1 | <chapter name="New-Gauge-Boson Processes"> |
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2 | |
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3 | <h2>New-Gauge-Boson Processes</h2> |
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4 | |
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5 | This page contains the production of new <ei>Z'^0</ei> and |
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6 | <ei>W'^+-</ei> gauge bosons, e.g. within the context of a new |
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7 | <ei>U(1)</ei> or <ei>SU(2)</ei> gauge group, and also a |
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8 | (rather speculative) horizontal gauge boson <ei>R^0</ei>. |
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9 | Left-right-symmetry scenarios also contain new gauge bosons, |
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10 | but are described |
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11 | <aloc href"LeftRightSymmetryProcesses">separately</aloc>. |
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12 | |
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13 | <h3><ei>Z'^0</ei></h3> |
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14 | |
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15 | This group only contains one subprocess, with the full |
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16 | <ei>gamma^*/Z^0/Z'^0</ei> interference structure for couplings |
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17 | to fermion pairs. It is possible to pick only a subset, e.g, only |
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18 | the pure <ei>Z'^0</ei> piece. No higher-order processes are |
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19 | available explicitly, but the ISR showers contain automatic |
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20 | matching to the <ei>Z'^0</ei> + 1 jet matrix elements, as for |
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21 | the corresponding <ei>gamma^*/Z^0</ei> process. |
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22 | |
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23 | <flag name="NewGaugeBoson:ffbar2gmZZprime" default="off"> |
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24 | Scattering <ei>f fbar ->Z'^0</ei>. |
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25 | Code 3001. |
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26 | </flag> |
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27 | |
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28 | <modepick name="Zprime:gmZmode" default="0" min="0" max="6"> |
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29 | Choice of full <ei>gamma^*/Z^0/Z'^0</ei> structure or not in |
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30 | the above process. Note that, with the <ei>Z'^0</ei> part switched |
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31 | off, this process is reduced to what already exists among |
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32 | <aloc href="ElectroweakProcesses">electroweak processes</aloc>, |
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33 | so those options are here only for crosschecks. |
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34 | <option value="0">full <ei>gamma^*/Z^0/Z'^0</ei> structure, |
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35 | with interference included.</option> |
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36 | <option value="1">only pure <ei>gamma^*</ei> contribution.</option> |
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37 | <option value="2">only pure <ei>Z^0</ei> contribution.</option> |
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38 | <option value="3">only pure <ei>Z'^0</ei> contribution.</option> |
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39 | <option value="4">only the <ei>gamma^*/Z^0</ei> contribution, |
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40 | including interference.</option> |
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41 | <option value="5">only the <ei>gamma^*/Z'^0</ei> contribution, |
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42 | including interference.</option> |
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43 | <option value="6">only the <ei>Z^0/Z'^0</ei> contribution, |
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44 | including interference.</option> |
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45 | <note>Note</note>: irrespective of the option used, the particle produced |
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46 | will always be assigned code 32 for <ei>Z'^0</ei>, and open decay channels |
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47 | is purely dictated by what is set for the <ei>Z'^0</ei>. |
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48 | </modepick> |
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49 | |
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50 | <p/> |
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51 | The couplings of the <ei>Z'^0</ei> to quarks and leptons can |
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52 | either be assumed universal, i.e. generation-independent, or not. |
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53 | In the former case eight numbers parametrize the vector and axial |
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54 | couplings of down-type quarks, up-type quarks, leptons and neutrinos, |
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55 | respectively. Depending on your assumed neutrino nature you may |
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56 | want to restrict your freedom in that sector, but no limitations |
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57 | are enforced by the program. The default corresponds to the same |
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58 | couplings as that of the Standard Model <ei>Z^0</ei>, with axial |
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59 | couplings <ei>a_f = +-1</ei> and vector couplings |
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60 | <ei>v_f = a_f - 4 e_f sin^2(theta_W)</ei>, with |
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61 | <ei>sin^2(theta_W) = 0.23</ei>. Without universality |
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62 | the same eight numbers have to be set separately also for the |
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63 | second and the third generation. The choice of fixed axial and |
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64 | vector couplings implies a resonance width that increases linearly |
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65 | with the <ei>Z'^0</ei> mass. |
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66 | |
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67 | <p/> |
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68 | By a suitable choice of the parameters, it is possible to simulate |
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69 | just about any imaginable <ei>Z'^0</ei> scenario, with full |
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70 | interference effects in cross sections and decay angular |
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71 | distributions and generation-dependent couplings; the default values |
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72 | should mainly be viewed as placeholders. The conversion |
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73 | from the coupling conventions in a set of different <ei>Z'^0</ei> |
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74 | models in the literature to those used in PYTHIA is described by |
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75 | <a href="http://www.hep.uiuc.edu/home/catutza/nota12.ps">C. |
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76 | Ciobanu et al.</a> |
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77 | |
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78 | <flag name="Zprime:universality" default="on"> |
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79 | If on then you need only set the first-generation couplings |
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80 | below, and these are automatically also used for the second and |
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81 | third generation. If off, then couplings can be chosen separately |
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82 | for each generation. |
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83 | </flag> |
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84 | |
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85 | <p/> |
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86 | Here are the couplings always valid for the first generation, |
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87 | and normally also for the second and third by trivial analogy: |
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88 | |
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89 | <parm name="Zprime:vd" default="-0.693"> |
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90 | vector coupling of <ei>d</ei> quarks. |
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91 | </parm> |
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92 | |
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93 | <parm name="Zprime:ad" default="-1."> |
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94 | axial coupling of <ei>d</ei> quarks. |
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95 | </parm> |
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96 | |
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97 | <parm name="Zprime:vu" default="0.387"> |
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98 | vector coupling of <ei>u</ei> quarks. |
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99 | </parm> |
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100 | |
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101 | <parm name="Zprime:au" default="1."> |
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102 | axial coupling of <ei>u</ei> quarks. |
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103 | </parm> |
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104 | |
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105 | <parm name="Zprime:ve" default="-0.08"> |
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106 | vector coupling of <ei>e</ei> leptons. |
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107 | </parm> |
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108 | |
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109 | <parm name="Zprime:ae" default="-1."> |
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110 | axial coupling of <ei>e</ei> leptons. |
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111 | </parm> |
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112 | |
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113 | <parm name="Zprime:vnue" default="1."> |
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114 | vector coupling of <ei>nu_e</ei> neutrinos. |
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115 | </parm> |
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116 | |
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117 | <parm name="Zprime:anue" default="1."> |
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118 | axial coupling of <ei>nu_e</ei> neutrinos. |
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119 | </parm> |
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120 | |
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121 | <p/> |
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122 | Here are the further couplings that are specific for |
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123 | a scenario with <code>Zprime:universality</code> swiched off: |
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124 | |
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125 | <parm name="Zprime:vs" default="-0.693"> |
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126 | vector coupling of <ei>s</ei> quarks. |
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127 | </parm> |
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128 | |
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129 | <parm name="Zprime:as" default="-1."> |
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130 | axial coupling of <ei>s</ei> quarks. |
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131 | </parm> |
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132 | |
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133 | <parm name="Zprime:vc" default="0.387"> |
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134 | vector coupling of <ei>c</ei> quarks. |
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135 | </parm> |
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136 | |
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137 | <parm name="Zprime:ac" default="1."> |
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138 | axial coupling of <ei>c</ei> quarks. |
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139 | </parm> |
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140 | |
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141 | <parm name="Zprime:vmu" default="-0.08"> |
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142 | vector coupling of <ei>mu</ei> leptons. |
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143 | </parm> |
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144 | |
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145 | <parm name="Zprime:amu" default="-1."> |
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146 | axial coupling of <ei>mu</ei> leptons. |
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147 | </parm> |
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148 | |
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149 | <parm name="Zprime:vnumu" default="1."> |
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150 | vector coupling of <ei>nu_mu</ei> neutrinos. |
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151 | </parm> |
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152 | |
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153 | <parm name="Zprime:anumu" default="1."> |
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154 | axial coupling of <ei>nu_mu</ei> neutrinos. |
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155 | </parm> |
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156 | |
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157 | <parm name="Zprime:vb" default="-0.693"> |
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158 | vector coupling of <ei>b</ei> quarks. |
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159 | </parm> |
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160 | |
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161 | <parm name="Zprime:ab" default="-1."> |
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162 | axial coupling of <ei>b</ei> quarks. |
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163 | </parm> |
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164 | |
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165 | <parm name="Zprime:vt" default="0.387"> |
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166 | vector coupling of <ei>t</ei> quarks. |
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167 | </parm> |
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168 | |
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169 | <parm name="Zprime:at" default="1."> |
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170 | axial coupling of <ei>t</ei> quarks. |
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171 | </parm> |
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172 | |
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173 | <parm name="Zprime:vtau" default="-0.08"> |
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174 | vector coupling of <ei>tau</ei> leptons. |
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175 | </parm> |
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176 | |
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177 | <parm name="Zprime:atau" default="-1."> |
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178 | axial coupling of <ei>tau</ei> leptons. |
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179 | </parm> |
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180 | |
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181 | <parm name="Zprime:vnutau" default="1."> |
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182 | vector coupling of <ei>nu_tau</ei> neutrinos. |
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183 | </parm> |
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184 | |
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185 | <parm name="Zprime:anutau" default="1."> |
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186 | axial coupling of <ei>nu_tau</ei> neutrinos. |
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187 | </parm> |
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188 | |
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189 | <p/> |
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190 | The coupling to the decay channel <ei>Z'^0 -> W^+ W^-</ei> is |
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191 | more model-dependent. By default it is therefore off, but can be |
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192 | switched on as follows. Furthermore, we have left some amount of |
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193 | freedom in the choice of decay angular correlations in this |
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194 | channel, but obviously alternative shapes could be imagined. |
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195 | |
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196 | <parm name="Zprime:coup2WW" default="0." min="0."> |
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197 | the coupling <ei>Z'^0 -> W^+ W^-</ei> is taken to be this number |
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198 | times <ei>m_W^2 / m_Z'^2</ei> times the <ei>Z^0 -> W^+ W^-</ei> |
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199 | coupling. Thus a unit value corresponds to the |
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200 | <ei>Z^0 -> W^+ W^-</ei> coupling, scaled down by a factor |
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201 | <ei>m_W^2 / m_Z'^2</ei>, and gives a <ei>Z'^0</ei> partial |
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202 | width into this channel that again increases linearly. If you |
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203 | cancel this behaviour, by letting <code>Zprime:coup2WW</code> be |
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204 | proportional to <ei>m_Z'^2 / m_W^2</ei>, you instead obtain a |
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205 | partial width that goes like the fifth power of the <ei>Z'^0</ei> |
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206 | mass. These two extremes correspond to the "extended gauge model" |
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207 | and the "reference model", respectively, of <ref>Alt89</ref>. |
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208 | Note that this channel only includes the pure <ei>Z'</ei> part, |
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209 | while <ei>f fbar -> gamma^*/Z^*0 -> W^+ W^-</ei> is available |
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210 | as a separate electroweak process. |
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211 | </parm> |
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212 | |
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213 | <parm name="Zprime:anglesWW" default="0." min="0." max="1."> |
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214 | in the decay chain <ei>Z'^0 -> W^+ W^- ->f_1 fbar_2 f_3 fbar_4</ei> |
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215 | the decay angular distributions is taken to be a mixture of two |
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216 | possible shapes. This parameter gives the fraction that is distributed |
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217 | as in Higgs <ei>h^0 -> W^+ W^-</ei> (longitudinal bosons), |
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218 | with the remainder (by default all) is taken to be the same as for |
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219 | <ei>Z^0 -> W^+ W^-</ei> (a mixture of transverse and longitudinal |
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220 | bosons). |
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221 | </parm> |
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222 | |
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223 | <p/> |
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224 | A massive <ei>Z'^0</ei> is also likely to decay into Higgses |
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225 | and potentially into other now unknown particles. Such possibilities |
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226 | clearly are quite model-dependent, and have not been included |
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227 | for now. |
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228 | |
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229 | <h3><ei>W'^+-</ei></h3> |
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230 | |
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231 | The <ei>W'^+-</ei> implementation is less ambitious than the |
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232 | <ei>Z'^0</ei>. Specifically, while indirect detection of a |
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233 | <ei>Z'^0</ei> through its interference contribution is |
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234 | a possible discovery channel in lepton colliders, there is no |
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235 | equally compelling case for <ei>W^+-/W'^+-</ei> interference |
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236 | effects being of importance for discovery, and such interference |
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237 | has therefore not been implemented for now. Related to this, a |
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238 | <ei>Z'^0</ei> could appear on its own in a new <ei>U(1)</ei> group, |
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239 | while <ei>W'^+-</ei> would have to sit in a <ei>SU(2)</ei> group |
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240 | and thus have a <ei>Z'^0</ei> partner that is likely to be found |
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241 | first. Only one process is implemented but, like for the |
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242 | <ei>W^+-</ei>, the ISR showers contain automatic matching to the |
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243 | <ei>W'^+-</ei> + 1 jet matrix elements. |
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244 | |
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245 | <flag name="NewGaugeBoson:ffbar2Wprime" default="off"> |
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246 | Scattering <ei>f fbar' -> W'^+-</ei>. |
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247 | Code 3021. |
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248 | </flag> |
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249 | |
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250 | <p/> |
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251 | The couplings of the <ei>W'^+-</ei> are here assumed universal, |
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252 | i.e. the same for all generations. One may set vector and axial |
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253 | couplings freely, separately for the <ei>q qbar'</ei> and the |
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254 | <ei>l nu_l</ei> decay channels. The defaults correspond to the |
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255 | <ei>V - A</ei> structure and normalization of the Standard Model |
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256 | <ei>W^+-</ei>, but can be changed to simulate a wide selection |
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257 | of models. One limitation is that, for simplicity, the same |
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258 | Cabibbo--Kobayashi--Maskawa quark mixing matrix is assumed as for |
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259 | the standard <ei>W^+-</ei>. Depending on your assumed neutrino |
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260 | nature you may want to restrict your freedom in the lepton sector, |
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261 | but no limitations are enforced by the program. |
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262 | |
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263 | <parm name="Wprime:vq" default="1."> |
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264 | vector coupling of quarks. |
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265 | </parm> |
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266 | |
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267 | <parm name="Wprime:aq" default="-1."> |
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268 | axial coupling of quarks. |
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269 | </parm> |
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270 | |
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271 | <parm name="Wprime:vl" default="1."> |
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272 | vector coupling of leptons. |
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273 | </parm> |
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274 | |
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275 | <parm name="Wprime:al" default="-1."> |
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276 | axial coupling of leptons. |
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277 | </parm> |
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278 | |
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279 | <p/> |
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280 | The coupling to the decay channel <ei>W'^+- -> W^+- Z^0</ei> is |
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281 | more model-dependent, like for <ei>Z'^0 -> W^+ W^-</ei> described |
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282 | above. By default it is therefore off, but can be |
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283 | switched on as follows. Furthermore, we have left some amount of |
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284 | freedom in the choice of decay angular correlations in this |
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285 | channel, but obviously alternative shapes could be imagined. |
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286 | |
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287 | <parm name="Wprime:coup2WZ" default="0." min="0."> |
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288 | the coupling <ei>W'^0 -> W^+- Z^0</ei> is taken to be this number |
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289 | times <ei>m_W^2 / m_W'^2</ei> times the <ei>W^+- -> W^+- Z^0</ei> |
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290 | coupling. Thus a unit value corresponds to the |
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291 | <ei>W^+- -> W^+- Z^0</ei> coupling, scaled down by a factor |
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292 | <ei>m_W^2 / m_W'^2</ei>, and gives a <ei>W'^+-</ei> partial |
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293 | width into this channel that increases linearly with the |
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294 | <ei>W'^+-</ei> mass. If you cancel this behaviour, by letting |
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295 | <code>Wprime:coup2WZ</code> be proportional to <ei>m_W'^2 / m_W^2</ei>, |
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296 | you instead obtain a partial width that goes like the fifth power |
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297 | of the <ei>W'^+-</ei> mass. These two extremes correspond to the |
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298 | "extended gauge model" and the "reference model", respectively, |
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299 | of <ref>Alt89</ref>. |
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300 | </parm> |
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301 | |
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302 | <parm name="Wprime:anglesWZ" default="0." min="0." max="1."> |
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303 | in the decay chain <ei>W'^+- -> W^+- Z^0 ->f_1 fbar_2 f_3 fbar_4</ei> |
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304 | the decay angular distributions is taken to be a mixture of two |
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305 | possible shapes. This parameter gives the fraction that is distributed |
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306 | as in Higgs <ei>H^+- -> W^+- Z^0</ei> (longitudinal bosons), |
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307 | with the remainder (by default all) is taken to be the same as for |
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308 | <ei>W^+- -> W^+- Z^0</ei> (a mixture of transverse and longitudinal |
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309 | bosons). |
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310 | </parm> |
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311 | |
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312 | <p/> |
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313 | A massive <ei>W'^+-</ei> is also likely to decay into Higgses |
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314 | and potentially into other now unknown particles. Such possibilities |
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315 | clearly are quite model-dependent, and have not been included |
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316 | for now. |
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317 | |
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318 | <h3><ei>R^0</ei></h3> |
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319 | |
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320 | The <ei>R^0</ei> boson (particle code 41) represents one possible |
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321 | scenario for a horizontal gauge boson, i.e. a gauge boson |
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322 | that couples between the generations, inducing processes like |
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323 | <ei>s dbar -> R^0 -> mu^- e^+</ei>. Experimental limits on |
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324 | flavour-changing neutral currents forces such a boson to be fairly |
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325 | heavy. In spite of being neutral the antiparticle is distinct from |
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326 | the particle: one carries a net positive generation number and |
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327 | the other a negative one. This particular model has no new |
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328 | parameters beyond the <ei>R^0</ei> mass. Decays are assumed isotropic. |
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329 | For further details see <ref>Ben85</ref>. |
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330 | |
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331 | <flag name="NewGaugeBoson:ffbar2R0" default="off"> |
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332 | Scattering <ei>f_1 fbar_2 -> R^0 -> f_3 fbar_4</ei>, where |
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333 | <ei>f_1</ei> and <ei>fbar_2</ei> are separated by <ei>+-</ei> one |
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334 | generation and similarly for <ei>f_3</ei> and <ei>fbar_4</ei>. |
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335 | Thus possible final states are e.g. <ei>d sbar</ei>, <ei>u cbar</ei> |
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336 | <ei>s bbar</ei>, <ei>c tbar</ei>, <ei>e- mu+</ei> and |
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337 | <ei>mu- tau+</ei>. |
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338 | Code 3041. |
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339 | </flag> |
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340 | |
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341 | </chapter> |
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342 | |
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343 | <!-- Copyright (C) 2012 Torbjorn Sjostrand --> |
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344 | |
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