3 <title>Flavour Selection</title>
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9 <h2>Flavour Selection</h2>
11 The <code>StringFlav</code> class handles the choice of a new flavour
12 in the fragmentation process, and the production of a new hadron
13 from a set of input flavours. It is mainly used by the string
14 fragmentation machinery (including ministrings), but also e.g.
15 in some particle decays and for some beam-remnant cases. The basic
16 concepts are in agreement with [<a href="Bibliography.html" target="page">And83</a>]. The baryon-sector
17 implementation is based on the <code>MSTJ(12)=3</code> option of
18 PYTHIA 6, i.e. new SU(6) weights scheme with at most one popcorn meson.
21 The relative production rates of different particle species is
22 influenced by the parameters below. Some have only an impact on
23 one specific quantity, but most directly or indirectly have
24 consequences for many observables. Therefore the values to use have
25 to be viewed in the context of a complete <a href="Tunes.html" target="page">tune</a>.
29 The main parameters of the selection of a new flavour are
31 <p/><code>parm </code><strong> StringFlav:probStoUD </strong>
32 (<code>default = <strong>0.19</strong></code>; <code>minimum = 0.0</code>; <code>maximum = 1.0</code>)<br/>
33 the suppression of <i>s</i> quark production relative to ordinary
34 <i>u</i> or <i>d</i> one.
37 <p/><code>parm </code><strong> StringFlav:probQQtoQ </strong>
38 (<code>default = <strong>0.09</strong></code>; <code>minimum = 0.0</code>; <code>maximum = 1.0</code>)<br/>
39 the suppression of diquark production relative to quark production,
40 i.e. of baryon relative to meson production.
43 <p/><code>parm </code><strong> StringFlav:probSQtoQQ </strong>
44 (<code>default = <strong>1.00</strong></code>; <code>minimum = 0.0</code>; <code>maximum = 1.0</code>)<br/>
45 the suppression of strange diquark production relative to light
46 diquark production, over and above the one already given by
47 <code>probStoU</code>.
50 <p/><code>parm </code><strong> StringFlav:probQQ1toQQ0 </strong>
51 (<code>default = <strong>0.027</strong></code>; <code>minimum = 0.0</code>; <code>maximum = 1.0</code>)<br/>
52 the suppression of spin 1 diquark production relative to spin 0 one,
53 apart from the factor of 3 enhancement of spin 0 from counting the
57 <h3>Standard-meson production</h3>
59 The bulk of the particle production corresponds to the lowest-lying
60 pseudoscalar and vector multiplets. Their production rates are
61 determined by the parameters in this section.
64 For a given set of flavours, produced according to the probabilities
65 outlined above, the ratio of vector-to-pseudocalar meson production
66 is described by the parameters below.
67 The maximum allowed rate for each case has been set according to
68 spin-counting rules, but we expect the real rates to be lower,
69 especially for lighter mesons, owing to the vector-pseudoscalar
72 <p/><code>parm </code><strong> StringFlav:mesonUDvector </strong>
73 (<code>default = <strong>0.62</strong></code>; <code>minimum = 0.</code>; <code>maximum = 3.</code>)<br/>
74 the relative production ratio vector/pseudoscalar for light
75 (<i>u</i>, <i>d</i>) mesons.
77 <p/><code>parm </code><strong> StringFlav:mesonSvector </strong>
78 (<code>default = <strong>0.725</strong></code>; <code>minimum = 0.</code>; <code>maximum = 3.</code>)<br/>
79 the relative production ratio vector/pseudoscalar for strange mesons.
81 <p/><code>parm </code><strong> StringFlav:mesonCvector </strong>
82 (<code>default = <strong>1.06</strong></code>; <code>minimum = 0.</code>; <code>maximum = 3.</code>)<br/>
83 the relative production ratio vector/pseudoscalar for charm mesons.
85 <p/><code>parm </code><strong> StringFlav:mesonBvector </strong>
86 (<code>default = <strong>3.0</strong></code>; <code>minimum = 0.</code>; <code>maximum = 3.</code>)<br/>
87 the relative production ratio vector/pseudoscalar for bottom mesons.
91 Inside each light-quark meson nonet, an octet-singlet mixing angle
92 describes the mixing of the two flavour-diagonal isoscalar = 0 states.
93 (For terminology and details see [<a href="Bibliography.html" target="page">Yao06</a>], chapter 14 on the
95 This angle is needed to specify the probability for such a <i>q qbar</i>
96 state to project onto a specific meson. More transparent formuale are
97 obtained by introducing the angle <i>alpha = theta + 54.7</i> degrees:
99 f = (uubar + ddbar)/sqrt(2) * sin(alpha) + ssbar * cos(alpha)<br/>
100 f' = (uubar + ddbar)/sqrt(2) * cos(alpha) - ssbar * sin(alpha)
103 <p/><code>parm </code><strong> StringFlav:thetaPS </strong>
104 (<code>default = <strong>-15.</strong></code>; <code>minimum = -90.</code>; <code>maximum = 90.</code>)<br/>
105 gives the mixing angle <i>theta_PS</i> in the pseudoscalar meson sector
106 (which is rather poorly determined), expressed in degrees.
107 Here <i>f</i> is associated with <i>eta'</i> and <i>f'</i> with
108 <i>eta</i>. (This standard but counterintuitive choice is fixed up
109 in the code by replacing <i>alpha -> 90^0 - alpha</i> so that
110 <i>eta <-> eta'</i>; relative signs do not matter since we are
111 interested in probabilities only.)
114 <p/><code>parm </code><strong> StringFlav:thetaV </strong>
115 (<code>default = <strong>36.</strong></code>; <code>minimum = -90.</code>; <code>maximum = 90.</code>)<br/>
116 gives the mixing angle <i>theta_V</i> in the vector meson sector
117 (which is somewhat better determined), expressed in degrees.
118 Here <i>f</i> is associated with <i>omega</i> and <i>f'</i>
123 Further, the simple model overestimates the production of <i>eta</i>
124 and, in particular, <i>eta'</i> mesons, which can be rectified by
126 <p/><code>parm </code><strong> StringFlav:etaSup </strong>
127 (<code>default = <strong>0.63</strong></code>; <code>minimum = 0.</code>; <code>maximum = 1.</code>)<br/>
128 the additional suppression of <i>eta</i> production, multiplying the
129 normal production probability. Thus 0 means no <i>eta</i> at all
130 are produced, while 1 means full rate.
133 <p/><code>parm </code><strong> StringFlav:etaPrimeSup </strong>
134 (<code>default = <strong>0.12</strong></code>; <code>minimum = 0.</code>; <code>maximum = 1.</code>)<br/>
135 the additional suppression of <i>eta'</i> production, multiplying the
136 normal production probability. Thus 0 means no <i>eta'</i> at all
137 are produced, while 1 means full rate.
140 <h3>Excited-meson production</h3>
142 Several excited mesons, ie. with radial or orbital excitations, have been
143 observed at non-negligible production rates. Extrapolated to all states
144 a fair fraction of all particle production might proceed through such
145 states. There are big uncertainties, however, since these excited
146 mesons in many cases are extremely poorly known. This also means that
147 the modelling of their production and decay is very primitive, and
148 even that the inclusion of the production of such states may lead to a
149 degraded agreement with data. Currently the default is that all such
150 production is switched off.
153 Parameters are provided to switch them on. By demand, this machinery
154 has been made more flexible than in the past. Therefore one parameter is
155 provided for each combination of heaviest flavour
156 (<i>u/d</i>, <i>s</i>, <i>c</i> or <i>b</i>) and
157 multiplet produced. In each case the production rate is normalized to
158 that of the lowest-lying pseudoscalar of the same flavour content, as for
159 the vector-meson rates introduced above. The multiplets available are the
160 four obtained for one unit of orbital angular momentum, in the
161 nonrelativistic classification. Using <i>J</i> to denote the sum of
162 quark spin <i>S</i> and orbital angular momentum <i>L</i>, i.e. what
163 would normally be called the spin of the meson, one has:
165 <li>a pseudovector multiplet with <i>L=1, S=0, J=1</i>;</li>
166 <li>a scalar multiplet with <i>L=1, S=1, J=0</i>;</li>
167 <li>a pseudovector multiplet with <i>L=1, S=1, J=1</i>;</li>
168 <li>a tensor multiplet with <i>L=1, S=1, J=2</i>.</li>
171 The maximum allowed rate for each case has been set according to
172 spin-counting rules, but we expect the real rates to be significantly
173 lower, owing to mass suppression.
175 <p/><code>parm </code><strong> StringFlav:mesonUDL1S0J1 </strong>
176 (<code>default = <strong>0.0</strong></code>; <code>minimum = 0.</code>; <code>maximum = 3.</code>)<br/>
177 the relative pseudovector production ratio
178 <i>(L=1,S=0,J=1)</i>/pseudoscalar
179 for light (<i>u</i>, <i>d</i>) mesons.
182 <p/><code>parm </code><strong> StringFlav:mesonUDL1S1J0 </strong>
183 (<code>default = <strong>0.0</strong></code>; <code>minimum = 0.</code>; <code>maximum = 1.</code>)<br/>
184 the relative scalar production ratio
185 <i>(L=1,S=1,J=0)</i>/pseudoscalar
186 for light (<i>u</i>, <i>d</i>) mesons.
189 <p/><code>parm </code><strong> StringFlav:mesonUDL1S1J1 </strong>
190 (<code>default = <strong>0.0</strong></code>; <code>minimum = 0.</code>; <code>maximum = 3.</code>)<br/>
191 the relative pseudovector production ratio
192 <i>(L=1,S=1,J=1)</i>/pseudoscalar
193 for light (<i>u</i>, <i>d</i>) mesons.
196 <p/><code>parm </code><strong> StringFlav:mesonUDL1S1J2 </strong>
197 (<code>default = <strong>0.0</strong></code>; <code>minimum = 0.</code>; <code>maximum = 5.</code>)<br/>
198 the relative tensor production ratio
199 <i>(L=1,S=1,J=2)</i>/pseudoscalar
200 for light (<i>u</i>, <i>d</i>) mesons.
203 <p/><code>parm </code><strong> StringFlav:mesonSL1S0J1 </strong>
204 (<code>default = <strong>0.0</strong></code>; <code>minimum = 0.</code>; <code>maximum = 3.</code>)<br/>
205 the relative pseudovector production ratio
206 <i>(L=1,S=0,J=1)</i>/pseudoscalar
210 <p/><code>parm </code><strong> StringFlav:mesonSL1S1J0 </strong>
211 (<code>default = <strong>0.0</strong></code>; <code>minimum = 0.</code>; <code>maximum = 1.</code>)<br/>
212 the relative scalar production ratio
213 <i>(L=1,S=1,J=0)</i>/pseudoscalar
217 <p/><code>parm </code><strong> StringFlav:mesonSL1S1J1 </strong>
218 (<code>default = <strong>0.0</strong></code>; <code>minimum = 0.</code>; <code>maximum = 3.</code>)<br/>
219 the relative pseudovector production ratio
220 <i>(L=1,S=1,J=1)</i>/pseudoscalar
224 <p/><code>parm </code><strong> StringFlav:mesonSL1S1J2 </strong>
225 (<code>default = <strong>0.0</strong></code>; <code>minimum = 0.</code>; <code>maximum = 5.</code>)<br/>
226 the relative tensor production ratio
227 <i>(L=1,S=1,J=2)</i>/pseudoscalar
231 <p/><code>parm </code><strong> StringFlav:mesonCL1S0J1 </strong>
232 (<code>default = <strong>0.0</strong></code>; <code>minimum = 0.</code>; <code>maximum = 3.</code>)<br/>
233 the relative pseudovector production ratio
234 <i>(L=1,S=0,J=1)</i>/pseudoscalar
238 <p/><code>parm </code><strong> StringFlav:mesonCL1S1J0 </strong>
239 (<code>default = <strong>0.0</strong></code>; <code>minimum = 0.</code>; <code>maximum = 1.</code>)<br/>
240 the relative scalar production ratio
241 <i>(L=1,S=1,J=0)</i>/pseudoscalar
245 <p/><code>parm </code><strong> StringFlav:mesonCL1S1J1 </strong>
246 (<code>default = <strong>0.0</strong></code>; <code>minimum = 0.</code>; <code>maximum = 3.</code>)<br/>
247 the relative pseudovector production ratio
248 <i>(L=1,S=1,J=1)</i>/pseudoscalar
252 <p/><code>parm </code><strong> StringFlav:mesonCL1S1J2 </strong>
253 (<code>default = <strong>0.0</strong></code>; <code>minimum = 0.</code>; <code>maximum = 5.</code>)<br/>
254 the relative tensor production ratio
255 <i>(L=1,S=1,J=2)</i>/pseudoscalar
259 <p/><code>parm </code><strong> StringFlav:mesonBL1S0J1 </strong>
260 (<code>default = <strong>0.0</strong></code>; <code>minimum = 0.</code>; <code>maximum = 3.</code>)<br/>
261 the relative pseudovector production ratio
262 <i>(L=1,S=0,J=1)</i>/pseudoscalar
266 <p/><code>parm </code><strong> StringFlav:mesonBL1S1J0 </strong>
267 (<code>default = <strong>0.0</strong></code>; <code>minimum = 0.</code>; <code>maximum = 1.</code>)<br/>
268 the relative scalar production ratio
269 <i>(L=1,S=1,J=0)</i>/pseudoscalar
273 <p/><code>parm </code><strong> StringFlav:mesonBL1S1J1 </strong>
274 (<code>default = <strong>0.0</strong></code>; <code>minimum = 0.</code>; <code>maximum = 3.</code>)<br/>
275 the relative pseudovector production ratio
276 <i>(L=1,S=1,J=1)</i>/pseudoscalar
280 <p/><code>parm </code><strong> StringFlav:mesonBL1S1J2 </strong>
281 (<code>default = <strong>0.0</strong></code>; <code>minimum = 0.</code>; <code>maximum = 5.</code>)<br/>
282 the relative tensor production ratio
283 <i>(L=1,S=1,J=2)</i>/pseudoscalar
288 In addition, an octet-singlet mixing angle is needed for each multiplet,
289 as for the pseudoscalar and vector multiplets above. Only for the
290 tensor multiplet does any determination exist; for the other multiplets
291 default has been chose so that <i>ssbar</i> does not mix with the light
292 quarks, and so that the <i>ssbar</i> state is the heavier of the two.
294 <p/><code>parm </code><strong> StringFlav:thetaL1S0J1 </strong>
295 (<code>default = <strong>35.3</strong></code>; <code>minimum = -90.</code>; <code>maximum = 90.</code>)<br/>
296 gives the mixing angle <i>theta</i> in the <i>(L=1,S=0,J=1)</i>
297 pseudovector meson sector, expressed in degrees.
300 <p/><code>parm </code><strong> StringFlav:thetaL1S1J0 </strong>
301 (<code>default = <strong>35.3</strong></code>; <code>minimum = -90.</code>; <code>maximum = 90.</code>)<br/>
302 gives the mixing angle <i>theta</i> in the <i>(L=1,S=1,J=0)</i>
303 scalar meson sector, expressed in degrees.
306 <p/><code>parm </code><strong> StringFlav:thetaL1S1J1 </strong>
307 (<code>default = <strong>35.3</strong></code>; <code>minimum = -90.</code>; <code>maximum = 90.</code>)<br/>
308 gives the mixing angle <i>theta</i> in the <i>(L=1,S=1,J=1)</i>
309 pseudovector meson sector, expressed in degrees.
312 <p/><code>parm </code><strong> StringFlav:thetaL1S1J2 </strong>
313 (<code>default = <strong>28.0</strong></code>; <code>minimum = -90.</code>; <code>maximum = 90.</code>)<br/>
314 gives the mixing angle <i>theta</i> in the <i>(L=1,S=1,J=2)</i>
315 tensor meson sector, expressed in degrees.
318 <h3>Baryon production</h3>
320 The relative rate of baryon production is mainly given by the quark
321 and diquark production parameters above, plus SU(6) Clebsch-Gordans.
322 The one modifiable parameter related to these coefficients is
324 <p/><code>parm </code><strong> StringFlav:decupletSup </strong>
325 (<code>default = <strong>1.0</strong></code>; <code>minimum = 0.0</code>; <code>maximum = 1.0</code>)<br/>
326 the suppression, relative to default SU(6) factors, of decuplet
327 baryon production. Default corresponds to no suppression, while 0
328 corresponds to no decuplet production at all.
332 In addition, if popcorn production is allowed, wherein a set of mesons
333 (<i>M</i>) may be producted in between the baryon (<i>B</i>) and
334 the antibaryon (<i>Bbar</i>), a set of further parameters is introduced.
335 Currently only the simplest scenario is implemented, wherein at most
336 one intermediate meson may be produced.
338 <p/><code>parm </code><strong> StringFlav:popcornRate </strong>
339 (<code>default = <strong>0.5</strong></code>; <code>minimum = 0.</code>; <code>maximum = 2.0</code>)<br/>
340 gives the relative rates of <i>B Bbar</i> and <i>B M Bbar</i>
341 production, roughly as
343 Prob(B M Bbar) / (Prob(B Bbar) + Prob(B M Bbar)) =
344 popcornRate / (0.5 + popcornRate)
346 (the complete expression depends on all the quark and diquark production
347 parameters and is therefore not so useful).
350 <p/><code>parm </code><strong> StringFlav:popcornSpair </strong>
351 (<code>default = <strong>0.5</strong></code>; <code>minimum = 0.</code>; <code>maximum = 1.0</code>)<br/>
352 extra suppression for having an <i>s sbar</i> pair shared between
353 the <i>B</i> and <i>Bbar</i> in a <i>B M Bbar</i> configuration.
356 <p/><code>parm </code><strong> StringFlav:popcornSmeson </strong>
357 (<code>default = <strong>0.5</strong></code>; <code>minimum = 0.</code>; <code>maximum = 1.0</code>)<br/>
358 extra suppression for having a strange meson <i>M</i> in a
359 <i>B M Bbar</i> configuration.
363 Finally, there are some indications that leading-baryon production
364 may be further suppressed. A proper description should probably be
365 based on a suppression of early production times [<a href="Bibliography.html" target="page">Ede97</a>],
366 but we here only implement a simpler version where production near
367 the end of a string, as defined by rank, is suppressed. The more
368 detailed studies suggest that leading <i>c</i> and <i>b</i> baryon
369 production will be less suppressed, so we leave it open to set
370 light- and heavy-baryon suppression separately.
372 <p/><code>flag </code><strong> StringFlav:suppressLeadingB </strong>
373 (<code>default = <strong>off</strong></code>)<br/>
374 Suppress leading-baryon production.
375 <br/><code>option </code><strong> off</strong> : No suppression.
376 <br/><code>option </code><strong> on</strong> : Suppress the production of a diquark in the string
377 breaking closest to a quark end of a string, by either of the factors
378 below. This suppresses the production of first-rank baryons by the same
379 amount. Indirectly also the second-rank and, if popcorn production is
380 switched on, third-rank (anti)baryon production is affected.
383 <p/><code>parm </code><strong> StringFlav:lightLeadingBSup </strong>
384 (<code>default = <strong>0.5</strong></code>; <code>minimum = 0.</code>; <code>maximum = 1.0</code>)<br/>
385 extra suppression of leading-baryon production for a light-quark
386 jet, i.e. <i>d</i>, <i>u</i> or <i>s</i>, when
387 <code>suppressLeadingB = on</code>. Thus 0 means no leading-baryon
388 production at all, while 1 means full rate.
391 <p/><code>parm </code><strong> StringFlav:heavyLeadingBSup </strong>
392 (<code>default = <strong>0.9</strong></code>; <code>minimum = 0.</code>; <code>maximum = 1.0</code>)<br/>
393 extra suppression of leading-baryon production for a heavy-quark
394 jet, i.e. <i>c</i> or <i>b</i>, when
395 <code>suppressLeadingB = on</code>. Thus 0 means no leading-baryon
396 production at all, while 1 means full rate.
402 <!-- Copyright (C) 2010 Torbjorn Sjostrand -->