h3>
85 This group contains the processes for QCD jet production above
86 some minimum <i>pT</i> threshold. The <i>pT_min</i> cut cannot be put
87 too low, or else unreasonably large jet cross sections will be obtained.
88 This is because the divergent perturbative QCD cross section is used
89 in this process group, without any regularization modifications.
90 An eikonalized description, intended to be valid at all <i>pT</i>,
91 is instead included as part of the multiple-interactions framework,
92 specifically in <code>SoftQCD:minBias</code> above.
93 <br/><b>Warning 1</b>: you <b>must</b> remember to set the
94 <code>PhaseSpace:pTHatMin</code> value if you use any of these
95 processes; there is no sensible default.
96 <br/><b>Warning 2</b>: you <b>must not</b> mix processes from the
97 <code>SoftQCD</code> and <code>HardQCD</code> process groups, since
98 this is likely to lead to doublecounting.
100 <p/><code>flag </code><strong> HardQCD:all </strong>
101 (<code>default = <strong>off</strong></code>)<br/>
102 Common switch for the group of all hard QCD processes,
103 as listed separately in the following.
106 <p/><code>flag </code><strong> HardQCD:gg2gg </strong>
107 (<code>default = <strong>off</strong></code>)<br/>
108 Scatterings <i>g g -> g g</i>.
112 <p/><code>flag </code><strong> HardQCD:gg2qqbar </strong>
113 (<code>default = <strong>off</strong></code>)<br/>
114 Scatterings <i>g g -> q qbar</i>, where <i>q</i> by default
115 is a light quark (<i>u, d, s</i>) (see below).
119 <p/><code>flag </code><strong> HardQCD:qg2qg </strong>
120 (<code>default = <strong>off</strong></code>)<br/>
121 Scatterings <i>q g -> q g</i> and <i>qbar g -> qbar g</i>.
125 <p/><code>flag </code><strong> HardQCD:qq2qq </strong>
126 (<code>default = <strong>off</strong></code>)<br/>
127 Scatterings <i>q q' -> q q'</i>, <i>q qbar' -> q qbar'</i>,
128 <i>qbar qbar' -> qbar qbar'</i>, where <i>q'</i> and <i>q</i>
129 may agree, but the outgoing flavours equals the incoming ones
133 <p/><code>flag </code><strong> HardQCD:qqbar2gg </strong>
134 (<code>default = <strong>off</strong></code>)<br/>
135 Scatterings <i>q qbar -> g g</i>.
139 <p/><code>flag </code><strong> HardQCD:qqbar2qqbarNew </strong>
140 (<code>default = <strong>off</strong></code>)<br/>
141 Scatterings <i>q qbar -> q' qbar'</i>, where <i>q'</i>
142 by default is a light quark (<i>u, d, s</i>) (see below).
146 <p/><code>mode </code><strong> HardQCD:nQuarkNew </strong>
147 (<code>default = <strong>3</strong></code>; <code>minimum = 0</code>; <code>maximum = 5</code>)<br/>
148 Number of allowed outgoing new quark flavours in the above
149 <i>g g -> q qbar</i> and <i>q qbar -> q' qbar'</i> processes,
150 where quarks are treated as massless in the matrix-element expressions
151 (but correctly in the phase space). It is thus assumed that <i>c cbar</i>
152 and <i>b bbar</i> are added separately with masses taken into account,
153 using the processes below. A change to 4 would also include <i>c cbar</i>
154 in the massless approximation, etc. In order to avoid doublecounting
155 the processes below should then not be used simultaneously.
158 <h3>Hard QCD processes: heavy-flavour subset</h3>
160 These processes form a natural part of the above class, but can
161 also be generated separately. Formally the heavy-quark mass makes
162 these matrix elements finite in the <i>pT -> 0</i> limit, but at
163 high energies one may still question the validity of the expressions
164 at low <i>pT</i> values, like for the other hard-QCD processes.
165 Also as above, an eikonalized description, intended to be valid at all
166 <i>pT</i>, is included as part of the multiple-interactions framework.
167 <br/>Note that the processes below only represent the "tip of the iceberg"
168 of charm and bottom production at high energies, where flavour excitation
169 and shower branchings provide major additional sources. All these sources
170 come together in the descriptions offered by <code>SoftQCD:minBias</code>
171 and <code>HardQCD:all</code>.
173 <p/><code>flag </code><strong> HardQCD:gg2ccbar </strong>
174 (<code>default = <strong>off</strong></code>)<br/>
175 Scatterings <i>g g -> c cbar</i>.
179 <p/><code>flag </code><strong> HardQCD:qqbar2ccbar </strong>
180 (<code>default = <strong>off</strong></code>)<br/>
181 Scatterings <i>q qbar -> c cbar</i>.
185 <p/><code>flag </code><strong> HardQCD:gg2bbbar </strong>
186 (<code>default = <strong>off</strong></code>)<br/>
187 Scatterings <i>g g -> b bbar</i>.
191 <p/><code>flag </code><strong> HardQCD:qqbar2bbbar </strong>
192 (<code>default = <strong>off</strong></code>)<br/>
193 Scatterings <i>q qbar -> b bbar</i>.
197 <h3>Hard QCD three-parton processes</h3>
199 Three-parton final states are generated by showers off two-parton
200 processes. Topologies then cannot be specified beforehand, beyond
201 what is provided by the two-parton hard process. For some checks
202 it may be convenient to have access to the dedicated three-parton
203 final states, which is what this set of processes allows.
204 Cross sections have been taken from [<a href="Bibliography.html" target="page">Ber81</a>].
205 <br/>Note that the processes in this section are <it>not</it>
206 affected by the <code>HardQCD:all</code> switch. In fact, it would
207 be doublecounting to include both the <code>HardQCD:all</code> and
208 the <code>HardQCD:3parton</code> processes in a run or study.
209 <br/><b>Warning:</b> this section is still incomplete, e.g. the
210 selection of colour flow is very simple, and so it should only
211 be used with caution.
213 <p/><code>flag </code><strong> HardQCD:3parton </strong>
214 (<code>default = <strong>off</strong></code>)<br/>
215 Common switch for the group of all hard QCD processes with three
216 partons in the final state, as listed separately in the following.
219 <p/><code>flag </code><strong> HardQCD:gg2ggg </strong>
220 (<code>default = <strong>off</strong></code>)<br/>
221 Scatterings <i>g g -> g g g</i>.
225 <p/><code>flag </code><strong> HardQCD:qqbar2ggg </strong>
226 (<code>default = <strong>off</strong></code>)<br/>
227 Scatterings <i>q qbar -> g g g</i>.
231 <p/><code>flag </code><strong> HardQCD:qg2qgg </strong>
232 (<code>default = <strong>off</strong></code>)<br/>
233 Scatterings <i>q g -> q g g</i> and <i>qbar g -> qbar g g</i>.
237 <p/><code>flag </code><strong> HardQCD:qq2qqgDiff </strong>
238 (<code>default = <strong>off</strong></code>)<br/>
239 Scatterings <i>q q' -> q q' g</i>, <i>q qbar' -> q qbar' g</i>,
240 and <i>qbar qbar' -> qbar qbar' g</i>.
244 <p/><code>flag </code><strong> HardQCD:qq2qqgSame </strong>
245 (<code>default = <strong>off</strong></code>)<br/>
246 Scatterings <i>q q -> q q g</i> and
247 <i>qbar qbar -> qbar qbar g</i>
248 (<i>q qbar -> q qbar g</i> scatterings are considered separately
249 below, see <code>HardQCD:qqbar2qqbargSame</code>).
253 <p/><code>flag </code><strong> HardQCD:qqbar2qqbargDiff </strong>
254 (<code>default = <strong>off</strong></code>)<br/>
255 Scatterings <i>q qbar -> q' qbar' g</i>, where <i>q'</i>
256 by default is a light quark (<i>u, d, s</i>)
257 (see <code>HardQCD:nQuarkNew</code> above).
261 <p/><code>flag </code><strong> HardQCD:qqbar2qqbargSame </strong>
262 (<code>default = <strong>off</strong></code>)<br/>
263 Scatterings <i>q qbar -> q qbar g</i>.
267 <p/><code>flag </code><strong> HardQCD:gg2qqbarg </strong>
268 (<code>default = <strong>off</strong></code>)<br/>
269 Scatterings <i>g g -> q qbar g</i>, where <i>q</i> by
270 default is a light quark (<i>u, d, s</i>)
271 (see <code>HardQCD:nQuarkNew</code> above).
275 <p/><code>flag </code><strong> HardQCD:qg2qqqbarDiff </strong>
276 (<code>default = <strong>off</strong></code>)<br/>
277 Scatterings <i>q g -> q q' qbar'</i> and
278 <i>qbar g -> qbar qbar' q'</i>, where <i>q'</i>
279 by default is a light quark (<i>u, d, s</i>)
280 (see <code>HardQCD:nQuarkNew</code> above).
284 <p/><code>flag </code><strong> HardQCD:qg2qqqbarSame </strong>
285 (<code>default = <strong>off</strong></code>)<br/>
286 Scatterings <i>q g -> q q qbar</i> and
287 <i>qbar g -> qbar qbar q</i>.
295 <!-- Copyright (C) 2010 Torbjorn Sjostrand -->