1 | <chapter name="The Event Record"> |
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2 | |
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3 | <h2>The Event Record</h2> |
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4 | |
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5 | A <code>Pythia</code> instance contains two members of the |
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6 | <code>Event</code> class. The one called <code>process</code> provides |
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7 | a brief summary of the main steps of the hard process, while the |
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8 | one called <code>event</code> contains the full history. The |
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9 | user would normally interact mainly with the second one, so |
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10 | we will examplify primarily with that one. |
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11 | |
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12 | <p/> |
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13 | The <code>Event</code> class to first approximation is a vector of |
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14 | <code>Particle</code>s, so that it can expand to fit the current |
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15 | event size. The index operator is overloaded, so that e.g. |
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16 | <code>event[i]</code> corresponds to the <ei>i</ei>'th particle |
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17 | of the object <code>event</code>. Thus <code>event[i].id()</code> |
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18 | returns the identity of the <ei>i</ei>'th particle, and so on. |
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19 | Therefore the methods of the |
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20 | <code><aloc href="ParticleProperties">Particle</aloc></code> class |
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21 | are at least as essential as those of the <code>Event</code> class |
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22 | itself. |
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23 | |
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24 | <p/> |
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25 | As used inside PYTHIA, some conventions are imposed on the structure |
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26 | of the event record. Entry 0 of the <code>vector<Particle></code> |
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27 | is used to represent the event as a whole, with its total four-momentum |
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28 | and invariant mass, but does not form part of the event history. |
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29 | Lines 1 and 2 contains the two incoming beams, and only from here on |
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30 | history tracing works as could be expected. That way unassigned mother |
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31 | and daughter indices can be put 0 without ambiguity. Depending on the |
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32 | task at hand, a loop may therefore start at index 1 rather than 0 |
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33 | without any loss. Specifically, for translation to other event record |
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34 | formats such as HepMC <ref>Dob01</ref>, where the first index is 1, the |
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35 | Pythia entry 0 definitely ought to be skipped in order to minimize the |
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36 | danger of indexing errors. |
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37 | |
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38 | <p/> |
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39 | In the following we will list the methods available. |
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40 | Only a few of them have a function to fill in normal user code. |
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41 | |
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42 | <h3>Basic output methods</h3> |
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43 | |
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44 | Some methods are available to read out information on the |
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45 | current event record: |
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46 | |
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47 | <method name="Particle& Event::operator[](int i)"> |
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48 | </method> |
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49 | <methodmore name="const Particle& Event::operator[](int i)"> |
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50 | </methodmore> |
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51 | <methodmore name="Particle& Event::at(int i)"> |
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52 | returns a (<code>const</code>) reference to the <ei>i</ei>'th particle |
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53 | in the event record, which can be used to get (or set) all the |
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54 | <aloc href="ParticleProperties">properties</aloc> of this particle. |
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55 | </methodmore> |
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56 | |
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57 | <method name="int Event::size()"> |
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58 | The event size, i.e. the sie of the <code>vector<Particle></code>. |
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59 | Thus valid particles, to be accessed by the above indexing operator, |
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60 | are stored in the range <ei>0 <= i < size()</ei>. See comment |
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61 | above about the (ir)relevance of entry 0. |
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62 | </method> |
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63 | |
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64 | <method name="void Event::list()"> |
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65 | </method> |
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66 | <methodmore name="void Event::list(ostream& os)"> |
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67 | </methodmore> |
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68 | <methodmore name="void Event::list(bool showScaleAndVertex, |
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69 | bool showMothersAndDaughters = false)"> |
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70 | </methodmore> |
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71 | <methodmore name="void Event::list(bool showScaleAndVertex, |
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72 | bool showMothersAndDaughters, ostream& os)"> |
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73 | Provide a listing of the whole event, i.e. of the |
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74 | <code>vector<Particle></code>. The methods with fewer arguments |
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75 | call the final one with the respective default values, and are |
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76 | non-inlined so they can be used in a debugger. The basic identity |
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77 | code, status, mother, daughter, colour, four-momentum and mass data |
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78 | are always given, but the methods can also be called with a few |
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79 | optional arguments for further information: |
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80 | <argument name="showScaleAndVertex" default="false"> optionally give a |
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81 | second line for each particle, with the production scale (in GeV), |
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82 | the particle polarization (dimensionless), the production vertex |
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83 | (in mm or mm/c) and the invariant lifetime (also in mm/c). |
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84 | </argument> |
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85 | <argument name="showMothersAndDaughters" default="false"> |
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86 | gives a list of all daughters and mothers of a particle, as defined by |
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87 | the <code>motherList(i)</code> and <code>daughterList(i)</code> methods |
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88 | described below. It is mainly intended for debug purposes. |
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89 | </argument> |
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90 | <argument name="os" default="cout"> a reference to the <code>ostream</code> |
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91 | object to which the event listing will be directed. |
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92 | </argument> |
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93 | |
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94 | </method> |
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95 | |
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96 | <p/> |
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97 | Each <code>Particle</code> has two mother and two daughter indices. |
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98 | These may be used to encode zero, one, two or more mothers/daughters, |
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99 | depending on the combination of values and status code, according to |
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100 | well-defined <aloc href="ParticleProperties">rules</aloc>. The |
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101 | two methods below can do this job easier for you. |
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102 | |
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103 | <method name="vector<int> Event::motherList(int i)"> |
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104 | returns a vector of all the mother indices of the particle at index |
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105 | <ei>i</ei>. This list is empty for entries 0, 1 and 2, |
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106 | i.e. the "system" in line 0 is not counted as part of the history. |
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107 | Normally the list contains one or two mothers, but it can also be more, |
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108 | e.g. in string fragmentation the whole fragmenting system is counted |
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109 | as mothers to the primary hadrons. Many particles may have the same |
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110 | <code>motherList</code>. Mothers are listed in ascending order. |
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111 | </method> |
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112 | |
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113 | <method name="vector<int> Event::daughterList(int i)"> |
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114 | returns a vector of all the daughter indices of the particle at index |
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115 | <ei>i</ei>. This list is empty for a particle that did |
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116 | not decay (or, if the evolution is stopped early enough, a parton |
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117 | that did not branch), while otherwise it can contain a list of |
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118 | varying length, from one to many. For the two incoming beam particles, |
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119 | all shower initiators and beam remnants are counted as daughters, |
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120 | with the one in slot 0 being the one leading up to the hardest |
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121 | interaction. The "system" in line 0 does not have any daughters, |
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122 | i.e. is not counted as part of the history. Many partons may have the |
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123 | same <code>daughterList</code>. Daughters are listed in ascending order. |
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124 | </method> |
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125 | |
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126 | <method name="int Event::statusHepMC(int i)"> |
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127 | returns the status code according to the HepMC conventions agreed in |
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128 | February 2009. This convention does not preserve the full information |
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129 | provided by the internal PYTHIA status code, as obtained by |
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130 | <code>Particle::status()</code>, but comes reasonably close. |
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131 | The allowed output values are: |
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132 | <ul> |
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133 | <li>0 : an empty entry, with no meaningful information and therefore |
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134 | to be skipped unconditionally (should not occur in PYTHIA);</li> |
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135 | <li>1 : a final-state particle, i.e. a particle that is not decayed |
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136 | further by the generator (may also include unstable particles that |
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137 | are to be decayed later, as part of the detector simulation);</li> |
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138 | <li>2 : a decayed Standard Model hadron or tau or mu lepton, excepting |
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139 | virtual intermediate states thereof (i.e. the particle must undergo |
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140 | a normal decay, not e.g. a shower branching);</li> |
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141 | <li>3 : a documentation entry (not used in PYTHIA);</li> |
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142 | <li>4 : an incoming beam particle;</li> |
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143 | <li>11 - 200 : an intermediate (decayed/branched/...) particle that does |
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144 | not fulfill the criteria of status code 2, with a generator-dependent |
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145 | classification of its nature; in PYTHIA the absolute value of the normal |
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146 | status code is used.</li> |
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147 | </ul> |
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148 | |
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149 | </method> |
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150 | |
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151 | <h3>Further output methods</h3> |
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152 | |
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153 | The above methods are the main ones that a normal user would make |
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154 | frequent use of. There are some further methods that also could come |
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155 | in handy, in the exploration of the history of an event, but where |
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156 | the outcome is not always obvious if one is not familiar with the |
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157 | detailed structure of an event record. |
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158 | |
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159 | <method name="int Event::iTopCopy(int i)"> |
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160 | </method> |
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161 | <methodmore name="int Event::iBotCopy(int i)"> |
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162 | are used to trace carbon copies of the particle at index <ei>i</ei> up |
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163 | to its top mother or down to its bottom daughter. If there are no such |
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164 | carbon copies, <ei>i</ei> itself will be returned. A carbon copy is |
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165 | when the "same" particle appears several times in the event record, but |
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166 | with changed momentum owing to recoil effects. |
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167 | </methodmore> |
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168 | |
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169 | <method name="int Event::iTopCopyId(int i)"> |
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170 | </method> |
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171 | <methodmore name="int Event::iBotCopyId(int i)"> |
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172 | also trace top mother and bottom daughter, but do not require carbon |
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173 | copies, only that one can find an unbroken chain, of mothers or daughters, |
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174 | with the same flavour <code>id</code> code. When it encounters ambiguities, |
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175 | say a <ei>g -> g g</ei> branching or a <ei>u u -> u u</ei> hard scattering, |
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176 | it will stop the tracing and return the current position. It can be confused |
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177 | by nontrivial flavour changes, e.g. a hard process <ei>u d -> d u</ei> |
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178 | by <ei>W^+-</ei> exchange will give the wrong answer. These methods |
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179 | therefore are of limited use for common particles, in particular for the |
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180 | gluon, but should work well for "rare" particles. |
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181 | </method> |
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182 | |
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183 | <method name="vector<int> Event::sisterList(int i)"> |
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184 | returns a vector of all the sister indices of the particle at index |
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185 | <ei>i</ei>, i.e. all the daughters of the first mother, except the |
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186 | particle itself. |
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187 | </method> |
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188 | |
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189 | <method name="vector<int> Event::sisterListTopBot(int i, |
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190 | bool widenSearch = true)"> |
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191 | returns a vector of all the sister indices of the particle at index |
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192 | <ei>i</ei>, tracking up and back down through carbon copies |
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193 | if required. That is, the particle is first traced up with |
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194 | <code>iTopCopy()</code> before its mother is found, and then all |
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195 | the particles in the <code>daughterList()</code> of this mother are |
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196 | traced down with <code>iBotCopy()</code>, omitting the original |
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197 | particle itself. Any non-final particles are removed from the list. |
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198 | Should this make the list empty the search criterion is widened so that |
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199 | all final daughters are allowed, not only carbon-copy ones. A second |
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200 | argument <code>false</code> inhibits the second step, and increases |
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201 | the risk that an empty list is returned. A typical example of this |
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202 | is for ISR cascades, e.g. <ei>e -> e gamma</ei> where the photon |
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203 | may not have any obvious sister in the final state if the bottom copy |
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204 | of the photon is an electron that annihilates and thus is not part of |
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205 | the final state. |
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206 | </method> |
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207 | |
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208 | <method name="bool Event::isAncestor(int i, int iAncestor)"> |
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209 | traces the particle <ei>i</ei> upwards through mother, grandmother, |
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210 | and so on, until either <ei>iAncestor</ei> is found or the top of |
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211 | the record is reached. Normally one unique mother is required, |
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212 | as is the case e.g. in decay chains or in parton showers, so that |
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213 | e.g. the tracing through a hard scattering would not work. For |
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214 | hadronization, first-rank hadrons are identified with the respective |
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215 | string endpoint quark, which may be useful e.g. for <ei>b</ei> physics, |
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216 | while higher-rank hadrons give <code>false</code>. Currently also |
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217 | ministrings that collapsed to one single hadron and junction topologies |
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218 | give <code>false</code>. |
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219 | </method> |
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220 | |
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221 | <p/> |
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222 | One data member in an <code>Event</code> object is used to keep track |
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223 | of the largest <code>col()</code> or <code>acol()</code> colour tag set |
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224 | so far, so that new ones do not clash. |
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225 | |
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226 | <modeopen name="Event:startColTag" default="100" min="0" max="1000"> |
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227 | This sets the initial colour tag value used, so that the first one |
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228 | assigned is <code>startColTag + 1</code>, etc. The Les Houches accord |
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229 | <ref>Boo01</ref> suggests this number to be 500, but 100 works equally |
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230 | well. |
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231 | </modeopen> |
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232 | |
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233 | <method name="void Event::initColTag(int colTag = 0)"> |
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234 | forces the current colour tag value to be the larger of the input |
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235 | <code>colTag</code> and the above <code>Event:startColTag</code> |
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236 | values. |
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237 | </method> |
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238 | |
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239 | <method name="int Event::lastColTag()"> |
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240 | returns the current maximum colour tag. |
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241 | </method> |
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242 | |
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243 | <method name="int Event::nextColTag()"> |
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244 | increases the current maximum colour tag by one and returns this |
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245 | new value. This method is used whenever a new colour tag is needed. |
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246 | </method> |
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247 | |
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248 | <p/> |
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249 | Many event properties are accessible via the <code>Info</code> class, |
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250 | <aloc href="EventInformation">see here</aloc>. Since they are used |
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251 | directly in the event generation, a few are stored directly in the |
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252 | <code>Event</code> class, however. |
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253 | |
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254 | <method name="void Event::scale( double scaleIn)"> |
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255 | </method> |
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256 | <methodmore name="double Event::scale()"> |
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257 | set or get the scale (in GeV) of the hardest process in the event. |
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258 | Matches the function of the <code>scale</code> variable in the |
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259 | <aloc href="LesHouchesAccord">Les Houches Accord</aloc>. |
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260 | </methodmore> |
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261 | |
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262 | <method name="void Event::scaleSecond( double scaleSecondIn)"> |
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263 | </method> |
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264 | <methodmore name="double Event::scaleSecond()"> |
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265 | set or get the scale (in GeV) of a second hard process in the event, |
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266 | in those cases where such a one |
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267 | <aloc href="SecondHardProcess">has been requested</aloc>. |
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268 | </methodmore> |
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269 | |
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270 | <h3>Constructors and modifications of the event record</h3> |
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271 | |
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272 | Although you would not normally need to create your own |
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273 | <code>Event</code> instance, there may be times where that |
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274 | could be convenient. The typical exampel would be if you want to |
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275 | create a new event record that is the sum of a few different ones, |
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276 | e.g. if you want to simulate pileup events. There may also be cases |
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277 | where you want to add one or a few particles to an existing event |
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278 | record. |
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279 | |
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280 | <method name="Event::Event(int capacity = 100)"> |
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281 | creates an empty event record, but with a reserved size |
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282 | <ei>capacity</ei> for the <code>Particle</code> vector. |
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283 | </method> |
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284 | |
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285 | <method name="Event& Event::operator=(const Event& oldEvent)"> |
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286 | copies the input event record. |
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287 | </method> |
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288 | |
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289 | <method name="Event& Event::operator+=(const Event& addEvent)"> |
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290 | appends an event to an existing one. For the appended particles |
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291 | mother, daughter and colour tags are shifted to make a consistent |
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292 | record. The zeroth particle of the appended event is not copied, |
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293 | but the zeroth particle of the combined event is updated to the |
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294 | full energy-momentum content. |
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295 | </method> |
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296 | |
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297 | <method name="void Event::init(string headerIn = "", |
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298 | ParticleData* particleDataPtrIn = 0, int startColTagIn = 100)"> |
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299 | initializes colour, the pointer to the particle database, and the |
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300 | header specification used for the event listing. We remind that a |
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301 | <code>Pythia</code> object contains two event records |
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302 | <code>process</code> and <code>event</code>. Thus one may e.g. |
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303 | call either <code>pythia.process.list()</code> or |
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304 | <code>pythia.event.list()</code>. To distinguish those two rapidly |
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305 | at visual inspection, the <code>"Pythia Event Listing"</code> header |
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306 | is printed out differently, in one case adding |
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307 | <code>"(hard process)"</code> and in the other |
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308 | <code>"(complete event)"</code>. When <code>+=</code> is used to |
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309 | append an event, the modified event is printed with |
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310 | <code>"(combination of several events)"</code> as a reminder. |
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311 | </method> |
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312 | |
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313 | <method name="void Event::clear()"> |
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314 | empties event record. Specifically the <code>Particle</code> vector |
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315 | size is reset to zero. |
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316 | </method> |
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317 | |
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318 | <method name="void Event::reset()"> |
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319 | empties the event record, as <code>clear()</code> above, but then |
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320 | fills the zero entry of the <code>Particle</code> vector with the |
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321 | pseudoparticle used to represent the event as a whole. At this point |
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322 | the pseudoparticle is not assigned any momentum or mass. |
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323 | </method> |
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324 | |
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325 | <method name="void Event::popBack(int n = 1)"> |
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326 | removes the last <ei>n</ei> particle entries; must be a positive |
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327 | number. |
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328 | </method> |
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329 | |
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330 | <method name="int Event::append(Particle entryIn)"> |
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331 | appends a particle to the bottom of the event record and |
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332 | returns the index of this position. |
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333 | </method> |
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334 | |
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335 | <method name="int Event::append(int id, int status, int mother1, |
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336 | int mother2, int daughter1, int daughter2, int col, int acol, |
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337 | double px, double py, double pz, double e, double m = 0., |
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338 | double scale = 0., double pol = 9.)"> |
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339 | appends a particle to the bottom of the event record and |
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340 | returns the index of this position; |
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341 | <aloc href="ParticleProperties">see here</aloc> for the meaning |
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342 | of the various particle properties. |
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343 | </method> |
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344 | |
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345 | <method name="int Event::append(int id, int status, int mother1, |
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346 | int mother2, int daughter1, int daughter2, int col, int acol, |
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347 | Vec4 p, double m = 0., double scale = 0., double pol = 9.)"> |
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348 | appends a particle to the bottom of the event record and |
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349 | returns the index of this position, as above but with four-momentum |
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350 | as a <code>Vec4</code>. |
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351 | </method> |
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352 | |
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353 | <method name="int Event::append(int id, int status, int col, int acol, |
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354 | double px, double py, double pz, double e, double m = 0., |
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355 | double scale = 0., double pol = 9.)"> |
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356 | </method> |
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357 | <methodmore name="int Event::append(int id, int status, int col, |
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358 | int acol, Vec4 p, double m = 0., double scale = 0., double pol = 9.)"> |
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359 | appends a particle to the bottom of the event record and |
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360 | returns the index of this position, as above but with vanishing |
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361 | (i.e. zero) mother and daughter indices. |
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362 | </method> |
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363 | |
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364 | <method name="int Event::setPDTPtr(int iSet = -1)"> |
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365 | send in a pointer to the <code>ParticleData</code> database for |
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366 | particle <code>iSet</code>, by default the most recently appended |
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367 | particle. Also generates a pointer to the |
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368 | <code>ParticleDataEntry</code> object of the identity code |
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369 | of the particle. |
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370 | </method> |
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371 | |
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372 | <method name="int Event::copy(int iCopy, int newStatus = 0)"> |
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373 | copies the existing particle in entry <code>iCopy</code> to the |
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374 | bottom of the event record and returns the index of this position. |
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375 | By default, i.e. with <code>newStatus = 0</code>, everything is |
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376 | copied precisely as it is, which means that history information |
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377 | has to be modified further by hand to make sense. With a positive |
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378 | <code>newStatus</code>, the new copy is set up to be the daughter of |
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379 | the old, with status code <code>newStatus</code>, while the status |
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380 | code of <code>iCopy</code> is negated. With a negative |
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381 | <code>newStatus</code>, the new copy is instead set up to be the |
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382 | mother of <code>iCopy</code>. An attempt to copy an out-of-range |
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383 | entry will return -1. |
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384 | </method> |
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385 | |
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386 | <method name="Particle& Event::back()"> |
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387 | returns a reference to the last particle in the event record. |
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388 | </method> |
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389 | |
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390 | <method name="void Event::restorePtrs()"> |
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391 | each particle in the event record has a pointer to the whole database |
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392 | and another to the particle species itself, used to find some particle |
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393 | properties. The latter pointer is automatically set/changed whenever |
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394 | the particle identity is set/changed by one of the normal methods. |
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395 | (It is the "changed" part that prompts the inclusion of a pointer |
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396 | to the whole database.) Of course the pointer values are specific to |
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397 | the memory locations of the current run, and so it has no sense to |
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398 | save them if events are written to file. Should you use some |
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399 | persistency scheme that bypasses the normal methods when the event is |
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400 | read back in, you can use <code>restorePtrs()</code> afterwards to set |
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401 | these pointers appropriately. |
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402 | </method> |
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403 | |
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404 | <p/> |
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405 | A few methods exist to rotate and boost events. These derive from the |
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406 | <aloc href="FourVectors">Vec4</aloc> methods, and affect both the |
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407 | momentum and the vertex (position) components of all particles. |
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408 | |
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409 | <method name="void Event::rot(double theta, double phi)"> |
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410 | rotate all particles in the event by this polar and azimuthal angle |
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411 | (expressed in radians). |
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412 | </method> |
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413 | |
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414 | <method name="void Event::bst(double betaX, double betaY, double betaZ)"> |
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415 | </method> |
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416 | <methodmore name="void Event::bst(double betaX, double betaY, |
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417 | double betaZ, double gamma)"> |
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418 | </methodmore> |
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419 | <methodmore name="void Event::bst(const Vec4& vec)"> |
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420 | boost all particles in the event by this three-vector. |
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421 | Optionally you may provide the <ei>gamma</ei> value as a fourth argument, |
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422 | which may help avoid roundoff errors for big boosts. You may alternatively |
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423 | supply a <code>Vec4</code> four-vector, in which case the boost vector |
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424 | becomes <ei>beta = p/E</ei>. |
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425 | </methodmore> |
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426 | |
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427 | <method name="void Event::rotbst(const RotBstMatrix& M)"> |
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428 | rotate and boost by the combined action encoded in the |
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429 | <code><aloc href="FourVectors">RotBstMatrix</aloc> M</code>. |
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430 | </method> |
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431 | |
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432 | <h3>The Junction Class</h3> |
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433 | |
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434 | The event record also contains a vector of junctions, which often |
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435 | is empty or else contains only a very few per event. Methods are |
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436 | available to add further junctions or query the current junction list. |
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437 | This is only for the expert user, however, and is not discussed |
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438 | further here, but only the main points. |
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439 | |
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440 | <p/> |
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441 | A junction stores the properites associated with a baryon number that |
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442 | is fully resolved, i.e. where three different colour indices are |
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443 | involved. There are two main applications, |
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444 | <ol> |
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445 | <li>baryon beams, where at least two valence quarks are kicked out, |
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446 | and so the motion of the baryon number is notrivial;</li> |
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447 | <li>baryon-number violating processes, e.g. in SUSY with broken |
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448 | <ei>R</ei>-parity.</li> |
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449 | </ol> |
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450 | Information on junctions is set, partly in the process generation, |
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451 | partly in the beam remnants machinery, and used by the fragmentation |
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452 | routines, but the normal user does not have to know the details. |
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453 | |
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454 | <p/> |
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455 | For each junction, information is stored on the kind of junction, and |
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456 | on the three (anti)colour indices that are involved in the junction. |
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457 | The possibilities foreseen are: |
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458 | <ul> |
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459 | <li><code>kind = 1</code> : incoming colourless particle to three |
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460 | outgoing colours (e.g. baryon beam remnant or |
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461 | <ei>neutralino -> q q q</ei>);</li> |
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462 | <li><code>kind = 2</code> : incoming colourless particle to three |
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463 | outgoing anticolours;</li> |
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464 | <li><code>kind = 3</code> : one incoming anticolour (stored first) |
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465 | and two outgoing colours (e.g. antisquark decaying to two quarks, or |
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466 | gluino decay to three quarks);</li> |
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467 | <li><code>kind = 4</code> : one incoming colour (stored first) and two |
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468 | outgoing anticolours (e.g. squark decaying to two antiquarks, or |
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469 | gluino decaying to three antiquarks);</li> |
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470 | <li><code>kind = 5</code> : two incoming anticolours (stored first) |
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471 | and one outgoing colour (e.g. resonant squark production through RPV);</li> |
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472 | <li><code>kind = 6</code> : two incoming colours (stored first) |
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473 | and one outgoing anticolour (e.g. resonant antisquark production |
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474 | through RPV); |
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475 | </li> |
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476 | </ul> |
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477 | The odd (even) <code>kind</code> codes corresponds to a +1 (-1) change in |
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478 | baryon number across the junction. |
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479 | |
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480 | <p/> |
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481 | The kind and colour information in the list of junctions can be set |
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482 | or read with methods of the <code>Event</code> class, but are not of |
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483 | common interest and so not described here. |
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484 | |
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485 | <p/> |
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486 | A listing of current junctions can be obtained with the |
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487 | <code>listJunctions()</code> method. |
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488 | |
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489 | <h3>Subsystems</h3> |
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490 | |
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491 | Separate from the event record as such, but closely tied to it is the |
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492 | <code><aloc href="AdvancedUsage">PartonSystems</aloc></code> class, |
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493 | which mainly stores the parton indices of incoming and outgoing partons, |
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494 | classified by collision subsystem. Such information is needed to |
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495 | interleave multiparton interactions, initial-state showers and final-state |
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496 | showers, and append beam remnants. It could also be used in other places. |
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497 | It is intended to be accessed only by experts, such as implementors of |
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498 | <aloc href="ImplementNewShowers">new showering models</aloc>. |
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499 | |
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500 | </chapter> |
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501 | |
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502 | <!-- Copyright (C) 2012 Torbjorn Sjostrand --> |
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