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