3 <title>QCD Processes</title>
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30 <h2>QCD Processes</h2>
32 This section is subdivided into soft and hard QCD processes, with
33 open charm and bottom production set aside as a special part of the
34 latter, and three-jet topologies as a special subset. Kindly note
35 that there is a considerable amount of overlap between the soft and
36 hard process classes, so that you are likely to doublecount
37 if you include both in a run.
39 <h3>Soft QCD processes</h3>
41 As a rule, the processes in this class should not be mixed with
42 the simulation of other processes. All by themselves, they are
43 intended to represent the total cross section of hadron collisions,
44 with the exception of the "rare processes" that one wishes to study
45 separately. In particular, jet physics at all scales occurs as part
46 of the minimum-bias description.
49 We here use the "minimum bias" expression as a shorthand for
50 inelastic, nondiffractive events. Strictly speaking, "minimum bias"
51 represents an experimental procedure of accepting "everything", with
52 some non-universal cuts to exclude elastic and diffractive topologies.
53 In practice, the experimental mimimum-bias sample may then contain
54 some contamination of what is in PYTHIA classified as diffractive,
55 especially (high-mass) double diffractive.
58 Some options to modify these cross sections are found on the
59 <?php $filepath = $_GET["filepath"];
60 echo "<a href='TotalCrossSections.php?filepath=".$filepath."' target='page'>";?>Total Cross Sections</a> page.
62 <br/><br/><strong>SoftQCD:all</strong> <input type="radio" name="1" value="on"><strong>On</strong>
63 <input type="radio" name="1" value="off" checked="checked"><strong>Off</strong>
64 (<code>default = <strong>off</strong></code>)<br/>
65 Common switch for the group of all soft QCD processes,
66 as listed separately in the following.
69 <br/><br/><strong>SoftQCD:minBias</strong> <input type="radio" name="2" value="on"><strong>On</strong>
70 <input type="radio" name="2" value="off" checked="checked"><strong>Off</strong>
71 (<code>default = <strong>off</strong></code>)<br/>
72 Minimum-bias events, based on an <?php $filepath = $_GET["filepath"];
73 echo "<a href='MultipartonInteractions.php?filepath=".$filepath."' target='page'>";?>
74 eikonalized description</a> of all the hard QCD processes, so
75 includes them in combinationation with low-<i>pT</i> events.
77 Since the current description is handled by the multiparton-interactions
78 machinery as part of the parton-level processing, no hard process at
79 all is defined at the process-level part of the event generation.
80 Fortunately, in this case a special
81 <code><?php $filepath = $_GET["filepath"];
82 echo "<a href='EventInformation.php?filepath=".$filepath."' target='page'>";?>codeSub()</a></code>
83 method provides information on the first, i.e. hardest, subprocess
84 selected by the multiparton-interactions machinery.
88 <br/><br/><strong>SoftQCD:elastic</strong> <input type="radio" name="3" value="on"><strong>On</strong>
89 <input type="radio" name="3" value="off" checked="checked"><strong>Off</strong>
90 (<code>default = <strong>off</strong></code>)<br/>
91 Elastic scattering <i>A B -> A B</i>.
92 Code 102. It is possible to include <?php $filepath = $_GET["filepath"];
93 echo "<a href='TotalCrossSections.php?filepath=".$filepath."' target='page'>";?>
94 Coulomb corrections</a>, but by default this is off.
97 <br/><br/><strong>SoftQCD:singleDiffractive</strong> <input type="radio" name="4" value="on"><strong>On</strong>
98 <input type="radio" name="4" value="off" checked="checked"><strong>Off</strong>
99 (<code>default = <strong>off</strong></code>)<br/>
100 Single diffractive scattering <i>A B -> X B</i> and
101 <i>A B -> A X</i>. See page on <?php $filepath = $_GET["filepath"];
102 echo "<a href='Diffraction.php?filepath=".$filepath."' target='page'>";?>
103 Diffraction</a> for details. Codes 103 and 104.
106 <br/><br/><strong>SoftQCD:doubleDiffractive</strong> <input type="radio" name="5" value="on"><strong>On</strong>
107 <input type="radio" name="5" value="off" checked="checked"><strong>Off</strong>
108 (<code>default = <strong>off</strong></code>)<br/>
109 Double diffractive scattering <i>A B -> X_1 X_2</i>.
110 See page on <?php $filepath = $_GET["filepath"];
111 echo "<a href='Diffraction.php?filepath=".$filepath."' target='page'>";?>Diffraction</a>
112 for details. Code 105.
115 <br/><br/><strong>SoftQCD:centralDiffractive</strong> <input type="radio" name="6" value="on"><strong>On</strong>
116 <input type="radio" name="6" value="off" checked="checked"><strong>Off</strong>
117 (<code>default = <strong>off</strong></code>)<br/>
118 Central diffractive scattering <i>A B -> A X B</i>
119 (a.k.a. double-Pomeron exchange, DPE). See pages on
120 <?php $filepath = $_GET["filepath"];
121 echo "<a href='TotalCrossSections.php?filepath=".$filepath."' target='page'>";?>Total Cross Sections</a>
122 and on <?php $filepath = $_GET["filepath"];
123 echo "<a href='Diffraction.php?filepath=".$filepath."' target='page'>";?>Diffraction</a> for details.
124 In particular note the <code>SigmaTotal:zeroAXB</code> flag,
125 which is off in most tunes, and that therefore would need to
126 be reset to on after the selection of a tune (even the default one).
130 <h3>Hard QCD processes</h3>
132 This group contains the processes for QCD jet production above
133 some minimum <i>pT</i> threshold. The <i>pT_min</i> cut cannot be put
134 too low, or else unreasonably large jet cross sections will be obtained.
135 This is because the divergent perturbative QCD cross section is used
136 in this process group, without any regularization modifications.
137 An eikonalized description, intended to be valid at all <i>pT</i>,
138 is instead included as part of the multiparton-interactions framework,
139 specifically in <code>SoftQCD:minBias</code> above.
140 <br/><b>Warning 1</b>: you <b>must</b> remember to set the
141 <code>PhaseSpace:pTHatMin</code> value if you use any of these
142 processes; there is no sensible default.
143 <br/><b>Warning 2</b>: you <b>must not</b> mix processes from the
144 <code>SoftQCD</code> and <code>HardQCD</code> process groups, since
145 this is likely to lead to doublecounting.
147 <br/><br/><strong>HardQCD:all</strong> <input type="radio" name="7" value="on"><strong>On</strong>
148 <input type="radio" name="7" value="off" checked="checked"><strong>Off</strong>
149 (<code>default = <strong>off</strong></code>)<br/>
150 Common switch for the group of all hard QCD processes,
151 as listed separately in the following.
154 <br/><br/><strong>HardQCD:gg2gg</strong> <input type="radio" name="8" value="on"><strong>On</strong>
155 <input type="radio" name="8" value="off" checked="checked"><strong>Off</strong>
156 (<code>default = <strong>off</strong></code>)<br/>
157 Scatterings <i>g g -> g g</i>.
161 <br/><br/><strong>HardQCD:gg2qqbar</strong> <input type="radio" name="9" value="on"><strong>On</strong>
162 <input type="radio" name="9" value="off" checked="checked"><strong>Off</strong>
163 (<code>default = <strong>off</strong></code>)<br/>
164 Scatterings <i>g g -> q qbar</i>, where <i>q</i> by default
165 is a light quark (<i>u, d, s</i>) (see below).
169 <br/><br/><strong>HardQCD:qg2qg</strong> <input type="radio" name="10" value="on"><strong>On</strong>
170 <input type="radio" name="10" value="off" checked="checked"><strong>Off</strong>
171 (<code>default = <strong>off</strong></code>)<br/>
172 Scatterings <i>q g -> q g</i> and <i>qbar g -> qbar g</i>.
176 <br/><br/><strong>HardQCD:qq2qq</strong> <input type="radio" name="11" value="on"><strong>On</strong>
177 <input type="radio" name="11" value="off" checked="checked"><strong>Off</strong>
178 (<code>default = <strong>off</strong></code>)<br/>
179 Scatterings <i>q q' -> q q'</i>, <i>q qbar' -> q qbar'</i>,
180 <i>qbar qbar' -> qbar qbar'</i>, where <i>q'</i> and <i>q</i>
181 may agree, but the outgoing flavours equals the incoming ones
185 <br/><br/><strong>HardQCD:qqbar2gg</strong> <input type="radio" name="12" value="on"><strong>On</strong>
186 <input type="radio" name="12" value="off" checked="checked"><strong>Off</strong>
187 (<code>default = <strong>off</strong></code>)<br/>
188 Scatterings <i>q qbar -> g g</i>.
192 <br/><br/><strong>HardQCD:qqbar2qqbarNew</strong> <input type="radio" name="13" value="on"><strong>On</strong>
193 <input type="radio" name="13" value="off" checked="checked"><strong>Off</strong>
194 (<code>default = <strong>off</strong></code>)<br/>
195 Scatterings <i>q qbar -> q' qbar'</i>, where <i>q'</i>
196 by default is a light quark (<i>u, d, s</i>) (see below).
200 <br/><br/><table><tr><td><strong>HardQCD:nQuarkNew </td><td></td><td> <input type="text" name="14" value="3" size="20"/> (<code>default = <strong>3</strong></code>; <code>minimum = 0</code>; <code>maximum = 5</code>)</td></tr></table>
201 Number of allowed outgoing new quark flavours in the above
202 <i>g g -> q qbar</i> and <i>q qbar -> q' qbar'</i> processes,
203 where quarks are treated as massless in the matrix-element expressions
204 (but correctly in the phase space). It is thus assumed that <i>c cbar</i>
205 and <i>b bbar</i> are added separately with masses taken into account,
206 using the processes below. A change to 4 would also include <i>c cbar</i>
207 in the massless approximation, etc. In order to avoid doublecounting
208 the processes below should then not be used simultaneously.
211 <h3>Hard QCD processes: heavy-flavour subset</h3>
213 These processes form a natural part of the above class, but can
214 also be generated separately. Formally the heavy-quark mass makes
215 these matrix elements finite in the <i>pT -> 0</i> limit, but at
216 high energies one may still question the validity of the expressions
217 at low <i>pT</i> values, like for the other hard-QCD processes.
218 Also as above, an eikonalized description, intended to be valid at all
219 <i>pT</i>, is included as part of the multiparton-interactions framework.
220 <br/>Note that the processes below only represent the "tip of the iceberg"
221 of charm and bottom production at high energies, where flavour excitation
222 and shower branchings provide major additional sources. All these sources
223 come together in the descriptions offered by <code>SoftQCD:minBias</code>
224 and <code>HardQCD:all</code>.
226 <br/><br/><strong>HardQCD:gg2ccbar</strong> <input type="radio" name="15" value="on"><strong>On</strong>
227 <input type="radio" name="15" value="off" checked="checked"><strong>Off</strong>
228 (<code>default = <strong>off</strong></code>)<br/>
229 Scatterings <i>g g -> c cbar</i>.
233 <br/><br/><strong>HardQCD:qqbar2ccbar</strong> <input type="radio" name="16" value="on"><strong>On</strong>
234 <input type="radio" name="16" value="off" checked="checked"><strong>Off</strong>
235 (<code>default = <strong>off</strong></code>)<br/>
236 Scatterings <i>q qbar -> c cbar</i>.
240 <br/><br/><strong>HardQCD:gg2bbbar</strong> <input type="radio" name="17" value="on"><strong>On</strong>
241 <input type="radio" name="17" value="off" checked="checked"><strong>Off</strong>
242 (<code>default = <strong>off</strong></code>)<br/>
243 Scatterings <i>g g -> b bbar</i>.
247 <br/><br/><strong>HardQCD:qqbar2bbbar</strong> <input type="radio" name="18" value="on"><strong>On</strong>
248 <input type="radio" name="18" value="off" checked="checked"><strong>Off</strong>
249 (<code>default = <strong>off</strong></code>)<br/>
250 Scatterings <i>q qbar -> b bbar</i>.
254 <h3>Hard QCD three-parton processes</h3>
256 Three-parton final states are generated by showers off two-parton
257 processes. Topologies then cannot be specified beforehand, beyond
258 what is provided by the two-parton hard process. For some checks
259 it may be convenient to have access to the dedicated three-parton
260 final states, which is what this set of processes allows.
261 Cross sections have been taken from [<a href="Bibliography.php" target="page">Ber81</a>].
262 <br/>Note that the processes in this section are <it>not</it>
263 affected by the <code>HardQCD:all</code> switch. In fact, it would
264 be doublecounting to include both the <code>HardQCD:all</code> and
265 the <code>HardQCD:3parton</code> processes in a run or study.
266 <br/><b>Warning:</b> this section is still incomplete, e.g. the
267 selection of colour flow is very simple, and so it should only
268 be used with caution.
270 <br/><br/><strong>HardQCD:3parton</strong> <input type="radio" name="19" value="on"><strong>On</strong>
271 <input type="radio" name="19" value="off" checked="checked"><strong>Off</strong>
272 (<code>default = <strong>off</strong></code>)<br/>
273 Common switch for the group of all hard QCD processes with three
274 partons in the final state, as listed separately in the following.
277 <br/><br/><strong>HardQCD:gg2ggg</strong> <input type="radio" name="20" value="on"><strong>On</strong>
278 <input type="radio" name="20" value="off" checked="checked"><strong>Off</strong>
279 (<code>default = <strong>off</strong></code>)<br/>
280 Scatterings <i>g g -> g g g</i>.
284 <br/><br/><strong>HardQCD:qqbar2ggg</strong> <input type="radio" name="21" value="on"><strong>On</strong>
285 <input type="radio" name="21" value="off" checked="checked"><strong>Off</strong>
286 (<code>default = <strong>off</strong></code>)<br/>
287 Scatterings <i>q qbar -> g g g</i>.
291 <br/><br/><strong>HardQCD:qg2qgg</strong> <input type="radio" name="22" value="on"><strong>On</strong>
292 <input type="radio" name="22" value="off" checked="checked"><strong>Off</strong>
293 (<code>default = <strong>off</strong></code>)<br/>
294 Scatterings <i>q g -> q g g</i> and <i>qbar g -> qbar g g</i>.
298 <br/><br/><strong>HardQCD:qq2qqgDiff</strong> <input type="radio" name="23" value="on"><strong>On</strong>
299 <input type="radio" name="23" value="off" checked="checked"><strong>Off</strong>
300 (<code>default = <strong>off</strong></code>)<br/>
301 Scatterings <i>q q' -> q q' g</i>, <i>q qbar' -> q qbar' g</i>,
302 and <i>qbar qbar' -> qbar qbar' g</i>.
306 <br/><br/><strong>HardQCD:qq2qqgSame</strong> <input type="radio" name="24" value="on"><strong>On</strong>
307 <input type="radio" name="24" value="off" checked="checked"><strong>Off</strong>
308 (<code>default = <strong>off</strong></code>)<br/>
309 Scatterings <i>q q -> q q g</i> and
310 <i>qbar qbar -> qbar qbar g</i>
311 (<i>q qbar -> q qbar g</i> scatterings are considered separately
312 below, see <code>HardQCD:qqbar2qqbargSame</code>).
316 <br/><br/><strong>HardQCD:qqbar2qqbargDiff</strong> <input type="radio" name="25" value="on"><strong>On</strong>
317 <input type="radio" name="25" value="off" checked="checked"><strong>Off</strong>
318 (<code>default = <strong>off</strong></code>)<br/>
319 Scatterings <i>q qbar -> q' qbar' g</i>, where <i>q'</i>
320 by default is a light quark (<i>u, d, s</i>)
321 (see <code>HardQCD:nQuarkNew</code> above).
325 <br/><br/><strong>HardQCD:qqbar2qqbargSame</strong> <input type="radio" name="26" value="on"><strong>On</strong>
326 <input type="radio" name="26" value="off" checked="checked"><strong>Off</strong>
327 (<code>default = <strong>off</strong></code>)<br/>
328 Scatterings <i>q qbar -> q qbar g</i>.
332 <br/><br/><strong>HardQCD:gg2qqbarg</strong> <input type="radio" name="27" value="on"><strong>On</strong>
333 <input type="radio" name="27" value="off" checked="checked"><strong>Off</strong>
334 (<code>default = <strong>off</strong></code>)<br/>
335 Scatterings <i>g g -> q qbar g</i>, where <i>q</i> by
336 default is a light quark (<i>u, d, s</i>)
337 (see <code>HardQCD:nQuarkNew</code> above).
341 <br/><br/><strong>HardQCD:qg2qqqbarDiff</strong> <input type="radio" name="28" value="on"><strong>On</strong>
342 <input type="radio" name="28" value="off" checked="checked"><strong>Off</strong>
343 (<code>default = <strong>off</strong></code>)<br/>
344 Scatterings <i>q g -> q q' qbar'</i> and
345 <i>qbar g -> qbar qbar' q'</i>, where <i>q'</i>
346 by default is a light quark (<i>u, d, s</i>)
347 (see <code>HardQCD:nQuarkNew</code> above).
351 <br/><br/><strong>HardQCD:qg2qqqbarSame</strong> <input type="radio" name="29" value="on"><strong>On</strong>
352 <input type="radio" name="29" value="off" checked="checked"><strong>Off</strong>
353 (<code>default = <strong>off</strong></code>)<br/>
354 Scatterings <i>q g -> q q qbar</i> and
355 <i>qbar g -> qbar qbar q</i>.
360 <input type="hidden" name="saved" value="1"/>
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527 <!-- Copyright (C) 2012 Torbjorn Sjostrand -->