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1<html>
2<head>
3<title>QCD Processes</title>
4<link rel="stylesheet" type="text/css" href="pythia.css"/>
5<link rel="shortcut icon" href="pythia32.gif"/>
6</head>
7<body>
8
9<h2>QCD Processes</h2>
10
11This section is subdivided into soft and hard QCD processes, with
12open charm and bottom production set aside as a special part of the
13latter, and three-jet topologies as a special subset. Kindly note
14that there is a considerable amount of overlap between the soft and
15hard process classes, so that you are likely to doublecount
16if you include both in a run.
17
18<h3>Soft QCD processes</h3>
19
20As a rule, the processes in this class should not be mixed with
21the simulation of other processes. All by themselves, they are
22intended to represent the total cross section of hadron collisions,
23with the exception of the "rare processes" that one wishes to study
24separately. In particular, jet physics at all scales occurs as part
25of the minimum-bias description.
26
27<p/>
28We here use the "minimum bias" expression as a shorthand for
29inelastic, nondiffractive events. Strictly speaking, "minimum bias"
30represents an experimental procedure of accepting "everything", with
31some non-universal cuts to exclude elastic and diffractive topologies.
32In practice, the experimental mimimum-bias sample may then contain
33some contamination of what is in PYTHIA classified as diffractive,
34especially (high-mass) double diffractive.
35
36<p/>
37Some options to modify these cross sections are found on the
38<a href="TotalCrossSections.html" target="page">Total Cross Sections</a> page. 
39
40<p/><code>flag&nbsp; </code><strong> SoftQCD:all &nbsp;</strong> 
41 (<code>default = <strong>off</strong></code>)<br/>
42Common switch for the group of all soft QCD processes,
43as listed separately in the following.
44 
45
46<p/><code>flag&nbsp; </code><strong> SoftQCD:minBias &nbsp;</strong> 
47 (<code>default = <strong>off</strong></code>)<br/>
48Minimum-bias events, based on an <a href="MultipartonInteractions.html" target="page">
49eikonalized description</a> of all the hard QCD processes, so
50includes them in combinationation with low-<i>pT</i> events.
51Code 101.<br/>
52Since the current description is handled by the multiparton-interactions
53machinery as part of the parton-level processing, no hard process at
54all is defined at the process-level part of the event generation.
55Fortunately, in this case a special
56<code><a href="EventInformation.html" target="page">codeSub()</a></code> 
57method provides information on the first, i.e. hardest, subprocess
58selected by the multiparton-interactions machinery.
59
60 
61
62<p/><code>flag&nbsp; </code><strong> SoftQCD:elastic &nbsp;</strong> 
63 (<code>default = <strong>off</strong></code>)<br/>
64Elastic scattering <i>A B -> A B</i>.
65Code 102. It is possible to include <a href="TotalCrossSections.html" target="page">
66Coulomb corrections</a>, but by default this is off.
67 
68
69<p/><code>flag&nbsp; </code><strong> SoftQCD:singleDiffractive &nbsp;</strong> 
70 (<code>default = <strong>off</strong></code>)<br/>
71Single 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">
73Diffraction</a> for details. Codes 103 and 104.
74 
75
76<p/><code>flag&nbsp; </code><strong> SoftQCD:doubleDiffractive &nbsp;</strong> 
77 (<code>default = <strong>off</strong></code>)<br/>
78Double diffractive scattering <i>A B -> X_1 X_2</i>.
79See page on <a href="Diffraction.html" target="page">Diffraction</a>
80for details. Code 105.
81 
82
83<p/><code>flag&nbsp; </code><strong> SoftQCD:centralDiffractive &nbsp;</strong> 
84 (<code>default = <strong>off</strong></code>)<br/>
85Central 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>
88and on <a href="Diffraction.html" target="page">Diffraction</a> for details.
89In particular note the <code>SigmaTotal:zeroAXB</code> flag,
90which is off in most tunes, and that therefore would need to
91be reset to on after the selection of a tune (even the default one).
92Code 106.
93 
94
95<h3>Hard QCD processes</h3>
96
97This group contains the processes for QCD jet production above
98some minimum <i>pT</i> threshold. The <i>pT_min</i> cut cannot be put
99too low, or else unreasonably large jet cross sections will be obtained.
100This is because the divergent perturbative QCD cross section is used
101in this process group, without any regularization modifications.
102An eikonalized description, intended to be valid at all <i>pT</i>,
103is instead included as part of the multiparton-interactions framework,
104specifically 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
107processes; 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
110this is likely to lead to doublecounting.
111 
112<p/><code>flag&nbsp; </code><strong> HardQCD:all &nbsp;</strong> 
113 (<code>default = <strong>off</strong></code>)<br/>
114Common switch for the group of all hard QCD processes,
115as listed separately in the following.
116 
117
118<p/><code>flag&nbsp; </code><strong> HardQCD:gg2gg &nbsp;</strong> 
119 (<code>default = <strong>off</strong></code>)<br/>
120Scatterings <i>g g -> g g</i>.
121Code 111.
122 
123
124<p/><code>flag&nbsp; </code><strong> HardQCD:gg2qqbar &nbsp;</strong> 
125 (<code>default = <strong>off</strong></code>)<br/>
126Scatterings <i>g g -> q qbar</i>, where <i>q</i> by default
127is a light quark (<i>u, d, s</i>) (see below).
128Code 112.
129 
130
131<p/><code>flag&nbsp; </code><strong> HardQCD:qg2qg &nbsp;</strong> 
132 (<code>default = <strong>off</strong></code>)<br/>
133Scatterings <i>q g -> q g</i> and <i>qbar g -> qbar g</i>.
134Code 113.
135 
136
137<p/><code>flag&nbsp; </code><strong> HardQCD:qq2qq &nbsp;</strong> 
138 (<code>default = <strong>off</strong></code>)<br/>
139Scatterings <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> 
141may agree, but the outgoing flavours equals the incoming ones
142Code 114.
143 
144
145<p/><code>flag&nbsp; </code><strong> HardQCD:qqbar2gg &nbsp;</strong> 
146 (<code>default = <strong>off</strong></code>)<br/>
147Scatterings <i>q qbar -> g g</i>.
148Code 115.
149 
150
151<p/><code>flag&nbsp; </code><strong> HardQCD:qqbar2qqbarNew &nbsp;</strong> 
152 (<code>default = <strong>off</strong></code>)<br/>
153Scatterings <i>q qbar -> q' qbar'</i>, where <i>q'</i> 
154by default is a light quark (<i>u, d, s</i>) (see below).
155Code 116.
156 
157
158<p/><code>mode&nbsp; </code><strong> HardQCD:nQuarkNew &nbsp;</strong> 
159 (<code>default = <strong>3</strong></code>; <code>minimum = 0</code>; <code>maximum = 5</code>)<br/>
160Number of allowed outgoing new quark flavours in the above
161<i>g g -> q qbar</i> and <i>q qbar -> q' qbar'</i> processes,
162where 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> 
164and <i>b bbar</i> are added separately with masses taken into account,
165using the processes below. A change to 4 would also include <i>c cbar</i> 
166in the massless approximation, etc. In order to avoid doublecounting
167the processes below should then not be used simultaneously.
168 
169
170<h3>Hard QCD processes: heavy-flavour subset</h3>
171
172These processes form a natural part of the above class, but can
173also be generated separately. Formally the heavy-quark mass makes
174these matrix elements finite in the <i>pT -> 0</i> limit, but at
175high energies one may still question the validity of the expressions
176at low <i>pT</i> values, like for the other hard-QCD processes.
177Also 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"
180of charm and bottom production at high energies, where flavour excitation
181and shower branchings provide major additional sources. All these sources
182come together in the descriptions offered by <code>SoftQCD:minBias</code>
183and <code>HardQCD:all</code>.
184
185<p/><code>flag&nbsp; </code><strong> HardQCD:gg2ccbar &nbsp;</strong> 
186 (<code>default = <strong>off</strong></code>)<br/>
187Scatterings <i>g g -> c cbar</i>.
188Code 121.
189 
190
191<p/><code>flag&nbsp; </code><strong> HardQCD:qqbar2ccbar &nbsp;</strong> 
192 (<code>default = <strong>off</strong></code>)<br/>
193Scatterings <i>q qbar -> c cbar</i>.
194Code 122.
195 
196
197<p/><code>flag&nbsp; </code><strong> HardQCD:gg2bbbar &nbsp;</strong> 
198 (<code>default = <strong>off</strong></code>)<br/>
199Scatterings <i>g g -> b bbar</i>.
200Code 123.
201 
202
203<p/><code>flag&nbsp; </code><strong> HardQCD:qqbar2bbbar &nbsp;</strong> 
204 (<code>default = <strong>off</strong></code>)<br/>
205Scatterings <i>q qbar -> b bbar</i>.
206Code 124.
207 
208
209<h3>Hard QCD three-parton processes</h3>
210
211Three-parton final states are generated by showers off two-parton
212processes. Topologies then cannot be specified beforehand, beyond
213what is provided by the two-parton hard process. For some checks
214it may be convenient to have access to the dedicated three-parton
215final states, which is what this set of processes allows.
216Cross 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> 
218affected by the <code>HardQCD:all</code> switch. In fact, it would
219be doublecounting to include both the <code>HardQCD:all</code> and
220the <code>HardQCD:3parton</code> processes in a run or study.
221<br/><b>Warning:</b> this section is still incomplete, e.g. the
222selection of colour flow is very simple, and so it should only
223be used with caution.
224 
225<p/><code>flag&nbsp; </code><strong> HardQCD:3parton &nbsp;</strong> 
226 (<code>default = <strong>off</strong></code>)<br/>
227Common switch for the group of all hard QCD processes with three
228partons in the final state, as listed separately in the following.
229 
230
231<p/><code>flag&nbsp; </code><strong> HardQCD:gg2ggg &nbsp;</strong> 
232 (<code>default = <strong>off</strong></code>)<br/>
233Scatterings <i>g g -> g g g</i>.
234Code 131.
235 
236
237<p/><code>flag&nbsp; </code><strong> HardQCD:qqbar2ggg &nbsp;</strong> 
238 (<code>default = <strong>off</strong></code>)<br/>
239Scatterings <i>q qbar -> g g g</i>.
240Code 132.
241 
242
243<p/><code>flag&nbsp; </code><strong> HardQCD:qg2qgg &nbsp;</strong> 
244 (<code>default = <strong>off</strong></code>)<br/>
245Scatterings <i>q g -> q g g</i> and <i>qbar g -> qbar g g</i>.
246Code 133.
247 
248
249<p/><code>flag&nbsp; </code><strong> HardQCD:qq2qqgDiff &nbsp;</strong> 
250 (<code>default = <strong>off</strong></code>)<br/>
251Scatterings <i>q q' -> q q' g</i>, <i>q qbar' -> q qbar' g</i>,
252and <i>qbar qbar' -> qbar qbar' g</i>.
253Code 134.
254 
255
256<p/><code>flag&nbsp; </code><strong> HardQCD:qq2qqgSame &nbsp;</strong> 
257 (<code>default = <strong>off</strong></code>)<br/>
258Scatterings <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
261below, see <code>HardQCD:qqbar2qqbargSame</code>).
262Code 135.
263 
264
265<p/><code>flag&nbsp; </code><strong> HardQCD:qqbar2qqbargDiff &nbsp;</strong> 
266 (<code>default = <strong>off</strong></code>)<br/>
267Scatterings <i>q qbar -> q' qbar' g</i>, where <i>q'</i> 
268by default is a light quark (<i>u, d, s</i>)
269(see <code>HardQCD:nQuarkNew</code> above).
270Code 136.
271 
272
273<p/><code>flag&nbsp; </code><strong> HardQCD:qqbar2qqbargSame &nbsp;</strong> 
274 (<code>default = <strong>off</strong></code>)<br/>
275Scatterings <i>q qbar -> q qbar g</i>.
276Code 137.
277 
278
279<p/><code>flag&nbsp; </code><strong> HardQCD:gg2qqbarg &nbsp;</strong> 
280 (<code>default = <strong>off</strong></code>)<br/>
281Scatterings <i>g g -> q qbar g</i>, where <i>q</i> by
282default is a light quark (<i>u, d, s</i>)
283(see <code>HardQCD:nQuarkNew</code> above).
284Code 138.
285 
286
287<p/><code>flag&nbsp; </code><strong> HardQCD:qg2qqqbarDiff &nbsp;</strong> 
288 (<code>default = <strong>off</strong></code>)<br/>
289Scatterings <i>q g -> q q' qbar'</i> and
290<i>qbar g -> qbar qbar' q'</i>, where <i>q'</i>
291by default is a light quark (<i>u, d, s</i>)
292(see <code>HardQCD:nQuarkNew</code> above).
293Code 139.
294 
295
296<p/><code>flag&nbsp; </code><strong> HardQCD:qg2qqqbarSame &nbsp;</strong> 
297 (<code>default = <strong>off</strong></code>)<br/>
298Scatterings <i>q g -> q q qbar</i> and
299<i>qbar g -> qbar qbar q</i>.
300Code 140.
301 
302
303
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