1 <chapter name="Flavour Selection">
3 <h2>Flavour Selection</h2>
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.
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>.
23 The main parameters of the selection of a new flavour are
25 <parm name="StringFlav:probStoUD" default="0.19" 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.
30 <parm name="StringFlav:probQQtoQ" default="0.09" 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.
35 <parm name="StringFlav:probSQtoQQ" default="1.00" 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>.
41 <parm name="StringFlav:probQQ1toQQ0" default="0.027" 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
47 <h3>Standard-meson production</h3>
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.
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
62 <parm name="StringFlav:mesonUDvector" default="0.62" min="0." max="3.">
63 the relative production ratio vector/pseudoscalar for light
64 (<ei>u</ei>, <ei>d</ei>) mesons.
66 <parm name="StringFlav:mesonSvector" default="0.725" min="0." max="3.">
67 the relative production ratio vector/pseudoscalar for strange mesons.
69 <parm name="StringFlav:mesonCvector" default="1.06" min="0." max="3.">
70 the relative production ratio vector/pseudoscalar for charm mesons.
72 <parm name="StringFlav:mesonBvector" default="3.0" min="0." max="3.">
73 the relative production ratio vector/pseudoscalar for bottom mesons.
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
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:
85 f = (uubar + ddbar)/sqrt(2) * sin(alpha) + ssbar * cos(alpha)<br/>
86 f' = (uubar + ddbar)/sqrt(2) * cos(alpha) - ssbar * sin(alpha)
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.)
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>
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
110 <parm name="StringFlav:etaSup" default="0.63" 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.
116 <parm name="StringFlav:etaPrimeSup" default="0.12" 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.
122 <h3>Excited-meson production</h3>
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.
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:
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>
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.
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.
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.
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.
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.
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
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
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
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
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
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
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
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
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
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
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
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
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.
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.
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.
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.
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.
280 <h3>Baryon production</h3>
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
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.
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.
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
303 Prob(B M Bbar) / (Prob(B Bbar) + Prob(B M Bbar)) =
304 popcornRate / (0.5 + popcornRate)
306 (the complete expression depends on all the quark and diquark production
307 parameters and is therefore not so useful).
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.
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.
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.
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>
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.
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.
356 <!-- Copyright (C) 2010 Torbjorn Sjostrand -->