1 | <chapter name="Beam Parameters"> |
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2 | |
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3 | <h2>Beam Parameters</h2> |
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4 | |
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5 | The settings on this page relate to the beam identities and energies, |
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6 | to a beam momentum spread and to a beam interaction spot. |
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7 | As always, momenta and energies are to be given in units of GeV, |
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8 | and of space and time in mm. |
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9 | |
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10 | <h3>Incoming beams</h3> |
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11 | |
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12 | There are two ways to set the identities and energies of the two incoming |
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13 | beam particles. One is to use the <code>init()</code> method with no |
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14 | arguments. Then the settings variables below will be read and used. The |
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15 | alternative is to call <code><aloc href="ProgramFlow">init(...)</aloc></code> |
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16 | with arguments that provide this information. Then you need not use the |
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17 | variables below (although it would still be possible). Note that, if nothing |
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18 | is done, you will default to LHC at 14 TeV. |
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19 | |
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20 | <p/> |
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21 | Currently the beam particles must be either a hadron pair or a lepton |
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22 | pair. In the former category <ei>p p</ei> and <ei>pbar p</ei> |
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23 | combinations dominate, but it is also possible to combine with |
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24 | <ei>pi^+</ei>, <ei>pi^-</ei> and <ei>pi^0</ei>. In the latter |
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25 | <ei>e^+ e^-</ei> and <ei>mu^+ mu^-</ei> would be the most useful |
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26 | combinations, but also others should work if combined with an |
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27 | appropriate hard process. |
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28 | |
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29 | <modeopen name="Beams:idA" default="2212"> |
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30 | The PDG <code>id</code> code for the first incoming particle. |
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31 | Allowed codes include |
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32 | <br/><ei>2212 = p</ei>, <ei>-2212 = pbar</ei>, |
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33 | <br/><ei>211 = pi^+</ei>, <ei>-211 = pi^-</ei>, <ei>111 = pi^0</ei>, |
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34 | <br/><ei>990 = Pomeron</ei> (used in diffractive machinery; |
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35 | here mainly for debug purposes), |
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36 | <br/><ei>11 = e^-</ei>, <ei>-11 = e^+</ei>, |
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37 | <br/><ei>13 = mu^-</ei>, <ei>-13 = mu^+</ei>, |
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38 | <br/>and a few more leptons/neutrinos in a few combinations. |
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39 | </modeopen> |
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40 | |
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41 | <modeopen name="Beams:idB" default="2212"> |
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42 | The PDG <code>id</code> code for the second incoming particle. |
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43 | </modeopen> |
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44 | |
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45 | <modepick name="Beams:frameType" default="1" min="1" max="5"> |
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46 | Choice of frame for the two colliding particles. For options |
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47 | 1 - 3 the beam identities are specified above, while they are |
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48 | obtained by the Les Houches information for options 4 and 5. |
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49 | <option value="1">the beams are colliding in their CM frame, |
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50 | and therefore only the CM energy needs to be provided, see |
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51 | <code>Beams:eCM</code> below. |
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52 | </option> |
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53 | <option value="2">the beams are back-to-back, but with different energies, |
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54 | see <code>Beams:eA</code> and <code>Beams:eB</code> below. |
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55 | This option could also be used for fixed-target configurations. |
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56 | </option> |
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57 | <option value="3">the beams are not back-to-back, and therefore the |
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58 | three-momentum of each incoming particle needs to be specified, see |
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59 | <code>Beams:pxA</code> through <code>Beams:pzB</code> below. |
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60 | </option> |
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61 | <option value="4">the beam and event information is stored in a |
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62 | <aloc href="LesHouchesAccord">Les Houches Event File</aloc>, |
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63 | see <code>Beams:LHEF</code> below. |
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64 | </option> |
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65 | <option value="5">the beam and event information is obtained by a |
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66 | pointer to an <code><aloc href="LesHouchesAccord">LHAup</aloc></code> |
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67 | class instance. |
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68 | </option> |
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69 | </modepick> |
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70 | |
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71 | <parm name="Beams:eCM" default="14000." min="10."> |
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72 | Collision CM energy, to be set if <code>Beams:frameType</code> = 1. |
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73 | </parm> |
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74 | |
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75 | <parm name="Beams:eA" default="7000." min="0."> |
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76 | The energy of the first incoming particle, moving in the |
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77 | <ei>+z </ei>direction, to be set if <code>Beams:frameType</code> = 2. |
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78 | If the particle energy is smaller than its mass |
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79 | it is assumed to be at rest. |
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80 | </parm> |
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81 | |
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82 | <parm name="Beams:eB" default="7000." min="0."> |
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83 | The energy of the second incoming particle, moving in the |
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84 | <ei>-z</ei> direction, to be set if <code>Beams:frameType</code> = 2. |
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85 | If the particle energy is smaller than its mass |
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86 | it is assumed to be at rest. |
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87 | </parm> |
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88 | |
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89 | <parm name="Beams:pxA" default="0."> |
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90 | The <ei>p_x</ei> component of the first incoming particle, |
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91 | to be set if <code>Beams:frameType</code> = 3. |
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92 | </parm> |
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93 | |
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94 | <parm name="Beams:pyA" default="0."> |
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95 | The <ei>p_y</ei> component of the first incoming particle, |
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96 | to be set if <code>Beams:frameType</code> = 3. |
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97 | </parm> |
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98 | |
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99 | <parm name="Beams:pzA" default="7000."> |
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100 | The <ei>p_z</ei> component of the first incoming particle, |
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101 | to be set if <code>Beams:frameType</code> = 3. |
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102 | </parm> |
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103 | |
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104 | <parm name="Beams:pxB" default="0."> |
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105 | The <ei>p_x</ei> component of the second incoming particle, |
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106 | to be set if <code>Beams:frameType</code> = 3. |
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107 | </parm> |
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108 | |
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109 | <parm name="Beams:pyB" default="0."> |
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110 | The <ei>p_y</ei> component of the second incoming particle, |
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111 | to be set if <code>Beams:frameType</code> = 3. |
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112 | </parm> |
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113 | |
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114 | <parm name="Beams:pzB" default="-7000."> |
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115 | The <ei>p_z</ei> component of the second incoming particle, |
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116 | to be set if <code>Beams:frameType</code> = 3. |
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117 | </parm> |
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118 | |
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119 | <word name="Beams:LHEF" default="void"> |
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120 | The name of a Les Houches Event File, |
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121 | to be set if <code>Beams:frameType</code> = 4. |
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122 | </word> |
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123 | |
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124 | <word name="Beams:LHEFheader" default="void"> |
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125 | As some information in a Les Houches Event File init block is only known |
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126 | at the end of generation, some programs choose to output this as a |
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127 | separate file. If <code>Beams:LHEFheader</code> is given, information up |
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128 | till the end of the init block is read from this file, with |
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129 | the events themselves read as usual from the file given in |
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130 | <code>Beams:LHEF</code>. |
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131 | </word> |
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132 | |
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133 | <flag name="Beams:newLHEFsameInit" default="off"> |
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134 | Allow to begin reading events from a new LHEF or or a new |
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135 | <code>LHAup</code> instance without a completely new initialization. |
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136 | Only useful when <code>Beams:frameType</code> = 4 or 5. |
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137 | </flag> |
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138 | |
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139 | <flag name="Beams:readLHEFheaders" default="on"> |
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140 | Read in LHEF header blocks and store them in the |
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141 | <aloc href="EventInformation">Info</aloc> class. See also |
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142 | <aloc href="LesHouchesAccord">LHAupLHEF</aloc> for more information. |
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143 | </flag> |
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144 | |
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145 | <mode name="Beams:nSkipLHEFatInit" default="0"> |
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146 | Skip the first <ei>nSkip</ei> events of the input stream |
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147 | (cf. the <code>LHAup::skipEvent(nSkip)</code> method). |
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148 | Only used when <code>Beams:frameType</code> = 4 or 5. |
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149 | </flag> |
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150 | |
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151 | <h3>Beam momentum spread</h3> |
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152 | |
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153 | This framework currently is intended for a modest beam spread, such as |
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154 | experienced at hadron colliders. Thus it can be safely assumed that the |
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155 | physics does not change over the CM energy range probed, so that the |
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156 | parameters of the physics initialization at the nominal energy can be |
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157 | used as is. Currently it can <b>not</b> be used for the more extensive |
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158 | energy spread expected at linear <ei>e^+ e^-</ei> colliders. Also, |
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159 | any attempt to combine it with external Les Houches input of |
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160 | parton-level events is at own risk. |
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161 | |
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162 | <p/> |
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163 | On this page you can set the momentum spread according to a simple |
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164 | Gaussian distribution. If you instead want a more sophisticated |
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165 | parametrization, you can write and link your own |
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166 | <code><aloc href="BeamShape">BeamShape</aloc></code> class. |
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167 | |
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168 | <flag name="Beams:allowMomentumSpread" default="off"> |
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169 | Allow the beam momenta to be smeared around their initialization |
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170 | nominal values. |
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171 | </flag> |
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172 | |
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173 | <parm name="Beams:sigmaPxA" default="0." min="0."> |
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174 | The width of a Gaussian distribution of the <ei>p_x</ei> spread of the |
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175 | first incoming particle. |
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176 | </parm> |
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177 | |
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178 | <parm name="Beams:sigmaPyA" default="0." min="0."> |
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179 | The width of a Gaussian distribution of the <ei>p_y</ei> spread of the |
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180 | first incoming particle. |
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181 | </parm> |
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182 | |
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183 | <parm name="Beams:sigmaPzA" default="0." min="0."> |
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184 | The width of a Gaussian distribution of the <ei>p_z</ei> spread of the |
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185 | first incoming particle. |
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186 | </parm> |
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187 | |
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188 | <parm name="Beams:maxDevA" default="5." min="0."> |
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189 | The triply Gaussian distribution <ei>(p_x, p_y, p_z)</ei> is restricted to |
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190 | a maximal total deviation from the nominal values <ei>(p_x0, p_y0, p_z0)</ei> |
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191 | for the first incoming particle, like |
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192 | <eq> |
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193 | (p_x - p_x0)^2/sigma_px^2 + (p_y - p_y0)^2/sigma_py^2 + |
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194 | (p_z - p_z0)^2/sigma_pz^2 < maxDev^2 |
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195 | </eq> |
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196 | (Note the absence of a factor 2 in the denominator, unlike the Gaussians |
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197 | used to pick <ei>(p_x, p_y, p_z)</ei>.) |
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198 | </parm> |
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199 | |
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200 | <parm name="Beams:sigmaPxB" default="0." min="0."> |
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201 | The width of a Gaussian distribution of the <ei>p_x</ei> spread of the |
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202 | second incoming particle. |
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203 | </parm> |
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204 | |
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205 | <parm name="Beams:sigmaPyB" default="0." min="0."> |
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206 | The width of a Gaussian distribution of the <ei>p_y</ei> spread of the |
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207 | second incoming particle. |
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208 | </parm> |
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209 | |
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210 | <parm name="Beams:sigmaPzB" default="0." min="0."> |
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211 | The width of a Gaussian distribution of the <ei>p_z</ei> spread of the |
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212 | second incoming particle. |
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213 | </parm> |
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214 | |
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215 | <parm name="Beams:maxDevB" default="5." min="0."> |
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216 | The triply Gaussian distribution <ei>(p_x, p_y, p_z)</ei> is restricted to |
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217 | a maximal total deviation from the nominal values <ei>(p_x0, p_y0, p_z0)</ei>, |
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218 | for the second incoming particle, like |
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219 | <eq> |
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220 | (p_x - p_x0)^2/sigma_px^2 + (p_y - p_y0)^2/sigma_py^2 + |
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221 | (p_z - p_z0)^2/sigma_pz^2 < maxDev^2 |
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222 | </eq> |
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223 | (Note the absence of a factor 2 in the denominator, unlike the Gaussians |
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224 | used to pick <ei>(p_x, p_y, p_z)</ei>.) |
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225 | </parm> |
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226 | |
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227 | <h3>Beam interaction vertex</h3> |
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228 | |
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229 | On this page you can set the spread of the interaction vertex according to |
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230 | a simple Gaussian distribution. If you instead want a more sophisticated |
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231 | parametrization, you can write and link your own |
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232 | <code><aloc href="BeamShape">BeamShape</aloc></code> class. |
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233 | |
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234 | <flag name="Beams:allowVertexSpread" default="off"> |
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235 | Allow the interaction vertex of the two colliding beams to be smeared. |
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236 | If off, then the vertex is set to be the origin. |
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237 | </flag> |
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238 | |
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239 | <parm name="Beams:sigmaVertexX" default="0." min="0."> |
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240 | The width of a Gaussian distribution of the <ei>x</ei> location of the |
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241 | interaction vertex. |
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242 | </parm> |
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243 | |
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244 | <parm name="Beams:sigmaVertexY" default="0." min="0."> |
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245 | The width of a Gaussian distribution of the <ei>y</ei> location of the |
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246 | interaction vertex. |
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247 | </parm> |
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248 | |
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249 | <parm name="Beams:sigmaVertexZ" default="0." min="0."> |
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250 | The width of a Gaussian distribution of the <ei>z</ei> location of the |
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251 | interaction vertex. |
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252 | </parm> |
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253 | |
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254 | <parm name="Beams:maxDevVertex" default="5." min="0."> |
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255 | The triply Gaussian distribution of interaction vertex position |
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256 | <ei>(x, y, z)</ei> is restricted to a maximal total deviation from the |
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257 | origin, like |
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258 | <eq> |
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259 | x^2/sigma_x^2 + y^2/sigma_y^2 + z^2/sigma_z^2 < maxDevVertex^2 |
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260 | </eq> |
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261 | (Note the absence of a factor 2 in the denominator, unlike the Gaussians |
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262 | used to pick <ei>(x, y, z)</ei>.) |
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263 | </parm> |
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264 | |
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265 | <parm name="Beams:sigmaTime" default="0." min="0."> |
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266 | The width of a Gaussian distribution of the collision time (in units of |
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267 | mm/c). Note that, if the above space parametrization is viewed as the |
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268 | effect of two incoming beams along the <ei>+-z</ei> axis, with each beam |
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269 | having a Gaussian spread, then the spread of the time would also become |
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270 | a Gaussian with the same width as the <ei>z</ei> one (times the |
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271 | velocity of the beams, which we expect is close to unity). For flexibility |
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272 | we have not enforced any such relation, however. |
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273 | </parm> |
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274 | |
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275 | <parm name="Beams:maxDevTime" default="5." min="0."> |
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276 | The collision time is restricted to be in the range |
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277 | <ei>|t| < sigma_t * maxDevTime</ei>. |
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278 | </parm> |
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279 | |
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280 | <p/> |
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281 | The distributions above are all centered at the origin. It is also |
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282 | possible to shift the above distributions to be centered around another |
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283 | nominal position. You must have <code>Beams:allowVertexSpread = on</code> |
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284 | to use this possibility. |
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285 | |
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286 | <parm name="Beams:offsetVertexX" default="0."> |
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287 | The <ei>x</ei> location of the interaction vertex is centered at this value. |
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288 | </parm> |
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289 | |
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290 | <parm name="Beams:offsetVertexY" default="0."> |
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291 | The <ei>y</ei> location of the interaction vertex is centered at this value. |
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292 | </parm> |
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293 | |
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294 | <parm name="Beams:offsetVertexZ" default="0."> |
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295 | The <ei>z</ei> location of the interaction vertex is centered at this value. |
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296 | </parm> |
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297 | |
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298 | <parm name="Beams:offsetTime" default="0."> |
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299 | The time <ei>t</ei> of the interaction vertex is centered at this value. |
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300 | </parm> |
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301 | |
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302 | </chapter> |
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303 | |
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304 | <!-- Copyright (C) 2012 Torbjorn Sjostrand --> |
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