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[u/mrichter/AliRoot.git] / PYTHIA8 / pythia8130 / xmldoc / FlavourSelection.xml
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5ad4eb21 1<chapter name="Flavour Selection">
2
3<h2>Flavour Selection</h2>
4
5The <code>StringFlav</code> class handles the choice of a new flavour
6in the fragmentation process, and the production of a new hadron
7from a set of input flavours. It is mainly used by the string
8fragmentation machinery (including ministrings), but also e.g.
9in some particle decays and for some beam-remnant cases. The basic
10concepts are in agreement with <ref>And83</ref>. The baryon-sector
11implementation is based on the <code>MSTJ(12)=3</code> option of
12PYTHIA 6, i.e. new SU(6) weights scheme with at most one popcorn meson.
13
14<p/>
15The relative production rates of different particle species is
16influenced by the parameters below. Some have only an impact on
17one specific quantity, but most directly or indirectly have
18consequences for many observables. Therefore the values to use have
19to be viewed in the context of a complete <aloc href="Tunes">tune</aloc>.
20
21<h3>New flavours</h3>
22
23The main parameters of the selection of a new flavour are
24
25<parm name="StringFlav:probStoUD" default="0.30" min="0.0" max="1.0">
26the suppression of <ei>s</ei> quark production relative to ordinary
27<ei>u</ei> or <ei>d</ei> one.
28</parm>
29
30<parm name="StringFlav:probQQtoQ" default="0.10" min="0.0" max="1.0">
31the suppression of diquark production relative to quark production,
32i.e. of baryon relative to meson production.
33</parm>
34
35<parm name="StringFlav:probSQtoQQ" default="0.4" min="0.0" max="1.0">
36the suppression of strange diquark production relative to light
37diquark production, over and above the one already given by
38<code>probStoU</code>.
39</parm>
40
41<parm name="StringFlav:probQQ1toQQ0" default="0.05" min="0.0" max="1.0">
42the suppression of spin 1 diquark production relative to spin 0 one,
43apart from the factor of 3 enhancement of spin 0 from counting the
44number of states.
45</parm>
46
47<h3>Standard-meson production</h3>
48
49The bulk of the particle production corresponds to the lowest-lying
50pseudoscalar and vector multiplets. Their production rates are
51determined by the parameters in this section.
52
53<p/>
54For a given set of flavours, produced according to the probabilities
55outlined above, the ratio of vector-to-pseudocalar meson production
56is described by the parameters below.
57The maximum allowed rate for each case has been set according to
58spin-counting rules, but we expect the real rates to be lower,
59especially for lighter mesons, owing to the vector-pseudoscalar
60mass splitting.
61
62<parm name="StringFlav:mesonUDvector" default="1.0" min="0." max="3.">
63the relative production ratio vector/pseudoscalar for light
64(<ei>u</ei>, <ei>d</ei>) mesons.
65</parm>
66<parm name="StringFlav:mesonSvector" default="1.5" min="0." max="3.">
67the relative production ratio vector/pseudoscalar for strange mesons.
68</parm>
69<parm name="StringFlav:mesonCvector" default="2.5" min="0." max="3.">
70the relative production ratio vector/pseudoscalar for charm mesons.
71</parm>
72<parm name="StringFlav:mesonBvector" default="3.0" min="0." max="3.">
73the relative production ratio vector/pseudoscalar for bottom mesons.
74</parm>
75
76<p/>
77Inside each light-quark meson nonet, an octet-singlet mixing angle
78describes the mixing of the two flavour-diagonal isoscalar = 0 states.
79(For terminology and details see <ref>Yao06</ref>, chapter 14 on the
80quark model.)
81This angle is needed to specify the probability for such a <ei>q qbar</ei>
82state to project onto a specific meson. More transparent formuale are
83obtained by introducing the angle <ei>alpha = theta + 54.7</ei> degrees:
84<eq>
85 f = (uubar + ddbar)/sqrt(2) * sin(alpha) + ssbar * cos(alpha)<br/>
86 f' = (uubar + ddbar)/sqrt(2) * cos(alpha) - ssbar * sin(alpha)
87</eq>
88
89<parm name="StringFlav:thetaPS" default="-15." min="-90." max="90.">
90gives the mixing angle <ei>theta_PS</ei> in the pseudoscalar meson sector
91(which is rather poorly determined), expressed in degrees.
92Here <ei>f</ei> is associated with <ei>eta'</ei> and <ei>f'</ei> with
93<ei>eta</ei>. (This standard but counterintuitive choice is fixed up
94in the code by replacing <ei>alpha -> 90^0 - alpha</ei> so that
95<ei>eta &lt;-> eta'</ei>; relative signs do not matter since we are
96interested in probabilities only.)
97</parm>
98
99<parm name="StringFlav:thetaV" default="36." min="-90." max="90.">
100gives the mixing angle <ei>theta_V</ei> in the vector meson sector
101(which is somewhat better determined), expressed in degrees.
102Here <ei>f</ei> is associated with <ei>omega</ei> and <ei>f'</ei>
103with <ei>phi</ei>.
104</parm>
105
106<p/>
107Further, the simple model overestimates the production of <ei>eta</ei>
108and, in particular, <ei>eta'</ei> mesons, which can be rectified by
109
110<parm name="StringFlav:etaSup" default="1.0" min="0." max="1.">
111the additional suppression of <ei>eta</ei> production, multiplying the
112normal production probability. Thus 0 means no <ei>eta</ei> at all
113are produced, while 1 means full rate.
114</parm>
115
116<parm name="StringFlav:etaPrimeSup" default="0.4" min="0." max="1.">
117the additional suppression of <ei>eta'</ei> production, multiplying the
118normal production probability. Thus 0 means no <ei>eta'</ei> at all
119are produced, while 1 means full rate.
120</parm>
121
122<h3>Excited-meson production</h3>
123
124Several excited mesons, ie. with radial or orbital excitations, have been
125observed at non-negligible production rates. Extrapolated to all states
126a fair fraction of all particle production might proceed through such
127states. There are big uncertainties, however, since these excited
128mesons in many cases are extremely poorly known. This also means that
129the modelling of their production and decay is very primitive, and
130even that the inclusion of the production of such states may lead to a
131degraded agreement with data. Currently the default is that all such
132production is switched off.
133
134<p/>
135Parameters are provided to switch them on. By demand, this machinery
136has been made more flexible than in the past. Therefore one parameter is
137provided for each combination of heaviest flavour
138(<ei>u/d</ei>, <ei>s</ei>, <ei>c</ei> or <ei>b</ei>) and
139multiplet produced. In each case the production rate is normalized to
140that of the lowest-lying pseudoscalar of the same flavour content, as for
141the vector-meson rates introduced above. The multiplets available are the
142four obtained for one unit of orbital angular momentum, in the
143nonrelativistic classification. Using <ei>J</ei> to denote the sum of
144quark spin <ei>S</ei> and orbital angular momentum <ei>L</ei>, i.e. what
145would normally be called the spin of the meson, one has:
146<ul>
147<li>a pseudovector multiplet with <ei>L=1, S=0, J=1</ei>;</li>
148<li>a scalar multiplet with <ei>L=1, S=1, J=0</ei>;</li>
149<li>a pseudovector multiplet with <ei>L=1, S=1, J=1</ei>;</li>
150<li>a tensor multiplet with <ei>L=1, S=1, J=2</ei>.</li>
151</ul>
152
153The maximum allowed rate for each case has been set according to
154spin-counting rules, but we expect the real rates to be significantly
155lower, owing to mass suppression.
156
157<parm name="StringFlav:mesonUDL1S0J1" default="0.0" min="0." max="3.">
158the relative pseudovector production ratio
159<ei>(L=1,S=0,J=1)</ei>/pseudoscalar
160for light (<ei>u</ei>, <ei>d</ei>) mesons.
161</parm>
162
163<parm name="StringFlav:mesonUDL1S1J0" default="0.0" min="0." max="1.">
164the relative scalar production ratio
165<ei>(L=1,S=1,J=0)</ei>/pseudoscalar
166for light (<ei>u</ei>, <ei>d</ei>) mesons.
167</parm>
168
169<parm name="StringFlav:mesonUDL1S1J1" default="0.0" min="0." max="3.">
170the relative pseudovector production ratio
171<ei>(L=1,S=1,J=1)</ei>/pseudoscalar
172for light (<ei>u</ei>, <ei>d</ei>) mesons.
173</parm>
174
175<parm name="StringFlav:mesonUDL1S1J2" default="0.0" min="0." max="5.">
176the relative tensor production ratio
177<ei>(L=1,S=1,J=2)</ei>/pseudoscalar
178for light (<ei>u</ei>, <ei>d</ei>) mesons.
179</parm>
180
181<parm name="StringFlav:mesonSL1S0J1" default="0.0" min="0." max="3.">
182the relative pseudovector production ratio
183<ei>(L=1,S=0,J=1)</ei>/pseudoscalar
184for strange mesons.
185</parm>
186
187<parm name="StringFlav:mesonSL1S1J0" default="0.0" min="0." max="1.">
188the relative scalar production ratio
189<ei>(L=1,S=1,J=0)</ei>/pseudoscalar
190for strange mesons.
191</parm>
192
193<parm name="StringFlav:mesonSL1S1J1" default="0.0" min="0." max="3.">
194the relative pseudovector production ratio
195<ei>(L=1,S=1,J=1)</ei>/pseudoscalar
196for strange mesons.
197</parm>
198
199<parm name="StringFlav:mesonSL1S1J2" default="0.0" min="0." max="5.">
200the relative tensor production ratio
201<ei>(L=1,S=1,J=2)</ei>/pseudoscalar
202for strange mesons.
203</parm>
204
205<parm name="StringFlav:mesonCL1S0J1" default="0.0" min="0." max="3.">
206the relative pseudovector production ratio
207<ei>(L=1,S=0,J=1)</ei>/pseudoscalar
208for charm mesons.
209</parm>
210
211<parm name="StringFlav:mesonCL1S1J0" default="0.0" min="0." max="1.">
212the relative scalar production ratio
213<ei>(L=1,S=1,J=0)</ei>/pseudoscalar
214for charm mesons.
215</parm>
216
217<parm name="StringFlav:mesonCL1S1J1" default="0.0" min="0." max="3.">
218the relative pseudovector production ratio
219<ei>(L=1,S=1,J=1)</ei>/pseudoscalar
220for charm mesons.
221</parm>
222
223<parm name="StringFlav:mesonCL1S1J2" default="0.0" min="0." max="5.">
224the relative tensor production ratio
225<ei>(L=1,S=1,J=2)</ei>/pseudoscalar
226for charm mesons.
227</parm>
228
229<parm name="StringFlav:mesonBL1S0J1" default="0.0" min="0." max="3.">
230the relative pseudovector production ratio
231<ei>(L=1,S=0,J=1)</ei>/pseudoscalar
232for bottom mesons.
233</parm>
234
235<parm name="StringFlav:mesonBL1S1J0" default="0.0" min="0." max="1.">
236the relative scalar production ratio
237<ei>(L=1,S=1,J=0)</ei>/pseudoscalar
238for bottom mesons.
239</parm>
240
241<parm name="StringFlav:mesonBL1S1J1" default="0.0" min="0." max="3.">
242the relative pseudovector production ratio
243<ei>(L=1,S=1,J=1)</ei>/pseudoscalar
244for bottom mesons.
245</parm>
246
247<parm name="StringFlav:mesonBL1S1J2" default="0.0" min="0." max="5.">
248the relative tensor production ratio
249<ei>(L=1,S=1,J=2)</ei>/pseudoscalar
250for bottom mesons.
251</parm>
252
253<p/>
254In addition, an octet-singlet mixing angle is needed for each multiplet,
255as for the pseudoscalar and vector multiplets above. Only for the
256tensor multiplet does any determination exist; for the other multiplets
257default has been chose so that <ei>ssbar</ei> does not mix with the light
258quarks, and so that the <ei>ssbar</ei> state is the heavier of the two.
259
260<parm name="StringFlav:thetaL1S0J1" default="35.3" min="-90." max="90.">
261gives the mixing angle <ei>theta</ei> in the <ei>(L=1,S=0,J=1)</ei>
262pseudovector meson sector, expressed in degrees.
263</parm>
264
265<parm name="StringFlav:thetaL1S1J0" default="35.3" min="-90." max="90.">
266gives the mixing angle <ei>theta</ei> in the <ei>(L=1,S=1,J=0)</ei>
267scalar meson sector, expressed in degrees.
268</parm>
269
270<parm name="StringFlav:thetaL1S1J1" default="35.3" min="-90." max="90.">
271gives the mixing angle <ei>theta</ei> in the <ei>(L=1,S=1,J=1)</ei>
272pseudovector meson sector, expressed in degrees.
273</parm>
274
275<parm name="StringFlav:thetaL1S1J2" default="28.0" min="-90." max="90.">
276gives the mixing angle <ei>theta</ei> in the <ei>(L=1,S=1,J=2)</ei>
277tensor meson sector, expressed in degrees.
278</parm>
279
280<h3>Baryon production</h3>
281
282The relative rate of baryon production is mainly given by the quark
283and diquark production parameters above, plus SU(6) Clebsch-Gordans.
284The one modifiable parameter related to these coefficients is
285
286<parm name="StringFlav:decupletSup" default="1.0" min="0.0" max="1.0">
287the suppression, relative to default SU(6) factors, of decuplet
288baryon production. Default corresponds to no suppression, while 0
289corresponds to no decuplet production at all.
290</parm>
291
292<p/>
293In addition, if popcorn production is allowed, wherein a set of mesons
294(<ei>M</ei>) may be producted in between the baryon (<ei>B</ei>) and
295the antibaryon (<ei>Bbar</ei>), a set of further parameters is introduced.
296Currently only the simplest scenario is implemented, wherein at most
297one intermediate meson may be produced.
298
299<parm name="StringFlav:popcornRate" default="0.5" min="0." max="2.0">
300gives the relative rates of <ei>B Bbar</ei> and <ei>B M Bbar</ei>
301production, roughly as
302<eq>
303Prob(B M Bbar) / (Prob(B Bbar) + Prob(B M Bbar)) =
304popcornRate / (0.5 + popcornRate)
305</eq>
306(the complete expression depends on all the quark and diquark production
307parameters and is therefore not so useful).
308</parm>
309
310<parm name="StringFlav:popcornSpair" default="0.5" min="0." max="1.0">
311extra suppression for having an <ei>s sbar</ei> pair shared between
312the <ei>B</ei> and <ei>Bbar</ei> in a <ei>B M Bbar</ei> configuration.
313</parm>
314
315<parm name="StringFlav:popcornSmeson" default="0.5" min="0." max="1.0">
316extra suppression for having a strange meson <ei>M</ei> in a
317<ei>B M Bbar</ei> configuration.
318</parm>
319
320<p/>
321Finally, there are some indications that leading-baryon production
322may be further suppressed. A proper description should probably be
323based on a suppression of early production times <ref>Ede97</ref>,
324but we here only implement a simpler version where production near
325the end of a string, as defined by rank, is suppressed. The more
326detailed studies suggest that leading <ei>c</ei> and <ei>b</ei> baryon
327production will be less suppressed, so we leave it open to set
328light- and heavy-baryon suppression separately.
329
330<flag name="StringFlav:suppressLeadingB" default="off">
331Suppress leading-baryon production.
332<option value="off">No suppression.</option>
333<option value="on">Suppress the production of a diquark in the string
334breaking closest to a quark end of a string, by either of the factors
335below. This suppresses the production of first-rank baryons by the same
336amount. Indirectly also the second-rank and, if popcorn production is
337switched on, third-rank (anti)baryon production is affected. </option>
338</flag>
339
340<parm name="StringFlav:lightLeadingBSup" default="0.5" min="0." max="1.0">
341extra suppression of leading-baryon production for a light-quark
342jet, i.e. <ei>d</ei>, <ei>u</ei> or <ei>s</ei>, when
343<code>suppressLeadingB = on</code>. Thus 0 means no leading-baryon
344production at all, while 1 means full rate.
345</parm>
346
347<parm name="StringFlav:heavyLeadingBSup" default="0.9" min="0." max="1.0">
348extra suppression of leading-baryon production for a heavy-quark
349jet, i.e. <ei>c</ei> or <ei>b</ei>, when
350<code>suppressLeadingB = on</code>. Thus 0 means no leading-baryon
351production at all, while 1 means full rate.
352</parm>
353
354</chapter>
355
356<!-- Copyright (C) 2008 Torbjorn Sjostrand -->