1 | <html> |
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2 | <head> |
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3 | <title>SUSY Processes</title> |
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4 | <link rel="stylesheet" type="text/css" href="pythia.css"/> |
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5 | <link rel="shortcut icon" href="pythia32.gif"/> |
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6 | </head> |
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7 | <body> |
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8 | |
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9 | <h2>SUSY</h2> |
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10 | <p> |
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11 | Here is collected processes involving supersymmetric particle |
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12 | production, with the exception of the (extended) Higgs sector. |
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13 | Since the number of separate but closely related processes is so big, |
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14 | there will not be switches for each separate process but only for a |
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15 | reasonable set of subgroups. However, the general |
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16 | switches <code>SUSY:idA</code> and <code>SUSY:idB</code> may be used in |
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17 | conjunction with any of these groups to provide some additional |
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18 | flexibility to concentrate on processes involving only specific (s)particle |
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19 | final states, see below. |
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20 | </p> |
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21 | |
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22 | <p> |
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23 | Most of the SUSY implementation in PYTHIA 8 has been written by |
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24 | N. Desai, in combination with an MCnet studentship at CERN in |
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25 | 2010, with further work having been carried out during 2011. |
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26 | The implementation is documented in [<a href="Bibliography.html" target="page">Des11</a>]. Please give due |
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27 | credit to external contributions to PYTHIA 8, such as this one, by |
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28 | including the original work in your list of references when using this |
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29 | implementation. |
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30 | </p> |
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31 | |
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32 | <p>Since the implementation of SUSY processes was only recently |
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33 | completed [<a href="Bibliography.html" target="page">Des11</a>], case-by-case validations against other codes |
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34 | are still recommended. In all cases, a set of default validations have |
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35 | already been carried out by the authors. These validations compared to |
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36 | the PYTHIA 6 SUSY implementation, using an sps1a spectrum. Most of the |
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37 | SLHA2-specific extensions have not been explicitly validated, |
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38 | however, with the exception of the R-parity violating single-sparticle |
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39 | production cross sections. |
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40 | </p> |
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41 | |
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42 | <br/><b>Important Note on SLHA:</b> |
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43 | In order to simulate SUSY processes it is required to read in the |
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44 | couplings and masses relevant for the scenario to be studied. This |
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45 | is done with the help of the SUSY Les Houches Accord (SLHA), including |
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46 | the SLHA2 extensions and generalizations. (Internally, the SLHA2 |
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47 | conventions are used. SLHA1 spectra are automatically translated into |
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48 | SLHA2 notation during initialization.) The |
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49 | reading of a relevant SLHA file <b>must</b> be set up, as described |
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50 | on <a href="SUSYLesHouchesAccord.html" target="page">the SLHA page</a>. |
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51 | Attempting to generate SUSY processes without a properly initialized |
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52 | SLHA spectrum is strongly discouraged and may lead to unexpected |
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53 | results. Always check for warnings and errors reported by the SLHA |
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54 | reader during the initialization stage. |
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55 | |
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56 | <h3>SUSY Processes</h3> |
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57 | |
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58 | <br/><b>Note 1:</b> Decays of SUSY particles are described |
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59 | separately <a href="#decays">below</a>. |
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60 | |
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61 | <br/><b>Note 2:</b> One special possibility is that the gluino or |
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62 | some squark(s) are sufficiently long-lived to hadronize. See |
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63 | <a href="RHadrons.html" target="page">the R-hadrons page</a> for further details. |
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64 | |
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65 | <br/><b>Note 3:</b> lepton- and photon-initial states are not yet available. |
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66 | Only quark/gluon-initiated <i>2 -> 2</i> and <i>2 -> 1</i> (RPV) |
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67 | processes have been implemented. Likewise, direct slepton production |
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68 | has not yet been implemented (i.e., <i>2 -> 2</i> processes |
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69 | involving sleptons in the final state). Sleptons will of course still be |
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70 | produced through cascade decays of heavier (s)particles. |
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71 | |
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72 | <br/><b>Note 4:</b> cross sections will be correctly folded with open |
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73 | branching fractions of cascade decays, but at present any difference between |
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74 | particle and antiparticle decay tables is not taken into account. This |
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75 | possibility will be included in a future update. |
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76 | |
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77 | <p/><code>flag </code><strong> SUSY:all </strong> |
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78 | (<code>default = <strong>off</strong></code>)<br/> |
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79 | Common switch for production of supersymmetric particles, i.e. |
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80 | particles with R-parity -1. |
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81 | |
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82 | |
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83 | <p/><code>mode </code><strong> SUSY:idA </strong> |
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84 | (<code>default = <strong>0</strong></code>; <code>minimum = 0</code>)<br/> |
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85 | Option to limit the sum over possible outgoing states in SUSY |
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86 | <i>2 -> 2</i> processes to ones including a specific particle |
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87 | identity code. The default corresponds to summing over all possible |
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88 | indices. A non-zero value of <code>SUSY:idA</code> selects only processes |
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89 | that contain the state corresponding to that particular particle identity |
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90 | code in the fundamental <i>2 -> 2</i> scattering process (symmetrized |
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91 | over particle/antiparticle). It is the user's responsibility to ensure |
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92 | that (a subset of) the processes be to simulated actually include this |
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93 | particle at the <i>2 -> 2</i> level; thus, asking for the lightest |
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94 | neutralino (code 1000021) to be present in a squark-squark production |
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95 | process will give no match. |
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96 | |
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97 | |
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98 | <p/><code>mode </code><strong> SUSY:idB </strong> |
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99 | (<code>default = <strong>0</strong></code>; <code>minimum = 0</code>)<br/> |
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100 | As for <code>SUSY:idA</code>, but requires an additional particle |
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101 | with PDG code <code>SUSY:idB</code> to be present in the <i>2 -> 2</i> |
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102 | process. Thus, using <code>SUSY:idA</code> and <code>SUSY:idB</code> |
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103 | a specific subprocess can be selected. Again only the absolute sign is |
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104 | used, i.e. the summation over particle and antiparticle is retained. |
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105 | |
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106 | |
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107 | <p/><code>mode </code><strong> SUSY:sin2thetaWMode </strong> |
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108 | (<code>default = <strong>2</strong></code>; <code>minimum = 1</code>; <code>maximum = 3</code>)<br/> |
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109 | The value of <i>sin2(thetaW)</i> should be taken from |
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110 | <br/><code>option </code><strong> 1</strong> : SM value, defined at <i>M_Z</i>, taken from |
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111 | PYTHIA's <code>StandardModel:sin2thetaW</code> parameter. |
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112 | <br/><code>option </code><strong> 2</strong> : SUSY value, defined at <i>M_SUSY</i>, derived from the |
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113 | running gauge couplings in <code>BLOCK GAUGE</code> in the SLHA |
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114 | file. Note: if no such block is present in the input file, this option |
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115 | will default back to option 1 above, i.e., the SM value. |
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116 | <br/><code>option </code><strong> 3</strong> : Pole value, defined by <i>1 - M_W^2/M_Z^2</i>, |
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117 | using the pole masses stored in the SLHA <code>BLOCK MASS</code>, or, |
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118 | alternatively, PYTHIA's internal pole masses if no such block is |
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119 | present. |
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120 | |
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121 | |
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122 | <h4>Gluino Pair Production</h4> |
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123 | |
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124 | <p/><code>flag </code><strong> SUSY:gg2gluinogluino </strong> |
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125 | (<code>default = <strong>off</strong></code>)<br/> |
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126 | Pair production of gluinos by gluon-gluon initial states. |
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127 | |
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128 | |
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129 | <p/><code>flag </code><strong> SUSY:qqbar2gluinogluino </strong> |
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130 | (<code>default = <strong>off</strong></code>)<br/> |
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131 | Pair production of gluinos by quark-antiquark annihilation and |
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132 | <i>t</i>-channel squark exchange. So far, these cross sections assume the |
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133 | squark CKM is aligned with the quark CKM and that all quantities are |
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134 | real, so effects of non-minimal flavour violation and/or CP violation |
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135 | are not yet included. |
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136 | |
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137 | |
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138 | <h4>Associated Squark-Gluino Production</h4> |
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139 | |
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140 | <p/><code>flag </code><strong> SUSY:qg2squarkgluino </strong> |
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141 | (<code>default = <strong>off</strong></code>)<br/> |
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142 | Associated production of a squark with a gluino. Only implemented for the |
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143 | flavor-diagonal case. So far, these cross sections assume the |
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144 | squark CKM is aligned with the quark CKM and that all quantities are |
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145 | real, so effects of non-minimal flavour violation and/or CP violation |
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146 | are not yet included. |
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147 | |
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148 | |
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149 | <h4>Squark Pair Production</h4> |
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150 | |
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151 | <p/><code>flag </code><strong> SUSY:gg2squarkantisquark </strong> |
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152 | (<code>default = <strong>off</strong></code>)<br/> |
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153 | Pair production of a scalar quark together with a scalar antiquark by |
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154 | gluon annhilation via <i>s</i>-channel gluon exhange, <i>t</i>- and |
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155 | <i>u</i>-channel squark exchange, and the direct 4-point coupling. |
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156 | The cross section expression follows [<a href="Bibliography.html" target="page">Boz07</a>]. |
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157 | Validation of the FLV and CPV cases has not yet been completed. |
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158 | |
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159 | |
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160 | <p/><code>flag </code><strong> SUSY:qqbar2squarkantisquark </strong> |
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161 | (<code>default = <strong>off</strong></code>)<br/> |
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162 | Pair production of a scalar quark together with a scalar antiquark |
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163 | by quark-antiquark annihilation. |
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164 | For same-isospin <i>~q~q*</i> production (i.e., <i>~u~u*</i>, |
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165 | <i>~u~c*</i>, ...), the <i>s</i>-channel gluon, photon, and |
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166 | <i>Z</i> and <i>t</i>-channel gluino contributions have so far |
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167 | been implemented (i.e., the <i>t</i>-channel neutralino contributions |
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168 | are neglected). For opposite-isospin <i>~q~q*</i> production |
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169 | (<i>~u~d*</i>, <i>~u~s*</i>, ...), the <i>s</i>-channel <i>W</i> |
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170 | and <i>t</i>-channel gluino contributions have been implemented |
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171 | (i.e., the <i>t</i>-channel neutralino contributions are neglected). |
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172 | The cross section expressions follow [<a href="Bibliography.html" target="page">Boz07</a>]. |
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173 | Validation of the FLV and CPV cases has not yet been completed. |
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174 | (Note to PYTHIA 6 users: |
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175 | in older PYTHIA 6 versions, a bug caused the <i>~t1~t2*</i> cross to be |
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176 | overcounted by a factor of 2. Starting from version 6.4.24, that |
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177 | generator now agrees with the implementation here.) |
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178 | |
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179 | |
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180 | <p/><code>flag </code><strong> SUSY:qqbar2squarkantisquark:onlyQCD </strong> |
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181 | (<code>default = <strong>off</strong></code>)<br/> |
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182 | When switched <code>on</code> this flag switches off all but the |
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183 | <i>s</i>-channel gluon contribution in the |
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184 | calculation of same-isospin squark-antisquark production cross |
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185 | sections. Intended for reference only. For the most |
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186 | accurate physics simulation, leave this flag in the <code>off</code> |
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187 | position. |
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188 | |
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189 | |
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190 | <p/><code>flag </code><strong> SUSY:qq2squarksquark </strong> |
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191 | (<code>default = <strong>off</strong></code>)<br/> |
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192 | Pair production of scalar quarks (squark-squark and its charge |
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193 | conjugate process; for squark-antisquark production see above) |
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194 | by <i>t</i>- and <i>u</i>-channel gluino, neutralino, and |
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195 | chargino exchange. The cross section expressions follow [<a href="Bibliography.html" target="page">Boz07</a>]. |
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196 | Validation of the FLV and CPV cases has not yet been completed. |
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197 | (Note to PYTHIA 6 users: PYTHIA 6 only included the gluino exchange |
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198 | contribution, which typically dominates due to the size of the strong |
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199 | coupling; for counterchecks, |
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200 | the flag <code>SUSY:qq2squarksquark:onlyQCD</code> |
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201 | below can be switched on to eliminate the chargino and neutralino |
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202 | contributions.) |
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203 | |
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204 | |
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205 | <p/><code>flag </code><strong> SUSY:qq2squarksquark:onlyQCD </strong> |
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206 | (<code>default = <strong>off</strong></code>)<br/> |
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207 | When switched <code>on</code> this flag causes the <i>t</i>- or |
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208 | <i>u</i>-channel neutralino and chargino contributions to be |
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209 | ignored in the calculation of squark pair production cross sections. |
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210 | Intended for reference only. For the most accurate physics simulation, |
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211 | leave this flag in the <code>off</code> position. |
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212 | |
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213 | |
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214 | <h4>Neutralino and Chargino Pair Production</h4> |
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215 | |
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216 | <p/><code>flag </code><strong> SUSY:qqbar2chi0chi0 </strong> |
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217 | (<code>default = <strong>off</strong></code>)<br/> |
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218 | Pair production of neutralinos by quark-antiquark annihilation. With |
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219 | four neutralino species this gives ten separate processes, codes |
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220 | 1201 - 1210. The cross section expressions follow [<a href="Bibliography.html" target="page">Boz07</a>]. |
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221 | Validation of the FLV and CPV cases has not yet been completed. |
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222 | |
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223 | |
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224 | <p/><code>flag </code><strong> SUSY:qqbar2chi+-chi0 </strong> |
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225 | (<code>default = <strong>off</strong></code>)<br/> |
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226 | Associated chargino-neutralino production by quark-antiquark |
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227 | annihilation. With four neutralino species, two chargino ones, and |
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228 | maintaining charge conjugate proceeses separate, this gives 16 |
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229 | separate processes, codes 1221 - 1236. The cross section expressions |
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230 | follow [<a href="Bibliography.html" target="page">Boz07</a>]. |
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231 | Validation of the FLV and CPV cases has not yet been completed. |
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232 | |
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233 | |
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234 | <p/><code>flag </code><strong> SUSY:qqbar2chi+chi- </strong> |
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235 | (<code>default = <strong>off</strong></code>)<br/> |
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236 | Pair production of charginos by quark-antiquark annihilation. With |
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237 | two chargino species and maintaining mutually charge conjugate |
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238 | processes separate, this gives four separate processes, codes |
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239 | 1241 - 1244. The cross section expressions follow [<a href="Bibliography.html" target="page">Boz07</a>]. |
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240 | Validation of the FLV and CPV cases has not yet been completed. |
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241 | |
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242 | |
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243 | <h4>Associated Neutralino/Chargino + Squark/Gluino Production</h4> |
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244 | |
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245 | <p/><code>flag </code><strong> SUSY:qg2chi0squark </strong> |
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246 | (<code>default = <strong>off</strong></code>)<br/> |
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247 | Pair production of neutralinos from quark-gluon initial states. |
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248 | The cross section expressions follow [<a href="Bibliography.html" target="page">Boz07</a>]. |
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249 | Validation of the FLV and CPV cases has not yet been completed. |
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250 | |
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251 | |
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252 | <p/><code>flag </code><strong> SUSY:qg2chi+-squark </strong> |
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253 | (<code>default = <strong>off</strong></code>)<br/> |
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254 | Associated chargino-squark production from quark-gluon initial states. |
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255 | annihilation. The cross section expressions |
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256 | follow [<a href="Bibliography.html" target="page">Boz07</a>]. |
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257 | Validation of the FLV and CPV cases has not yet been completed. |
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258 | |
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259 | |
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260 | <p/><code>flag </code><strong> SUSY:qqbar2chi0gluino </strong> |
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261 | (<code>default = <strong>off</strong></code>)<br/> |
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262 | Associated neutralino-gluino production by quark-antiquark |
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263 | annihilation. Status: not implemented yet. |
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264 | |
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265 | |
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266 | <p/><code>flag </code><strong> SUSY:qqbar2chi+-gluino </strong> |
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267 | (<code>default = <strong>off</strong></code>)<br/> |
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268 | Associated chargino-gluino production by quark-antiquark |
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269 | annihilation. Status: not implemented yet. |
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270 | |
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271 | |
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272 | <h4>Slepton Production</h4> |
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273 | |
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274 | No 2->2 slepton pair production or associated slepton production |
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275 | cross sections have been implemented yet. |
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276 | |
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277 | <h4>R-parity violating squark production</h4> |
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278 | |
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279 | <p/><code>flag </code><strong> SUSY:qq2antisquark </strong> |
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280 | (<code>default = <strong>off</strong></code>)<br/> |
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281 | Resonant squark production via R-parity violating UDD couplings. The |
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282 | couplings must be input using the SLHA2 structure. |
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283 | |
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284 | |
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285 | <a name="decays"></a> |
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286 | <h3>Decays of SUSY Particles</h3> |
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287 | |
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288 | Based on the parameters read in from the SLHA, PYTHIA 8 will normally |
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289 | compute the decay modes of SUSY particles automatically, using the |
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290 | <code>SusyResonanceDecays</code> class(es). Essentially all tree-level |
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291 | 2-body decays in the MSSM |
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292 | have been implemented this way, excepting so far only those involving |
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293 | Higgs bosons (either in the in- or out-state) or gravitinos. |
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294 | Available channels so far include: |
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295 | <ul> |
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296 | <li>~q → q + ~chi</li> |
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297 | <li>~q → ~q + W/Z</li> |
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298 | <li>~q → q + q (RPV UDD)</li> |
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299 | <li>~q → l + q (RPV LQD)</li> |
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300 | <li>~g → ~q + q</li> |
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301 | <li>~chi → ~chi + Z/W</li> |
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302 | <li>~chi → ~q + q</li> |
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303 | <li>~chi → ~l/~nu + l/nu</li> |
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304 | <li>~chi0 → q + q + q (RPV UDD)</li> |
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305 | <li>~l/~nu → l/nu + ~chi</li> |
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306 | <li>~l/~nu → ~l/~nu + W/Z</li> |
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307 | </ul> |
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308 | All channels are still undergoing validation, so this |
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309 | implementation should be considered preliminary. |
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310 | Still missing but to be included in a forthcoming update |
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311 | are: 3-body decays of charginos (via RPV), and 2-body decays of squarks and |
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312 | gauginos with Higgs as one of the decay products. |
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313 | |
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314 | </body> |
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315 | </html> |
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316 | |
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317 | <!-- Copyright (C) 2012 Torbjorn Sjostrand --> |
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318 | |
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