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="MultipartonInteractions.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 multiparton-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 multiparton-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 <p/><code>flag </code><strong> SoftQCD:centralDiffractive </strong>
84 (<code>default = <strong>off</strong></code>)<br/>
85 Central diffractive scattering <i>A B -> A X B</i>
86 (a.k.a. double-Pomeron exchange, DPE). See pages on
87 <a href="TotalCrossSections.html" target="page">Total Cross Sections</a>
88 and on <a href="Diffraction.html" target="page">Diffraction</a> for details.
89 In particular note the <code>SigmaTotal:zeroAXB</code> flag,
90 which is off in most tunes, and that therefore would need to
91 be reset to on after the selection of a tune (even the default one).
95 <h3>Hard QCD processes</h3>
97 This group contains the processes for QCD jet production above
98 some minimum <i>pT</i> threshold. The <i>pT_min</i> cut cannot be put
99 too low, or else unreasonably large jet cross sections will be obtained.
100 This is because the divergent perturbative QCD cross section is used
101 in this process group, without any regularization modifications.
102 An eikonalized description, intended to be valid at all <i>pT</i>,
103 is instead included as part of the multiparton-interactions framework,
104 specifically in <code>SoftQCD:minBias</code> above.
105 <br/><b>Warning 1</b>: you <b>must</b> remember to set the
106 <code>PhaseSpace:pTHatMin</code> value if you use any of these
107 processes; there is no sensible default.
108 <br/><b>Warning 2</b>: you <b>must not</b> mix processes from the
109 <code>SoftQCD</code> and <code>HardQCD</code> process groups, since
110 this is likely to lead to doublecounting.
112 <p/><code>flag </code><strong> HardQCD:all </strong>
113 (<code>default = <strong>off</strong></code>)<br/>
114 Common switch for the group of all hard QCD processes,
115 as listed separately in the following.
118 <p/><code>flag </code><strong> HardQCD:gg2gg </strong>
119 (<code>default = <strong>off</strong></code>)<br/>
120 Scatterings <i>g g -> g g</i>.
124 <p/><code>flag </code><strong> HardQCD:gg2qqbar </strong>
125 (<code>default = <strong>off</strong></code>)<br/>
126 Scatterings <i>g g -> q qbar</i>, where <i>q</i> by default
127 is a light quark (<i>u, d, s</i>) (see below).
131 <p/><code>flag </code><strong> HardQCD:qg2qg </strong>
132 (<code>default = <strong>off</strong></code>)<br/>
133 Scatterings <i>q g -> q g</i> and <i>qbar g -> qbar g</i>.
137 <p/><code>flag </code><strong> HardQCD:qq2qq </strong>
138 (<code>default = <strong>off</strong></code>)<br/>
139 Scatterings <i>q q' -> q q'</i>, <i>q qbar' -> q qbar'</i>,
140 <i>qbar qbar' -> qbar qbar'</i>, where <i>q'</i> and <i>q</i>
141 may agree, but the outgoing flavours equals the incoming ones
145 <p/><code>flag </code><strong> HardQCD:qqbar2gg </strong>
146 (<code>default = <strong>off</strong></code>)<br/>
147 Scatterings <i>q qbar -> g g</i>.
151 <p/><code>flag </code><strong> HardQCD:qqbar2qqbarNew </strong>
152 (<code>default = <strong>off</strong></code>)<br/>
153 Scatterings <i>q qbar -> q' qbar'</i>, where <i>q'</i>
154 by default is a light quark (<i>u, d, s</i>) (see below).
158 <p/><code>mode </code><strong> HardQCD:nQuarkNew </strong>
159 (<code>default = <strong>3</strong></code>; <code>minimum = 0</code>; <code>maximum = 5</code>)<br/>
160 Number of allowed outgoing new quark flavours in the above
161 <i>g g -> q qbar</i> and <i>q qbar -> q' qbar'</i> processes,
162 where quarks are treated as massless in the matrix-element expressions
163 (but correctly in the phase space). It is thus assumed that <i>c cbar</i>
164 and <i>b bbar</i> are added separately with masses taken into account,
165 using the processes below. A change to 4 would also include <i>c cbar</i>
166 in the massless approximation, etc. In order to avoid doublecounting
167 the processes below should then not be used simultaneously.
170 <h3>Hard QCD processes: heavy-flavour subset</h3>
172 These processes form a natural part of the above class, but can
173 also be generated separately. Formally the heavy-quark mass makes
174 these matrix elements finite in the <i>pT -> 0</i> limit, but at
175 high energies one may still question the validity of the expressions
176 at low <i>pT</i> values, like for the other hard-QCD processes.
177 Also as above, an eikonalized description, intended to be valid at all
178 <i>pT</i>, is included as part of the multiparton-interactions framework.
179 <br/>Note that the processes below only represent the "tip of the iceberg"
180 of charm and bottom production at high energies, where flavour excitation
181 and shower branchings provide major additional sources. All these sources
182 come together in the descriptions offered by <code>SoftQCD:minBias</code>
183 and <code>HardQCD:all</code>.
185 <p/><code>flag </code><strong> HardQCD:gg2ccbar </strong>
186 (<code>default = <strong>off</strong></code>)<br/>
187 Scatterings <i>g g -> c cbar</i>.
191 <p/><code>flag </code><strong> HardQCD:qqbar2ccbar </strong>
192 (<code>default = <strong>off</strong></code>)<br/>
193 Scatterings <i>q qbar -> c cbar</i>.
197 <p/><code>flag </code><strong> HardQCD:gg2bbbar </strong>
198 (<code>default = <strong>off</strong></code>)<br/>
199 Scatterings <i>g g -> b bbar</i>.
203 <p/><code>flag </code><strong> HardQCD:qqbar2bbbar </strong>
204 (<code>default = <strong>off</strong></code>)<br/>
205 Scatterings <i>q qbar -> b bbar</i>.
209 <h3>Hard QCD three-parton processes</h3>
211 Three-parton final states are generated by showers off two-parton
212 processes. Topologies then cannot be specified beforehand, beyond
213 what is provided by the two-parton hard process. For some checks
214 it may be convenient to have access to the dedicated three-parton
215 final states, which is what this set of processes allows.
216 Cross sections have been taken from [<a href="Bibliography.html" target="page">Ber81</a>].
217 <br/>Note that the processes in this section are <it>not</it>
218 affected by the <code>HardQCD:all</code> switch. In fact, it would
219 be doublecounting to include both the <code>HardQCD:all</code> and
220 the <code>HardQCD:3parton</code> processes in a run or study.
221 <br/><b>Warning:</b> this section is still incomplete, e.g. the
222 selection of colour flow is very simple, and so it should only
223 be used with caution.
225 <p/><code>flag </code><strong> HardQCD:3parton </strong>
226 (<code>default = <strong>off</strong></code>)<br/>
227 Common switch for the group of all hard QCD processes with three
228 partons in the final state, as listed separately in the following.
231 <p/><code>flag </code><strong> HardQCD:gg2ggg </strong>
232 (<code>default = <strong>off</strong></code>)<br/>
233 Scatterings <i>g g -> g g g</i>.
237 <p/><code>flag </code><strong> HardQCD:qqbar2ggg </strong>
238 (<code>default = <strong>off</strong></code>)<br/>
239 Scatterings <i>q qbar -> g g g</i>.
243 <p/><code>flag </code><strong> HardQCD:qg2qgg </strong>
244 (<code>default = <strong>off</strong></code>)<br/>
245 Scatterings <i>q g -> q g g</i> and <i>qbar g -> qbar g g</i>.
249 <p/><code>flag </code><strong> HardQCD:qq2qqgDiff </strong>
250 (<code>default = <strong>off</strong></code>)<br/>
251 Scatterings <i>q q' -> q q' g</i>, <i>q qbar' -> q qbar' g</i>,
252 and <i>qbar qbar' -> qbar qbar' g</i>.
256 <p/><code>flag </code><strong> HardQCD:qq2qqgSame </strong>
257 (<code>default = <strong>off</strong></code>)<br/>
258 Scatterings <i>q q -> q q g</i> and
259 <i>qbar qbar -> qbar qbar g</i>
260 (<i>q qbar -> q qbar g</i> scatterings are considered separately
261 below, see <code>HardQCD:qqbar2qqbargSame</code>).
265 <p/><code>flag </code><strong> HardQCD:qqbar2qqbargDiff </strong>
266 (<code>default = <strong>off</strong></code>)<br/>
267 Scatterings <i>q qbar -> q' qbar' g</i>, where <i>q'</i>
268 by default is a light quark (<i>u, d, s</i>)
269 (see <code>HardQCD:nQuarkNew</code> above).
273 <p/><code>flag </code><strong> HardQCD:qqbar2qqbargSame </strong>
274 (<code>default = <strong>off</strong></code>)<br/>
275 Scatterings <i>q qbar -> q qbar g</i>.
279 <p/><code>flag </code><strong> HardQCD:gg2qqbarg </strong>
280 (<code>default = <strong>off</strong></code>)<br/>
281 Scatterings <i>g g -> q qbar g</i>, where <i>q</i> by
282 default is a light quark (<i>u, d, s</i>)
283 (see <code>HardQCD:nQuarkNew</code> above).
287 <p/><code>flag </code><strong> HardQCD:qg2qqqbarDiff </strong>
288 (<code>default = <strong>off</strong></code>)<br/>
289 Scatterings <i>q g -> q q' qbar'</i> and
290 <i>qbar g -> qbar qbar' q'</i>, where <i>q'</i>
291 by default is a light quark (<i>u, d, s</i>)
292 (see <code>HardQCD:nQuarkNew</code> above).
296 <p/><code>flag </code><strong> HardQCD:qg2qqqbarSame </strong>
297 (<code>default = <strong>off</strong></code>)<br/>
298 Scatterings <i>q g -> q q qbar</i> and
299 <i>qbar g -> qbar qbar q</i>.
307 <!-- Copyright (C) 2012 Torbjorn Sjostrand -->