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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