[1] | 1 | <chapter name="Flavour Selection"> |
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| 2 | |
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| 3 | <h2>Flavour Selection</h2> |
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| 4 | |
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| 5 | The <code>StringFlav</code> class handles the choice of a new flavour |
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| 6 | in the fragmentation process, and the production of a new hadron |
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| 7 | from a set of input flavours. It is mainly used by the string |
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| 8 | fragmentation machinery (including ministrings), but also e.g. |
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| 9 | in some particle decays and for some beam-remnant cases. The basic |
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| 10 | concepts are in agreement with <ref>And83</ref>. The baryon-sector |
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| 11 | implementation is based on the <code>MSTJ(12)=3</code> option of |
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| 12 | PYTHIA 6, i.e. new SU(6) weights scheme with at most one popcorn meson. |
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| 13 | |
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| 14 | <p/> |
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| 15 | The relative production rates of different particle species is |
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| 16 | influenced by the parameters below. Some have only an impact on |
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| 17 | one specific quantity, but most directly or indirectly have |
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| 18 | consequences for many observables. Therefore the values to use have |
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| 19 | to be viewed in the context of a complete <aloc href="Tunes">tune</aloc>. |
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| 20 | |
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| 21 | <h3>New flavours</h3> |
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| 22 | |
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| 23 | The main parameters of the selection of a new flavour are |
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| 24 | |
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| 25 | <parm name="StringFlav:probStoUD" default="0.19" min="0.0" max="1.0"> |
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| 26 | the suppression of <ei>s</ei> quark production relative to ordinary |
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| 27 | <ei>u</ei> or <ei>d</ei> one. |
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| 28 | </parm> |
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| 29 | |
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| 30 | <parm name="StringFlav:probQQtoQ" default="0.09" min="0.0" max="1.0"> |
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| 31 | the suppression of diquark production relative to quark production, |
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| 32 | i.e. of baryon relative to meson production. |
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| 33 | </parm> |
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| 34 | |
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| 35 | <parm name="StringFlav:probSQtoQQ" default="1.00" min="0.0" max="1.0"> |
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| 36 | the suppression of strange diquark production relative to light |
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| 37 | diquark production, over and above the one already given by |
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| 38 | <code>probStoU</code>. |
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| 39 | </parm> |
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| 40 | |
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| 41 | <parm name="StringFlav:probQQ1toQQ0" default="0.027" min="0.0" max="1.0"> |
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| 42 | the suppression of spin 1 diquark production relative to spin 0 one, |
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| 43 | apart from the factor of 3 enhancement of spin 1 from counting the |
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| 44 | number of states. |
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| 45 | </parm> |
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| 46 | |
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| 47 | <h3>Standard-meson production</h3> |
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| 48 | |
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| 49 | The bulk of the particle production corresponds to the lowest-lying |
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| 50 | pseudoscalar and vector multiplets. Their production rates are |
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| 51 | determined by the parameters in this section. |
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| 52 | |
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| 53 | <p/> |
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| 54 | For a given set of flavours, produced according to the probabilities |
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| 55 | outlined above, the ratio of vector-to-pseudocalar meson production |
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| 56 | is described by the parameters below. |
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| 57 | The maximum allowed rate for each case has been set according to |
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| 58 | spin-counting rules, but we expect the real rates to be lower, |
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| 59 | especially for lighter mesons, owing to the vector-pseudoscalar |
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| 60 | mass splitting. |
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| 61 | |
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| 62 | <parm name="StringFlav:mesonUDvector" default="0.62" min="0." max="3."> |
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| 63 | the relative production ratio vector/pseudoscalar for light |
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| 64 | (<ei>u</ei>, <ei>d</ei>) mesons. |
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| 65 | </parm> |
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| 66 | <parm name="StringFlav:mesonSvector" default="0.725" min="0." max="3."> |
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| 67 | the relative production ratio vector/pseudoscalar for strange mesons. |
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| 68 | </parm> |
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| 69 | <parm name="StringFlav:mesonCvector" default="1.06" min="0." max="3."> |
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| 70 | the relative production ratio vector/pseudoscalar for charm mesons. |
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| 71 | </parm> |
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| 72 | <parm name="StringFlav:mesonBvector" default="3.0" min="0." max="3."> |
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| 73 | the relative production ratio vector/pseudoscalar for bottom mesons. |
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| 74 | </parm> |
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| 75 | |
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| 76 | <p/> |
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| 77 | Inside each light-quark meson nonet, an octet-singlet mixing angle |
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| 78 | describes the mixing of the two flavour-diagonal isoscalar = 0 states. |
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| 79 | (For terminology and details see <ref>Yao06</ref>, chapter 14 on the |
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| 80 | quark model.) |
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| 81 | This angle is needed to specify the probability for such a <ei>q qbar</ei> |
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| 82 | state to project onto a specific meson. More transparent formuale are |
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| 83 | obtained by introducing the angle <ei>alpha = theta + 54.7</ei> degrees: |
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| 84 | <eq> |
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| 85 | f = (uubar + ddbar)/sqrt(2) * sin(alpha) + ssbar * cos(alpha)<br/> |
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| 86 | f' = (uubar + ddbar)/sqrt(2) * cos(alpha) - ssbar * sin(alpha) |
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| 87 | </eq> |
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| 88 | |
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| 89 | <parm name="StringFlav:thetaPS" default="-15." min="-90." max="90."> |
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| 90 | gives the mixing angle <ei>theta_PS</ei> in the pseudoscalar meson sector |
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| 91 | (which is rather poorly determined), expressed in degrees. |
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| 92 | Here <ei>f</ei> is associated with <ei>eta'</ei> and <ei>f'</ei> with |
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| 93 | <ei>eta</ei>. (This standard but counterintuitive choice is fixed up |
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| 94 | in the code by replacing <ei>alpha -> 90^0 - alpha</ei> so that |
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| 95 | <ei>eta <-> eta'</ei>; relative signs do not matter since we are |
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| 96 | interested in probabilities only.) |
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| 97 | </parm> |
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| 98 | |
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| 99 | <parm name="StringFlav:thetaV" default="36." min="-90." max="90."> |
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| 100 | gives the mixing angle <ei>theta_V</ei> in the vector meson sector |
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| 101 | (which is somewhat better determined), expressed in degrees. |
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| 102 | Here <ei>f</ei> is associated with <ei>omega</ei> and <ei>f'</ei> |
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| 103 | with <ei>phi</ei>. |
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| 104 | </parm> |
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| 105 | |
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| 106 | <p/> |
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| 107 | Further, the simple model overestimates the production of <ei>eta</ei> |
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| 108 | and, in particular, <ei>eta'</ei> mesons, which can be rectified by |
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| 109 | |
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| 110 | <parm name="StringFlav:etaSup" default="0.63" min="0." max="1."> |
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| 111 | the additional suppression of <ei>eta</ei> production, multiplying the |
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| 112 | normal production probability. Thus 0 means no <ei>eta</ei> at all |
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| 113 | are produced, while 1 means full rate. |
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| 114 | </parm> |
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| 115 | |
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| 116 | <parm name="StringFlav:etaPrimeSup" default="0.12" min="0." max="1."> |
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| 117 | the additional suppression of <ei>eta'</ei> production, multiplying the |
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| 118 | normal production probability. Thus 0 means no <ei>eta'</ei> at all |
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| 119 | are produced, while 1 means full rate. |
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| 120 | </parm> |
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| 121 | |
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| 122 | <h3>Excited-meson production</h3> |
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| 123 | |
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| 124 | Several excited mesons, ie. with radial or orbital excitations, have been |
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| 125 | observed at non-negligible production rates. Extrapolated to all states |
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| 126 | a fair fraction of all particle production might proceed through such |
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| 127 | states. There are big uncertainties, however, since these excited |
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| 128 | mesons in many cases are extremely poorly known. This also means that |
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| 129 | the modelling of their production and decay is very primitive, and |
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| 130 | even that the inclusion of the production of such states may lead to a |
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| 131 | degraded agreement with data. Currently the default is that all such |
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| 132 | production is switched off. |
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| 133 | |
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| 134 | <p/> |
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| 135 | Parameters are provided to switch them on. By demand, this machinery |
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| 136 | has been made more flexible than in the past. Therefore one parameter is |
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| 137 | provided for each combination of heaviest flavour |
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| 138 | (<ei>u/d</ei>, <ei>s</ei>, <ei>c</ei> or <ei>b</ei>) and |
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| 139 | multiplet produced. In each case the production rate is normalized to |
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| 140 | that of the lowest-lying pseudoscalar of the same flavour content, as for |
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| 141 | the vector-meson rates introduced above. The multiplets available are the |
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| 142 | four obtained for one unit of orbital angular momentum, in the |
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| 143 | nonrelativistic classification. Using <ei>J</ei> to denote the sum of |
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| 144 | quark spin <ei>S</ei> and orbital angular momentum <ei>L</ei>, i.e. what |
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| 145 | would normally be called the spin of the meson, one has: |
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| 146 | <ul> |
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| 147 | <li>a pseudovector multiplet with <ei>L=1, S=0, J=1</ei>;</li> |
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| 148 | <li>a scalar multiplet with <ei>L=1, S=1, J=0</ei>;</li> |
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| 149 | <li>a pseudovector multiplet with <ei>L=1, S=1, J=1</ei>;</li> |
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| 150 | <li>a tensor multiplet with <ei>L=1, S=1, J=2</ei>.</li> |
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| 151 | </ul> |
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| 152 | |
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| 153 | The maximum allowed rate for each case has been set according to |
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| 154 | spin-counting rules, but we expect the real rates to be significantly |
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| 155 | lower, owing to mass suppression. |
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| 156 | |
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| 157 | <parm name="StringFlav:mesonUDL1S0J1" default="0.0" min="0." max="3."> |
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| 158 | the relative pseudovector production ratio |
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| 159 | <ei>(L=1,S=0,J=1)</ei>/pseudoscalar |
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| 160 | for light (<ei>u</ei>, <ei>d</ei>) mesons. |
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| 161 | </parm> |
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| 162 | |
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| 163 | <parm name="StringFlav:mesonUDL1S1J0" default="0.0" min="0." max="1."> |
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| 164 | the relative scalar production ratio |
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| 165 | <ei>(L=1,S=1,J=0)</ei>/pseudoscalar |
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| 166 | for light (<ei>u</ei>, <ei>d</ei>) mesons. |
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| 167 | </parm> |
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| 168 | |
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| 169 | <parm name="StringFlav:mesonUDL1S1J1" default="0.0" min="0." max="3."> |
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| 170 | the relative pseudovector production ratio |
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| 171 | <ei>(L=1,S=1,J=1)</ei>/pseudoscalar |
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| 172 | for light (<ei>u</ei>, <ei>d</ei>) mesons. |
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| 173 | </parm> |
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| 174 | |
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| 175 | <parm name="StringFlav:mesonUDL1S1J2" default="0.0" min="0." max="5."> |
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| 176 | the relative tensor production ratio |
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| 177 | <ei>(L=1,S=1,J=2)</ei>/pseudoscalar |
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| 178 | for light (<ei>u</ei>, <ei>d</ei>) mesons. |
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| 179 | </parm> |
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| 180 | |
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| 181 | <parm name="StringFlav:mesonSL1S0J1" default="0.0" min="0." max="3."> |
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| 182 | the relative pseudovector production ratio |
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| 183 | <ei>(L=1,S=0,J=1)</ei>/pseudoscalar |
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| 184 | for strange mesons. |
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| 185 | </parm> |
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| 186 | |
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| 187 | <parm name="StringFlav:mesonSL1S1J0" default="0.0" min="0." max="1."> |
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| 188 | the relative scalar production ratio |
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| 189 | <ei>(L=1,S=1,J=0)</ei>/pseudoscalar |
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| 190 | for strange mesons. |
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| 191 | </parm> |
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| 192 | |
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| 193 | <parm name="StringFlav:mesonSL1S1J1" default="0.0" min="0." max="3."> |
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| 194 | the relative pseudovector production ratio |
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| 195 | <ei>(L=1,S=1,J=1)</ei>/pseudoscalar |
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| 196 | for strange mesons. |
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| 197 | </parm> |
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| 198 | |
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| 199 | <parm name="StringFlav:mesonSL1S1J2" default="0.0" min="0." max="5."> |
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| 200 | the relative tensor production ratio |
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| 201 | <ei>(L=1,S=1,J=2)</ei>/pseudoscalar |
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| 202 | for strange mesons. |
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| 203 | </parm> |
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| 204 | |
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| 205 | <parm name="StringFlav:mesonCL1S0J1" default="0.0" min="0." max="3."> |
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| 206 | the relative pseudovector production ratio |
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| 207 | <ei>(L=1,S=0,J=1)</ei>/pseudoscalar |
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| 208 | for charm mesons. |
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| 209 | </parm> |
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| 210 | |
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| 211 | <parm name="StringFlav:mesonCL1S1J0" default="0.0" min="0." max="1."> |
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| 212 | the relative scalar production ratio |
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| 213 | <ei>(L=1,S=1,J=0)</ei>/pseudoscalar |
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| 214 | for charm mesons. |
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| 215 | </parm> |
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| 216 | |
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| 217 | <parm name="StringFlav:mesonCL1S1J1" default="0.0" min="0." max="3."> |
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| 218 | the relative pseudovector production ratio |
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| 219 | <ei>(L=1,S=1,J=1)</ei>/pseudoscalar |
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| 220 | for charm mesons. |
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| 221 | </parm> |
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| 222 | |
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| 223 | <parm name="StringFlav:mesonCL1S1J2" default="0.0" min="0." max="5."> |
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| 224 | the relative tensor production ratio |
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| 225 | <ei>(L=1,S=1,J=2)</ei>/pseudoscalar |
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| 226 | for charm mesons. |
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| 227 | </parm> |
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| 228 | |
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| 229 | <parm name="StringFlav:mesonBL1S0J1" default="0.0" min="0." max="3."> |
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| 230 | the relative pseudovector production ratio |
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| 231 | <ei>(L=1,S=0,J=1)</ei>/pseudoscalar |
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| 232 | for bottom mesons. |
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| 233 | </parm> |
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| 234 | |
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| 235 | <parm name="StringFlav:mesonBL1S1J0" default="0.0" min="0." max="1."> |
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| 236 | the relative scalar production ratio |
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| 237 | <ei>(L=1,S=1,J=0)</ei>/pseudoscalar |
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| 238 | for bottom mesons. |
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| 239 | </parm> |
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| 240 | |
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| 241 | <parm name="StringFlav:mesonBL1S1J1" default="0.0" min="0." max="3."> |
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| 242 | the relative pseudovector production ratio |
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| 243 | <ei>(L=1,S=1,J=1)</ei>/pseudoscalar |
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| 244 | for bottom mesons. |
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| 245 | </parm> |
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| 246 | |
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| 247 | <parm name="StringFlav:mesonBL1S1J2" default="0.0" min="0." max="5."> |
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| 248 | the relative tensor production ratio |
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| 249 | <ei>(L=1,S=1,J=2)</ei>/pseudoscalar |
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| 250 | for bottom mesons. |
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| 251 | </parm> |
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| 252 | |
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| 253 | <p/> |
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| 254 | In addition, an octet-singlet mixing angle is needed for each multiplet, |
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| 255 | as for the pseudoscalar and vector multiplets above. Only for the |
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| 256 | tensor multiplet does any determination exist; for the other multiplets |
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| 257 | default has been chose so that <ei>ssbar</ei> does not mix with the light |
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| 258 | quarks, and so that the <ei>ssbar</ei> state is the heavier of the two. |
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| 259 | |
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| 260 | <parm name="StringFlav:thetaL1S0J1" default="35.3" min="-90." max="90."> |
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| 261 | gives the mixing angle <ei>theta</ei> in the <ei>(L=1,S=0,J=1)</ei> |
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| 262 | pseudovector meson sector, expressed in degrees. |
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| 263 | </parm> |
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| 264 | |
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| 265 | <parm name="StringFlav:thetaL1S1J0" default="35.3" min="-90." max="90."> |
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| 266 | gives the mixing angle <ei>theta</ei> in the <ei>(L=1,S=1,J=0)</ei> |
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| 267 | scalar meson sector, expressed in degrees. |
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| 268 | </parm> |
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| 269 | |
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| 270 | <parm name="StringFlav:thetaL1S1J1" default="35.3" min="-90." max="90."> |
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| 271 | gives the mixing angle <ei>theta</ei> in the <ei>(L=1,S=1,J=1)</ei> |
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| 272 | pseudovector meson sector, expressed in degrees. |
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| 273 | </parm> |
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| 274 | |
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| 275 | <parm name="StringFlav:thetaL1S1J2" default="28.0" min="-90." max="90."> |
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| 276 | gives the mixing angle <ei>theta</ei> in the <ei>(L=1,S=1,J=2)</ei> |
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| 277 | tensor meson sector, expressed in degrees. |
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| 278 | </parm> |
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| 279 | |
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| 280 | <h3>Baryon production</h3> |
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| 281 | |
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| 282 | The relative rate of baryon production is mainly given by the quark |
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| 283 | and diquark production parameters above, plus SU(6) Clebsch-Gordans. |
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| 284 | The one modifiable parameter related to these coefficients is |
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| 285 | |
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| 286 | <parm name="StringFlav:decupletSup" default="1.0" min="0.0" max="1.0"> |
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| 287 | the suppression, relative to default SU(6) factors, of decuplet |
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| 288 | baryon production. Default corresponds to no suppression, while 0 |
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| 289 | corresponds to no decuplet production at all. |
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| 290 | </parm> |
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| 291 | |
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| 292 | <p/> |
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| 293 | In addition, if popcorn production is allowed, wherein a set of mesons |
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| 294 | (<ei>M</ei>) may be producted in between the baryon (<ei>B</ei>) and |
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| 295 | the antibaryon (<ei>Bbar</ei>), a set of further parameters is introduced. |
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| 296 | Currently only the simplest scenario is implemented, wherein at most |
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| 297 | one intermediate meson may be produced. |
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| 298 | |
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| 299 | <parm name="StringFlav:popcornRate" default="0.5" min="0." max="2.0"> |
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| 300 | gives the relative rates of <ei>B Bbar</ei> and <ei>B M Bbar</ei> |
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| 301 | production, roughly as |
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| 302 | <eq> |
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| 303 | Prob(B M Bbar) / (Prob(B Bbar) + Prob(B M Bbar)) = |
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| 304 | popcornRate / (0.5 + popcornRate) |
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| 305 | </eq> |
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| 306 | (the complete expression depends on all the quark and diquark production |
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| 307 | parameters and is therefore not so useful). |
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| 308 | </parm> |
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| 309 | |
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| 310 | <parm name="StringFlav:popcornSpair" default="0.5" min="0." max="1.0"> |
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| 311 | extra suppression for having an <ei>s sbar</ei> pair shared between |
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| 312 | the <ei>B</ei> and <ei>Bbar</ei> in a <ei>B M Bbar</ei> configuration. |
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| 313 | </parm> |
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| 314 | |
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| 315 | <parm name="StringFlav:popcornSmeson" default="0.5" min="0." max="1.0"> |
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| 316 | extra suppression for having a strange meson <ei>M</ei> in a |
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| 317 | <ei>B M Bbar</ei> configuration. |
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| 318 | </parm> |
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| 319 | |
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| 320 | <p/> |
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| 321 | Finally, there are some indications that leading-baryon production |
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| 322 | may be further suppressed. A proper description should probably be |
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| 323 | based on a suppression of early production times <ref>Ede97</ref>, |
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| 324 | but we here only implement a simpler version where production near |
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| 325 | the end of a string, as defined by rank, is suppressed. The more |
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| 326 | detailed studies suggest that leading <ei>c</ei> and <ei>b</ei> baryon |
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| 327 | production will be less suppressed, so we leave it open to set |
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| 328 | light- and heavy-baryon suppression separately. |
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| 329 | |
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| 330 | <flag name="StringFlav:suppressLeadingB" default="off"> |
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| 331 | Suppress leading-baryon production. |
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| 332 | <option value="off">No suppression.</option> |
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| 333 | <option value="on">Suppress the production of a diquark in the string |
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| 334 | breaking closest to a quark end of a string, by either of the factors |
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| 335 | below. This suppresses the production of first-rank baryons by the same |
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| 336 | amount. Indirectly also the second-rank and, if popcorn production is |
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| 337 | switched on, third-rank (anti)baryon production is affected. </option> |
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| 338 | </flag> |
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| 339 | |
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| 340 | <parm name="StringFlav:lightLeadingBSup" default="0.5" min="0." max="1.0"> |
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| 341 | extra suppression of leading-baryon production for a light-quark |
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| 342 | jet, i.e. <ei>d</ei>, <ei>u</ei> or <ei>s</ei>, when |
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| 343 | <code>suppressLeadingB = on</code>. Thus 0 means no leading-baryon |
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| 344 | production at all, while 1 means full rate. |
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| 345 | </parm> |
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| 346 | |
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| 347 | <parm name="StringFlav:heavyLeadingBSup" default="0.9" min="0." max="1.0"> |
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| 348 | extra suppression of leading-baryon production for a heavy-quark |
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| 349 | jet, i.e. <ei>c</ei> or <ei>b</ei>, when |
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| 350 | <code>suppressLeadingB = on</code>. Thus 0 means no leading-baryon |
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| 351 | production at all, while 1 means full rate. |
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| 352 | </parm> |
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| 353 | |
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| 354 | </chapter> |
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| 355 | |
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| 356 | <!-- Copyright (C) 2012 Torbjorn Sjostrand --> |
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