--- /dev/null
+<chapter name="Flavour Selection">
+
+<h2>Flavour Selection</h2>
+
+The <code>StringFlav</code> class handles the choice of a new flavour
+in the fragmentation process, and the production of a new hadron
+from a set of input flavours. It is mainly used by the string
+fragmentation machinery (including ministrings), but also e.g.
+in some particle decays and for some beam-remnant cases. The basic
+concepts are in agreement with <ref>And83</ref>. The baryon-sector
+implementation is based on the <code>MSTJ(12)=3</code> option of
+PYTHIA 6, i.e. new SU(6) weights scheme with at most one popcorn meson.
+
+<p/>
+The relative production rates of different particle species is
+influenced by the parameters below. Some have only an impact on
+one specific quantity, but most directly or indirectly have
+consequences for many observables. Therefore the values to use have
+to be viewed in the context of a complete <aloc href="Tunes">tune</aloc>.
+
+<h3>New flavours</h3>
+
+The main parameters of the selection of a new flavour are
+
+<parm name="StringFlav:probStoUD" default="0.19" min="0.0" max="1.0">
+the suppression of <ei>s</ei> quark production relative to ordinary
+<ei>u</ei> or <ei>d</ei> one.
+</parm>
+
+<parm name="StringFlav:probQQtoQ" default="0.09" min="0.0" max="1.0">
+the suppression of diquark production relative to quark production,
+i.e. of baryon relative to meson production.
+</parm>
+
+<parm name="StringFlav:probSQtoQQ" default="1.00" min="0.0" max="1.0">
+the suppression of strange diquark production relative to light
+diquark production, over and above the one already given by
+<code>probStoU</code>.
+</parm>
+
+<parm name="StringFlav:probQQ1toQQ0" default="0.027" min="0.0" max="1.0">
+the suppression of spin 1 diquark production relative to spin 0 one,
+apart from the factor of 3 enhancement of spin 1 from counting the
+number of states.
+</parm>
+
+<h3>Standard-meson production</h3>
+
+The bulk of the particle production corresponds to the lowest-lying
+pseudoscalar and vector multiplets. Their production rates are
+determined by the parameters in this section.
+
+<p/>
+For a given set of flavours, produced according to the probabilities
+outlined above, the ratio of vector-to-pseudocalar meson production
+is described by the parameters below.
+The maximum allowed rate for each case has been set according to
+spin-counting rules, but we expect the real rates to be lower,
+especially for lighter mesons, owing to the vector-pseudoscalar
+mass splitting.
+
+<parm name="StringFlav:mesonUDvector" default="0.62" min="0." max="3.">
+the relative production ratio vector/pseudoscalar for light
+(<ei>u</ei>, <ei>d</ei>) mesons.
+</parm>
+<parm name="StringFlav:mesonSvector" default="0.725" min="0." max="3.">
+the relative production ratio vector/pseudoscalar for strange mesons.
+</parm>
+<parm name="StringFlav:mesonCvector" default="1.06" min="0." max="3.">
+the relative production ratio vector/pseudoscalar for charm mesons.
+</parm>
+<parm name="StringFlav:mesonBvector" default="3.0" min="0." max="3.">
+the relative production ratio vector/pseudoscalar for bottom mesons.
+</parm>
+
+<p/>
+Inside each light-quark meson nonet, an octet-singlet mixing angle
+describes the mixing of the two flavour-diagonal isoscalar = 0 states.
+(For terminology and details see <ref>Yao06</ref>, chapter 14 on the
+quark model.)
+This angle is needed to specify the probability for such a <ei>q qbar</ei>
+state to project onto a specific meson. More transparent formuale are
+obtained by introducing the angle <ei>alpha = theta + 54.7</ei> degrees:
+<eq>
+ f = (uubar + ddbar)/sqrt(2) * sin(alpha) + ssbar * cos(alpha)<br/>
+ f' = (uubar + ddbar)/sqrt(2) * cos(alpha) - ssbar * sin(alpha)
+</eq>
+
+<parm name="StringFlav:thetaPS" default="-15." min="-90." max="90.">
+gives the mixing angle <ei>theta_PS</ei> in the pseudoscalar meson sector
+(which is rather poorly determined), expressed in degrees.
+Here <ei>f</ei> is associated with <ei>eta'</ei> and <ei>f'</ei> with
+<ei>eta</ei>. (This standard but counterintuitive choice is fixed up
+in the code by replacing <ei>alpha -> 90^0 - alpha</ei> so that
+<ei>eta <-> eta'</ei>; relative signs do not matter since we are
+interested in probabilities only.)
+</parm>
+
+<parm name="StringFlav:thetaV" default="36." min="-90." max="90.">
+gives the mixing angle <ei>theta_V</ei> in the vector meson sector
+(which is somewhat better determined), expressed in degrees.
+Here <ei>f</ei> is associated with <ei>omega</ei> and <ei>f'</ei>
+with <ei>phi</ei>.
+</parm>
+
+<p/>
+Further, the simple model overestimates the production of <ei>eta</ei>
+and, in particular, <ei>eta'</ei> mesons, which can be rectified by
+
+<parm name="StringFlav:etaSup" default="0.63" min="0." max="1.">
+the additional suppression of <ei>eta</ei> production, multiplying the
+normal production probability. Thus 0 means no <ei>eta</ei> at all
+are produced, while 1 means full rate.
+</parm>
+
+<parm name="StringFlav:etaPrimeSup" default="0.12" min="0." max="1.">
+the additional suppression of <ei>eta'</ei> production, multiplying the
+normal production probability. Thus 0 means no <ei>eta'</ei> at all
+are produced, while 1 means full rate.
+</parm>
+
+<h3>Excited-meson production</h3>
+
+Several excited mesons, ie. with radial or orbital excitations, have been
+observed at non-negligible production rates. Extrapolated to all states
+a fair fraction of all particle production might proceed through such
+states. There are big uncertainties, however, since these excited
+mesons in many cases are extremely poorly known. This also means that
+the modelling of their production and decay is very primitive, and
+even that the inclusion of the production of such states may lead to a
+degraded agreement with data. Currently the default is that all such
+production is switched off.
+
+<p/>
+Parameters are provided to switch them on. By demand, this machinery
+has been made more flexible than in the past. Therefore one parameter is
+provided for each combination of heaviest flavour
+(<ei>u/d</ei>, <ei>s</ei>, <ei>c</ei> or <ei>b</ei>) and
+multiplet produced. In each case the production rate is normalized to
+that of the lowest-lying pseudoscalar of the same flavour content, as for
+the vector-meson rates introduced above. The multiplets available are the
+four obtained for one unit of orbital angular momentum, in the
+nonrelativistic classification. Using <ei>J</ei> to denote the sum of
+quark spin <ei>S</ei> and orbital angular momentum <ei>L</ei>, i.e. what
+would normally be called the spin of the meson, one has:
+<ul>
+<li>a pseudovector multiplet with <ei>L=1, S=0, J=1</ei>;</li>
+<li>a scalar multiplet with <ei>L=1, S=1, J=0</ei>;</li>
+<li>a pseudovector multiplet with <ei>L=1, S=1, J=1</ei>;</li>
+<li>a tensor multiplet with <ei>L=1, S=1, J=2</ei>.</li>
+</ul>
+
+The maximum allowed rate for each case has been set according to
+spin-counting rules, but we expect the real rates to be significantly
+lower, owing to mass suppression.
+
+<parm name="StringFlav:mesonUDL1S0J1" default="0.0" min="0." max="3.">
+the relative pseudovector production ratio
+<ei>(L=1,S=0,J=1)</ei>/pseudoscalar
+for light (<ei>u</ei>, <ei>d</ei>) mesons.
+</parm>
+
+<parm name="StringFlav:mesonUDL1S1J0" default="0.0" min="0." max="1.">
+the relative scalar production ratio
+<ei>(L=1,S=1,J=0)</ei>/pseudoscalar
+for light (<ei>u</ei>, <ei>d</ei>) mesons.
+</parm>
+
+<parm name="StringFlav:mesonUDL1S1J1" default="0.0" min="0." max="3.">
+the relative pseudovector production ratio
+<ei>(L=1,S=1,J=1)</ei>/pseudoscalar
+for light (<ei>u</ei>, <ei>d</ei>) mesons.
+</parm>
+
+<parm name="StringFlav:mesonUDL1S1J2" default="0.0" min="0." max="5.">
+the relative tensor production ratio
+<ei>(L=1,S=1,J=2)</ei>/pseudoscalar
+for light (<ei>u</ei>, <ei>d</ei>) mesons.
+</parm>
+
+<parm name="StringFlav:mesonSL1S0J1" default="0.0" min="0." max="3.">
+the relative pseudovector production ratio
+<ei>(L=1,S=0,J=1)</ei>/pseudoscalar
+for strange mesons.
+</parm>
+
+<parm name="StringFlav:mesonSL1S1J0" default="0.0" min="0." max="1.">
+the relative scalar production ratio
+<ei>(L=1,S=1,J=0)</ei>/pseudoscalar
+for strange mesons.
+</parm>
+
+<parm name="StringFlav:mesonSL1S1J1" default="0.0" min="0." max="3.">
+the relative pseudovector production ratio
+<ei>(L=1,S=1,J=1)</ei>/pseudoscalar
+for strange mesons.
+</parm>
+
+<parm name="StringFlav:mesonSL1S1J2" default="0.0" min="0." max="5.">
+the relative tensor production ratio
+<ei>(L=1,S=1,J=2)</ei>/pseudoscalar
+for strange mesons.
+</parm>
+
+<parm name="StringFlav:mesonCL1S0J1" default="0.0" min="0." max="3.">
+the relative pseudovector production ratio
+<ei>(L=1,S=0,J=1)</ei>/pseudoscalar
+for charm mesons.
+</parm>
+
+<parm name="StringFlav:mesonCL1S1J0" default="0.0" min="0." max="1.">
+the relative scalar production ratio
+<ei>(L=1,S=1,J=0)</ei>/pseudoscalar
+for charm mesons.
+</parm>
+
+<parm name="StringFlav:mesonCL1S1J1" default="0.0" min="0." max="3.">
+the relative pseudovector production ratio
+<ei>(L=1,S=1,J=1)</ei>/pseudoscalar
+for charm mesons.
+</parm>
+
+<parm name="StringFlav:mesonCL1S1J2" default="0.0" min="0." max="5.">
+the relative tensor production ratio
+<ei>(L=1,S=1,J=2)</ei>/pseudoscalar
+for charm mesons.
+</parm>
+
+<parm name="StringFlav:mesonBL1S0J1" default="0.0" min="0." max="3.">
+the relative pseudovector production ratio
+<ei>(L=1,S=0,J=1)</ei>/pseudoscalar
+for bottom mesons.
+</parm>
+
+<parm name="StringFlav:mesonBL1S1J0" default="0.0" min="0." max="1.">
+the relative scalar production ratio
+<ei>(L=1,S=1,J=0)</ei>/pseudoscalar
+for bottom mesons.
+</parm>
+
+<parm name="StringFlav:mesonBL1S1J1" default="0.0" min="0." max="3.">
+the relative pseudovector production ratio
+<ei>(L=1,S=1,J=1)</ei>/pseudoscalar
+for bottom mesons.
+</parm>
+
+<parm name="StringFlav:mesonBL1S1J2" default="0.0" min="0." max="5.">
+the relative tensor production ratio
+<ei>(L=1,S=1,J=2)</ei>/pseudoscalar
+for bottom mesons.
+</parm>
+
+<p/>
+In addition, an octet-singlet mixing angle is needed for each multiplet,
+as for the pseudoscalar and vector multiplets above. Only for the
+tensor multiplet does any determination exist; for the other multiplets
+default has been chose so that <ei>ssbar</ei> does not mix with the light
+quarks, and so that the <ei>ssbar</ei> state is the heavier of the two.
+
+<parm name="StringFlav:thetaL1S0J1" default="35.3" min="-90." max="90.">
+gives the mixing angle <ei>theta</ei> in the <ei>(L=1,S=0,J=1)</ei>
+pseudovector meson sector, expressed in degrees.
+</parm>
+
+<parm name="StringFlav:thetaL1S1J0" default="35.3" min="-90." max="90.">
+gives the mixing angle <ei>theta</ei> in the <ei>(L=1,S=1,J=0)</ei>
+scalar meson sector, expressed in degrees.
+</parm>
+
+<parm name="StringFlav:thetaL1S1J1" default="35.3" min="-90." max="90.">
+gives the mixing angle <ei>theta</ei> in the <ei>(L=1,S=1,J=1)</ei>
+pseudovector meson sector, expressed in degrees.
+</parm>
+
+<parm name="StringFlav:thetaL1S1J2" default="28.0" min="-90." max="90.">
+gives the mixing angle <ei>theta</ei> in the <ei>(L=1,S=1,J=2)</ei>
+tensor meson sector, expressed in degrees.
+</parm>
+
+<h3>Baryon production</h3>
+
+The relative rate of baryon production is mainly given by the quark
+and diquark production parameters above, plus SU(6) Clebsch-Gordans.
+The one modifiable parameter related to these coefficients is
+
+<parm name="StringFlav:decupletSup" default="1.0" min="0.0" max="1.0">
+the suppression, relative to default SU(6) factors, of decuplet
+baryon production. Default corresponds to no suppression, while 0
+corresponds to no decuplet production at all.
+</parm>
+
+<p/>
+In addition, if popcorn production is allowed, wherein a set of mesons
+(<ei>M</ei>) may be producted in between the baryon (<ei>B</ei>) and
+the antibaryon (<ei>Bbar</ei>), a set of further parameters is introduced.
+Currently only the simplest scenario is implemented, wherein at most
+one intermediate meson may be produced.
+
+<parm name="StringFlav:popcornRate" default="0.5" min="0." max="2.0">
+gives the relative rates of <ei>B Bbar</ei> and <ei>B M Bbar</ei>
+production, roughly as
+<eq>
+Prob(B M Bbar) / (Prob(B Bbar) + Prob(B M Bbar)) =
+popcornRate / (0.5 + popcornRate)
+</eq>
+(the complete expression depends on all the quark and diquark production
+parameters and is therefore not so useful).
+</parm>
+
+<parm name="StringFlav:popcornSpair" default="0.5" min="0." max="1.0">
+extra suppression for having an <ei>s sbar</ei> pair shared between
+the <ei>B</ei> and <ei>Bbar</ei> in a <ei>B M Bbar</ei> configuration.
+</parm>
+
+<parm name="StringFlav:popcornSmeson" default="0.5" min="0." max="1.0">
+extra suppression for having a strange meson <ei>M</ei> in a
+<ei>B M Bbar</ei> configuration.
+</parm>
+
+<p/>
+Finally, there are some indications that leading-baryon production
+may be further suppressed. A proper description should probably be
+based on a suppression of early production times <ref>Ede97</ref>,
+but we here only implement a simpler version where production near
+the end of a string, as defined by rank, is suppressed. The more
+detailed studies suggest that leading <ei>c</ei> and <ei>b</ei> baryon
+production will be less suppressed, so we leave it open to set
+light- and heavy-baryon suppression separately.
+
+<flag name="StringFlav:suppressLeadingB" default="off">
+Suppress leading-baryon production.
+<option value="off">No suppression.</option>
+<option value="on">Suppress the production of a diquark in the string
+breaking closest to a quark end of a string, by either of the factors
+below. This suppresses the production of first-rank baryons by the same
+amount. Indirectly also the second-rank and, if popcorn production is
+switched on, third-rank (anti)baryon production is affected. </option>
+</flag>
+
+<parm name="StringFlav:lightLeadingBSup" default="0.5" min="0." max="1.0">
+extra suppression of leading-baryon production for a light-quark
+jet, i.e. <ei>d</ei>, <ei>u</ei> or <ei>s</ei>, when
+<code>suppressLeadingB = on</code>. Thus 0 means no leading-baryon
+production at all, while 1 means full rate.
+</parm>
+
+<parm name="StringFlav:heavyLeadingBSup" default="0.9" min="0." max="1.0">
+extra suppression of leading-baryon production for a heavy-quark
+jet, i.e. <ei>c</ei> or <ei>b</ei>, when
+<code>suppressLeadingB = on</code>. Thus 0 means no leading-baryon
+production at all, while 1 means full rate.
+</parm>
+
+</chapter>
+
+<!-- Copyright (C) 2012 Torbjorn Sjostrand -->
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