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29 | |
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30 | <h2>Tunes</h2> |
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31 | |
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32 | Since some physics aspects cannot be derived from first principles, |
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33 | this program contains many parameters that represent a true |
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34 | uncertainty in our understanding of nature. Particularly afflicted |
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35 | are the areas of hadronization and multiparton interactions, which both |
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36 | involve nonperturbative QCD physics. |
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37 | |
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38 | <p/> |
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39 | Technically, PYTHIA parameters can be varied independently of each |
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40 | other, but the physical requirement of a sensible description of a set |
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41 | of data leads to correlations and anticorrelations between the |
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42 | parameters. Hence the need to produce tunes, not of one parameter at |
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43 | a time, but simultaneously for a group of them. A well-known (separate) |
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44 | such example is parton densities, where combined tunes to a wide range |
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45 | of data have been produced, that can then be obtained prepackaged. |
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46 | |
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47 | <p/> |
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48 | Given the many PYTHIA parameters to be tuned, it is convenient to |
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49 | divide the task into subtasks. Firstly, if we assume jet universality, |
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50 | hadronization and final-state parton showers should be tuned to |
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51 | <i>e^+e^-</i> annihilation data, notably from LEP1, since this |
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52 | offers the cleanest environment. Secondly, with such parameters fixed, |
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53 | hadron collider data should be studied to pin down multiparton interactions |
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54 | and other further aspects, such as initial-state radiation. Ideally this |
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55 | would be done separately for diffractive and non-diffractive events, |
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56 | although it is not possible to have a clean separation. (Thirdly |
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57 | would come anything else, such as physics with photon beams, which |
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58 | involve further parameters, but that is beyond the current scope.) |
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59 | |
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60 | <p/> |
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61 | The first step in this program has now been taken, with a tune to LEP1 |
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62 | data by Hendrik Hoeth, using the Rivet + Professor framework. Starting |
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63 | with version 8.125 it defines the default values for hadronization |
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64 | parameters and timelike showers. |
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65 | |
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66 | <p/> |
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67 | The situation is more complicated for hadronic interactions in general |
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68 | and multiparton interactions in particular, where PYTHIA 8 is more |
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69 | different from PYTHIA 6, and therefore more work is needed. Specifically, |
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70 | it is <i>not</i> possible to "port" a PYTHIA 6 tune to PYTHIA 8. |
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71 | |
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72 | <p/> |
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73 | A first simple tune, appropriately called "Tune 1", became default |
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74 | starting with version 8.127. It was noted, in particular by Hendrik |
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75 | Hoeth, that this tune had a tension between parameters needed to |
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76 | describe minimum-bias and underlying-event activity. Therefore some |
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77 | further physics features were introduced in the code itself |
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78 | [<a href="Bibliography.php" target="page">Cor10a</a>], which were made default as of 8.140. This version |
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79 | also included two new tunes, 2C and 2M, based on the CTEQ 6L1 and the |
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80 | MRST LO** PDF sets, respectively. These have been made by hand, as a |
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81 | prequel to complete Professor-style tunings. |
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82 | |
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83 | <p/> |
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84 | The very first data to come out of the LHC showed a higher rapidity |
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85 | plateau than predicted for current PYTHIA 6 tunes, also for the lower |
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86 | energies. This may suggest some tension in the data. Two alternatives, |
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87 | 3C and 3M, were produced by a few brute-force changes of 2C and 2M. |
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88 | These were introduced in 8.140, but discontinued in 8.145 in favour of |
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89 | the new 4C tune, that is based on a more serious study of some early |
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90 | LHC data, see [<a href="Bibliography.php" target="page">Cor10a</a>]. Following the comparative studies in |
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91 | [<a href="Bibliography.php" target="page">Buc11</a>], which independently confirmed a reasonable agreement |
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92 | with LHC data, tune 4C was made the default as of 8.150. A variant is |
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93 | tune 4Cx, where the Gaussian matter profile has an <i>x</i>-dependent |
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94 | width [<a href="Bibliography.php" target="page">Cor11</a>]. |
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95 | |
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96 | <p/> |
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97 | Several ATLAS tunes have now been included, obtained with different PDFs |
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98 | and with different emphasis on minimum-bias and underlying-event data |
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99 | [<a href="Bibliography.php" target="page">ATL12</a>]. These typically require LHAPDF to be linked, but this |
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100 | can be avoided in cases where the same PDF set is implemented internally. |
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101 | |
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102 | <p/> |
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103 | Central diffraction is a recent addition to the "soft QCD" process palette, |
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104 | and is thus not yet included in tunes; indeed its cross section is actively |
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105 | zeroed. You can switch it back on <i>after</i> you have selected your tune, |
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106 | with <code>SigmaTotal:zeroAXB = off</code>. But note that, since the |
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107 | total cross section is assumed unchanged, the minbias cross section |
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108 | is reduced and thus also the MPI machinery affected, even if effects |
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109 | should not be big (for a small central diffractive cross section). |
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110 | |
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111 | <p/> |
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112 | Note that comparisons with data also require that other aspects agree, |
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113 | such as that decay chains are stopped at an agreed-on level. For instance, |
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114 | in the ATLAS tunes all particles with a lifetime above 10 mm |
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115 | are considered stable, <code>ParticleDecays:limitTau0 = on</code>, |
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116 | <code>ParticleDecays:tau0Max = 10</code>. We have chosen not to |
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117 | include this as part of the tune settings itself, since the tune as |
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118 | such could still be used with any other choice of stable and |
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119 | unstable particles. |
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120 | |
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121 | <p/> |
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122 | Further comparisons have been posted on the |
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123 | <a href="http://mcplots.cern.ch/">MCPLOTS</a> pages. |
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124 | They have been produced with help of the |
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125 | <a href="http://projects.hepforge.org/rivet/">Rivet</a> package |
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126 | [<a href="Bibliography.php" target="page">Buc10</a>]. |
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127 | |
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128 | <p/> |
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129 | In the future we hope to see further PYTHIA 8 tunes appear. Like with |
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130 | parton distributions, there is likely to be several tunes, because |
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131 | different sets of data will pull in different directions, by imperfections |
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132 | in the model or in the data, and by differences in the chosen |
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133 | tuning strategies. We therefore propose to collect some of these tunes |
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134 | here, in a prepackaged form. Of course, in all cases it is a matter |
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135 | of setting values for parameters already defined elsewhere, so the |
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136 | tunes offer no new functionality, only a more convenient setup. |
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137 | |
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138 | <p/> |
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139 | You should be aware that the evolution of the program will not guarantee |
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140 | complete backwards compatibility between versions. Most obviously this |
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141 | concerns bug fixes. But also for some other major changes, like the |
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142 | introduction of the new diffractive machinery, the default behaviour |
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143 | of old tunes has been changed retroactively. (Which should be fine for |
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144 | diffraction, since previous tunes were not based on data strongly |
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145 | influenced by diffraction.) |
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146 | |
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147 | <p/> |
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148 | The setup of the tunes is special, in that the choice of a tune forces |
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149 | the change of several different flags, modes and parameters. Furthermore |
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150 | a design principle has been that it should be possible to start out |
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151 | from a tune and then change a few of its settings. This gives power |
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152 | and flexibility at the expense of requiring a more careful ordering |
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153 | of commands. We therefore here sketch the order in which operations |
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154 | are carried out. |
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155 | <ol> |
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156 | <li>The constructor of a <code>Pythia</code> instance will read in |
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157 | all settings, and initialize them with their default values. |
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158 | </li> |
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159 | <li>At the end of this operation, the <code>Tune:ee</code> and |
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160 | <code>Tune:pp</code> modes (see further below) are checked. If either |
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161 | of them are positive the methods <code>Settings::initTuneEE(...)</code> |
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162 | and <code>Settings::initTunePP(...)</code>, respectively, are called |
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163 | to overwrite the whole collection of settings in the relevant tune. |
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164 | Zero (or negative) means that nothing will be done. |
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165 | </li> |
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166 | <li>After the <code>Pythia</code> constructor all the relevant values |
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167 | for the default tune(s) have thus been set up. |
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168 | </li> |
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169 | <li>You as a user can now start to overwrite the values at will, |
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170 | using <code>Pythia::readFile(...)</code> to read a configuration file, |
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171 | or a list of <code>Pythia::readString(...)</code> commands, |
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172 | or the lower-level <code>Settings</code> methods. All changes |
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173 | are made in the order in which the commands are encountered during |
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174 | the execution. A given variable can be changed multiparton times, |
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175 | but it is the latest change that sets the current value. |
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176 | </li> |
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177 | <li>The two <code>Tune:ee</code> and <code>Tune:pp</code> modes can also |
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178 | be changed in exactly the same way as described for all other settings |
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179 | above. Unique for them, however, is that when one of them is encountered |
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180 | it also initiates a call to the <code>initTuneEE(...)</code> or |
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181 | <code>initTunePP(...)</code> method, respectively. In such cases all |
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182 | settings affected by the <i>e^+e^-</i> or <i>pp/ppbar</i> tune |
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183 | are first reset to the default values (the <code>-1</code> options) |
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184 | and thereafter the relevant tune is set up. |
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185 | </li> |
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186 | <li>It is possible to mix commands of type 4 and 5 in any order; it |
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187 | is always the last change that counts. That is, any changes you have |
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188 | made to variables of a tune <i>before</i> a <code>Tune:ee</code> or |
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189 | <code>Tune:pp</code> command are overwritten by it, while variables |
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190 | you set <i>after</i> will overwrite the tune values. As a rule, |
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191 | therefore, you want to begin with the tune choice, and thereafter |
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192 | modify only a small part of its settings. |
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193 | </li> |
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194 | <li>Needless to say, the flexibility can lead to unwanted setups if |
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195 | you do not exercise some discipline. It is therefore recommended that |
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196 | you always check the listing obtained with |
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197 | <code>Pythia::settings.listChanged()</code> to confirm that the |
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198 | final set of changes is the intended one. |
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199 | </li> |
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200 | </ol> |
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201 | |
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202 | <br/><br/><table><tr><td><strong>Tune:ee </td><td> (<code>default = <strong>0</strong></code>; <code>minimum = -1</code>; <code>maximum = 3</code>)</td></tr></table> |
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203 | Choice of tune to <ei>e^+e^-</ei> data, mainly for the hadronization |
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204 | and timelike-showering aspects of PYTHIA. You should study the |
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205 | <code>Settings::initTuneEE(...)</code> method to find exactly which |
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206 | are the settings for the respective tune. |
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207 | <br/> |
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208 | <input type="radio" name="1" value="-1"><strong>-1 </strong>: reset all values that are affected by any of the <ei>e^+e^-</ei> tunes to the default values. This option can be used on its own, but is also automatically used as a first step for either of the positive tune values below, to undo the effect of previous tune settings. <br/> |
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209 | <input type="radio" name="1" value="0" checked="checked"><strong>0 </strong>: no values are overwritten during the initial setup, step 2 above. Note that changing to <code>0</code> in the user code has no effect; if you want to restore the individual settings you should instead use <code>-1</code>. <br/> |
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210 | <input type="radio" name="1" value="1"><strong>1 </strong>: the original PYTHIA 8 parameter set, based on some very old flavour studies (with JETSET around 1990) and a simple tune <ei>of alpha_strong</ei> to three-jet shapes to the new <ei>pT</ei>-ordered shower. These were the default values before version 8.125. <br/> |
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211 | <input type="radio" name="1" value="2"><strong>2 </strong>: a tune by Marc Montull to the LEP 1 particle composition, as published in the RPP (August 2007). No related (re)tune to event shapes has been performed, however. <br/> |
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212 | <input type="radio" name="1" value="3"><strong>3 </strong>: a tune to a wide selection of LEP1 data by Hendrik Hoeth within the Rivet + Professor framework, both to hadronization and timelike-shower parameters (June 2009). These are the default values starting from version 8.125, so currently there is no need for this option. <br/> |
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213 | |
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214 | <br/><br/><strong>Tune:preferLHAPDF</strong> <input type="radio" name="2" value="on" checked="checked"><strong>On</strong> |
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215 | <input type="radio" name="2" value="off"><strong>Off</strong> |
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216 | (<code>default = <strong>on</strong></code>)<br/> |
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217 | Tunes made by experimental collaborations typically use the LHAPDF |
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218 | package to obtain their PDF values, and so PYTHIA has to be linked |
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219 | accordingly. For PDFs implemanted natively in PYTHIA it is possible |
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220 | to use the respective tunes, without having to link to LHAPDF, if you |
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221 | set <code>Tune:preferLHAPDF = off</code> <i>before</i> the |
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222 | <code>Tune:pp</code> choice. |
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223 | |
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224 | |
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225 | <br/><br/><table><tr><td><strong>Tune:pp </td><td> (<code>default = <strong>5</strong></code>; <code>minimum = -1</code>; <code>maximum = 11</code>)</td></tr></table> |
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226 | Choice of tune to <ei>pp/ppbar</ei> data, mainly for the |
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227 | initial-state-radiation, multiparton-interactions and beam-remnants |
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228 | aspects of PYTHIA. Note that the previous crude (non-)tunes |
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229 | 3C and 3M are removed as of 8.145, superseded by the 4C tune. |
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230 | You should study the <code>Settings::initTunePP(...)</code> method |
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231 | to find exactly which are the settings for the respective tune. |
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232 | <br/> |
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233 | <input type="radio" name="3" value="-1"><strong>-1 </strong>: reset all values that are affected by any of the <ei>pp/ppbar</ei> tunes to the default values. This option can be used on its own, but is also automatically used as a first step for either of the positive tune values below, to undo the effect of previous tune settings. <br/> |
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234 | <input type="radio" name="3" value="0"><strong>0 </strong>: no values are overwritten during the initial setup, step 2 above. Note that changing to <code>0</code> in the user code has no effect; if you want to restore the individual settings you should instead use <code>-1</code>. <br/> |
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235 | <input type="radio" name="3" value="1"><strong>1 </strong>: default used up to version 8.126, based on some early and primitive comparisons with data. <br/> |
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236 | <input type="radio" name="3" value="2"><strong>2 </strong>: "Tune 1", default in 8.127 - 8.139, based on some data comparisons by Peter Skands. Largely but not wholly overlaps with the default option 0. <br/> |
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237 | <input type="radio" name="3" value="3"><strong>3 </strong>: "Tune 2C", introduced with 8.140 <ref>Cor10a</ref>. It uses the CTEQ 6L1 PDF, and is intended to give good agreement with much of the published CDF data. <br/> |
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238 | <input type="radio" name="3" value="4"><strong>4 </strong>: "Tune 2M", introduced with 8.140 <ref>Cor10a</ref>. It is uses the MRST LO** PDF, which has a momentum sum somewhat above unity, which is compensated by a smaller <ei>alpha_s</ei> than in the previous tune. Again it is intended to give good agreement with much of the published CDF data. <br/> |
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239 | <input type="radio" name="3" value="5" checked="checked"><strong>5 </strong>: "Tune 4C", new tune, introduced with 8.145 <ref>Cor10a</ref>. Starts out from tune 2C, but with a reduced cross section for diffraction, plus modified multiparton interactions parameters to give a higher and more rapidly increasing charged pseudorapidity plateau, for better agreement with some early key LHC numbers. See also the comparative study in <ref>Buc11</ref>. <br/> |
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240 | <input type="radio" name="3" value="6"><strong>6 </strong>: "Tune 4Cx", based on tune 4C, but using the x-dependent matter profile, <code>MultipartonInteractions:bProfile = 4</code> and an increased <code>MultipartonInteractions:pT0Ref</code> <ref>Cor11</ref>. <br/> |
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241 | <input type="radio" name="3" value="7"><strong>7 </strong>: "ATLAS MB Tune A2-CTEQ6L1", a minimum-bias tune based on tune 4Cx, but without rapidity-ordered spacelike emissions <ref>ATL12</ref>. Uses CTEQ 6L1, by default from LHAPDF. <br/> |
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242 | <input type="radio" name="3" value="8"><strong>8 </strong>: "ATLAS MB Tune A2-MSTW2008LO", as above, but uses MSTW 2008 LO, by default from LHAPDF. <br/> |
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243 | <input type="radio" name="3" value="9"><strong>9 </strong>: "ATLAS UE Tune AU2-CTEQ6L1", an underlying-event tune based on tune 4Cx, but without rapidity-ordered spacelike emissions <ref>ATL12</ref>. Uses CTEQ 6L1, by default from LHAPDF. <br/> |
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244 | <input type="radio" name="3" value="10"><strong>10 </strong>: "ATLAS UE Tune AU2-MSTW2008LO", as above, but uses MSTW 2008 LO, by default from LHAPDF. <br/> |
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245 | <input type="radio" name="3" value="11"><strong>11 </strong>: "ATLAS UE Tune AU2-CT10", as above, but uses CT 10, which is not currently implemented in PYTHIA, so you must link LHAPDF. <br/> |
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246 | <input type="radio" name="3" value="12"><strong>12 </strong>: "ATLAS UE Tune AU2-MRST2007LO*", as above, but uses MRST 2007 LO*, by default from LHAPDF. <br/> |
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247 | <input type="radio" name="3" value="13"><strong>13 </strong>: "ATLAS UE Tune AU2-MRST2007LO**", as above, but uses MRST 2007 LO**, by default from LHAPDF. <br/> |
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248 | |
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249 | <input type="hidden" name="saved" value="1"/> |
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250 | |
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251 | <?php |
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252 | echo "<input type='hidden' name='filepath' value='".$_GET["filepath"]."'/>"?> |
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253 | |
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254 | <table width="100%"><tr><td align="right"><input type="submit" value="Save Settings" /></td></tr></table> |
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255 | </form> |
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256 | |
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257 | <?php |
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258 | |
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259 | if($_POST["saved"] == 1) |
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260 | { |
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261 | $filepath = $_POST["filepath"]; |
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262 | $handle = fopen($filepath, 'a'); |
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263 | |
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264 | if($_POST["1"] != "0") |
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265 | { |
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266 | $data = "Tune:ee = ".$_POST["1"]."\n"; |
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267 | fwrite($handle,$data); |
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268 | } |
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269 | if($_POST["2"] != "on") |
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270 | { |
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271 | $data = "Tune:preferLHAPDF = ".$_POST["2"]."\n"; |
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272 | fwrite($handle,$data); |
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273 | } |
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274 | if($_POST["3"] != "5") |
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275 | { |
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276 | $data = "Tune:pp = ".$_POST["3"]."\n"; |
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277 | fwrite($handle,$data); |
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278 | } |
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279 | fclose($handle); |
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280 | } |
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281 | |
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282 | ?> |
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283 | </body> |
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284 | </html> |
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285 | |
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286 | <!-- Copyright (C) 2012 Torbjorn Sjostrand --> |
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