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| 2 | <head> |
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| 3 | <title>Tunes</title> |
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| 4 | <link rel="stylesheet" type="text/css" href="pythia.css"/> |
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| 5 | <link rel="shortcut icon" href="pythia32.gif"/> |
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| 6 | </head> |
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| 7 | <body> |
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| 8 | |
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| 9 | <h2>Tunes</h2> |
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| 10 | |
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| 11 | Since some physics aspects cannot be derived from first principles, |
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| 12 | this program contains many parameters that represent a true |
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| 13 | uncertainty in our understanding of nature. Particularly afflicted |
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| 14 | are the areas of hadronization and multiparton interactions, which both |
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| 15 | involve nonperturbative QCD physics. |
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| 16 | |
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| 17 | <p/> |
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| 18 | Technically, PYTHIA parameters can be varied independently of each |
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| 19 | other, but the physical requirement of a sensible description of a set |
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| 20 | of data leads to correlations and anticorrelations between the |
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| 21 | parameters. Hence the need to produce tunes, not of one parameter at |
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| 22 | a time, but simultaneously for a group of them. A well-known (separate) |
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| 23 | such example is parton densities, where combined tunes to a wide range |
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| 24 | of data have been produced, that can then be obtained prepackaged. |
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| 25 | |
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| 26 | <p/> |
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| 27 | Given the many PYTHIA parameters to be tuned, it is convenient to |
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| 28 | divide the task into subtasks. Firstly, if we assume jet universality, |
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| 29 | hadronization and final-state parton showers should be tuned to |
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| 30 | <i>e^+e^-</i> annihilation data, notably from LEP1, since this |
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| 31 | offers the cleanest environment. Secondly, with such parameters fixed, |
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| 32 | hadron collider data should be studied to pin down multiparton interactions |
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| 33 | and other further aspects, such as initial-state radiation. Ideally this |
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| 34 | would be done separately for diffractive and non-diffractive events, |
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| 35 | although it is not possible to have a clean separation. (Thirdly |
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| 36 | would come anything else, such as physics with photon beams, which |
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| 37 | involve further parameters, but that is beyond the current scope.) |
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| 38 | |
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| 39 | <p/> |
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| 40 | The first step in this program has now been taken, with a tune to LEP1 |
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| 41 | data by Hendrik Hoeth, using the Rivet + Professor framework. Starting |
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| 42 | with version 8.125 it defines the default values for hadronization |
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| 43 | parameters and timelike showers. |
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| 44 | |
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| 45 | <p/> |
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| 46 | The situation is more complicated for hadronic interactions in general |
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| 47 | and multiparton interactions in particular, where PYTHIA 8 is more |
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| 48 | different from PYTHIA 6, and therefore more work is needed. Specifically, |
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| 49 | it is <i>not</i> possible to "port" a PYTHIA 6 tune to PYTHIA 8. |
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| 50 | |
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| 51 | <p/> |
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| 52 | A first simple tune, appropriately called "Tune 1", became default |
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| 53 | starting with version 8.127. It was noted, in particular by Hendrik |
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| 54 | Hoeth, that this tune had a tension between parameters needed to |
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| 55 | describe minimum-bias and underlying-event activity. Therefore some |
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| 56 | further physics features were introduced in the code itself |
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| 57 | [<a href="Bibliography.html" target="page">Cor10a</a>], which were made default as of 8.140. This version |
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| 58 | also included two new tunes, 2C and 2M, based on the CTEQ 6L1 and the |
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| 59 | MRST LO** PDF sets, respectively. These have been made by hand, as a |
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| 60 | prequel to complete Professor-style tunings. |
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| 61 | |
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| 62 | <p/> |
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| 63 | The very first data to come out of the LHC showed a higher rapidity |
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| 64 | plateau than predicted for current PYTHIA 6 tunes, also for the lower |
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| 65 | energies. This may suggest some tension in the data. Two alternatives, |
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| 66 | 3C and 3M, were produced by a few brute-force changes of 2C and 2M. |
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| 67 | These were introduced in 8.140, but discontinued in 8.145 in favour of |
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| 68 | the new 4C tune, that is based on a more serious study of some early |
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| 69 | LHC data, see [<a href="Bibliography.html" target="page">Cor10a</a>]. Following the comparative studies in |
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| 70 | [<a href="Bibliography.html" target="page">Buc11</a>], which independently confirmed a reasonable agreement |
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| 71 | with LHC data, tune 4C was made the default as of 8.150. A variant is |
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| 72 | tune 4Cx, where the Gaussian matter profile has an <i>x</i>-dependent |
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| 73 | width [<a href="Bibliography.html" target="page">Cor11</a>]. |
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| 74 | |
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| 75 | <p/> |
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| 76 | Several ATLAS tunes have now been included, obtained with different PDFs |
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| 77 | and with different emphasis on minimum-bias and underlying-event data |
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| 78 | [<a href="Bibliography.html" target="page">ATL12</a>]. These typically require LHAPDF to be linked, but this |
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| 79 | can be avoided in cases where the same PDF set is implemented internally. |
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| 80 | |
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| 81 | <p/> |
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| 82 | Central diffraction is a recent addition to the "soft QCD" process palette, |
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| 83 | and is thus not yet included in tunes; indeed its cross section is actively |
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| 84 | zeroed. You can switch it back on <i>after</i> you have selected your tune, |
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| 85 | with <code>SigmaTotal:zeroAXB = off</code>. But note that, since the |
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| 86 | total cross section is assumed unchanged, the minbias cross section |
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| 87 | is reduced and thus also the MPI machinery affected, even if effects |
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| 88 | should not be big (for a small central diffractive cross section). |
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| 89 | |
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| 90 | <p/> |
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| 91 | Note that comparisons with data also require that other aspects agree, |
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| 92 | such as that decay chains are stopped at an agreed-on level. For instance, |
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| 93 | in the ATLAS tunes all particles with a lifetime above 10 mm |
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| 94 | are considered stable, <code>ParticleDecays:limitTau0 = on</code>, |
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| 95 | <code>ParticleDecays:tau0Max = 10</code>. We have chosen not to |
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| 96 | include this as part of the tune settings itself, since the tune as |
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| 97 | such could still be used with any other choice of stable and |
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| 98 | unstable particles. |
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| 99 | |
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| 100 | <p/> |
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| 101 | Further comparisons have been posted on the |
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| 102 | <a href="http://mcplots.cern.ch/">MCPLOTS</a> pages. |
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| 103 | They have been produced with help of the |
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| 104 | <a href="http://projects.hepforge.org/rivet/">Rivet</a> package |
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| 105 | [<a href="Bibliography.html" target="page">Buc10</a>]. |
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| 106 | |
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| 107 | <p/> |
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| 108 | In the future we hope to see further PYTHIA 8 tunes appear. Like with |
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| 109 | parton distributions, there is likely to be several tunes, because |
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| 110 | different sets of data will pull in different directions, by imperfections |
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| 111 | in the model or in the data, and by differences in the chosen |
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| 112 | tuning strategies. We therefore propose to collect some of these tunes |
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| 113 | here, in a prepackaged form. Of course, in all cases it is a matter |
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| 114 | of setting values for parameters already defined elsewhere, so the |
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| 115 | tunes offer no new functionality, only a more convenient setup. |
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| 116 | |
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| 117 | <p/> |
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| 118 | You should be aware that the evolution of the program will not guarantee |
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| 119 | complete backwards compatibility between versions. Most obviously this |
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| 120 | concerns bug fixes. But also for some other major changes, like the |
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| 121 | introduction of the new diffractive machinery, the default behaviour |
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| 122 | of old tunes has been changed retroactively. (Which should be fine for |
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| 123 | diffraction, since previous tunes were not based on data strongly |
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| 124 | influenced by diffraction.) |
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| 125 | |
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| 126 | <p/> |
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| 127 | The setup of the tunes is special, in that the choice of a tune forces |
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| 128 | the change of several different flags, modes and parameters. Furthermore |
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| 129 | a design principle has been that it should be possible to start out |
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| 130 | from a tune and then change a few of its settings. This gives power |
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| 131 | and flexibility at the expense of requiring a more careful ordering |
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| 132 | of commands. We therefore here sketch the order in which operations |
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| 133 | are carried out. |
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| 134 | <ol> |
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| 135 | <li>The constructor of a <code>Pythia</code> instance will read in |
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| 136 | all settings, and initialize them with their default values. |
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| 137 | </li> |
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| 138 | <li>At the end of this operation, the <code>Tune:ee</code> and |
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| 139 | <code>Tune:pp</code> modes (see further below) are checked. If either |
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| 140 | of them are positive the methods <code>Settings::initTuneEE(...)</code> |
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| 141 | and <code>Settings::initTunePP(...)</code>, respectively, are called |
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| 142 | to overwrite the whole collection of settings in the relevant tune. |
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| 143 | Zero (or negative) means that nothing will be done. |
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| 144 | </li> |
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| 145 | <li>After the <code>Pythia</code> constructor all the relevant values |
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| 146 | for the default tune(s) have thus been set up. |
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| 147 | </li> |
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| 148 | <li>You as a user can now start to overwrite the values at will, |
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| 149 | using <code>Pythia::readFile(...)</code> to read a configuration file, |
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| 150 | or a list of <code>Pythia::readString(...)</code> commands, |
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| 151 | or the lower-level <code>Settings</code> methods. All changes |
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| 152 | are made in the order in which the commands are encountered during |
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| 153 | the execution. A given variable can be changed multiparton times, |
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| 154 | but it is the latest change that sets the current value. |
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| 155 | </li> |
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| 156 | <li>The two <code>Tune:ee</code> and <code>Tune:pp</code> modes can also |
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| 157 | be changed in exactly the same way as described for all other settings |
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| 158 | above. Unique for them, however, is that when one of them is encountered |
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| 159 | it also initiates a call to the <code>initTuneEE(...)</code> or |
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| 160 | <code>initTunePP(...)</code> method, respectively. In such cases all |
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| 161 | settings affected by the <i>e^+e^-</i> or <i>pp/ppbar</i> tune |
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| 162 | are first reset to the default values (the <code>-1</code> options) |
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| 163 | and thereafter the relevant tune is set up. |
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| 164 | </li> |
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| 165 | <li>It is possible to mix commands of type 4 and 5 in any order; it |
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| 166 | is always the last change that counts. That is, any changes you have |
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| 167 | made to variables of a tune <i>before</i> a <code>Tune:ee</code> or |
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| 168 | <code>Tune:pp</code> command are overwritten by it, while variables |
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| 169 | you set <i>after</i> will overwrite the tune values. As a rule, |
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| 170 | therefore, you want to begin with the tune choice, and thereafter |
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| 171 | modify only a small part of its settings. |
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| 172 | </li> |
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| 173 | <li>Needless to say, the flexibility can lead to unwanted setups if |
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| 174 | you do not exercise some discipline. It is therefore recommended that |
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| 175 | you always check the listing obtained with |
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| 176 | <code>Pythia::settings.listChanged()</code> to confirm that the |
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| 177 | final set of changes is the intended one. |
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| 178 | </li> |
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| 179 | </ol> |
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| 180 | |
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| 181 | <p/><code>mode </code><strong> Tune:ee </strong> |
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| 182 | (<code>default = <strong>0</strong></code>; <code>minimum = -1</code>; <code>maximum = 3</code>)<br/> |
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| 183 | Choice of tune to <i>e^+e^-</i> data, mainly for the hadronization |
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| 184 | and timelike-showering aspects of PYTHIA. You should study the |
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| 185 | <code>Settings::initTuneEE(...)</code> method to find exactly which |
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| 186 | are the settings for the respective tune. |
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| 187 | <br/><code>option </code><strong> -1</strong> : reset all values that are affected by any of the |
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| 188 | <i>e^+e^-</i> tunes to the default values. This option can be used |
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| 189 | on its own, but is also automatically used as a first step for either |
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| 190 | of the positive tune values below, to undo the effect of previous tune |
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| 191 | settings. |
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| 192 | |
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| 193 | <br/><code>option </code><strong> 0</strong> : no values are overwritten during the initial setup, |
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| 194 | step 2 above. Note that changing to <code>0</code> in the user code |
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| 195 | has no effect; if you want to restore the individual settings you |
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| 196 | should instead use <code>-1</code>. |
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| 197 | |
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| 198 | <br/><code>option </code><strong> 1</strong> : the original PYTHIA 8 parameter set, based on some |
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| 199 | very old flavour studies (with JETSET around 1990) and a simple tune |
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| 200 | <i>of alpha_strong</i> to three-jet shapes to the new |
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| 201 | <i>pT</i>-ordered shower. These were the default values before |
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| 202 | version 8.125. |
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| 203 | |
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| 204 | <br/><code>option </code><strong> 2</strong> : a tune by Marc Montull to the LEP 1 particle |
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| 205 | composition, as published in the RPP (August 2007). No related (re)tune |
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| 206 | to event shapes has been performed, however. |
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| 207 | |
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| 208 | <br/><code>option </code><strong> 3</strong> : a tune to a wide selection of LEP1 data by Hendrik |
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| 209 | Hoeth within the Rivet + Professor framework, both to hadronization and |
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| 210 | timelike-shower parameters (June 2009). These are the default values |
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| 211 | starting from version 8.125, so currently there is no need for this |
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| 212 | option. |
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| 213 | |
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| 214 | |
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| 215 | |
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| 216 | <p/><code>flag </code><strong> Tune:preferLHAPDF </strong> |
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| 217 | (<code>default = <strong>on</strong></code>)<br/> |
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| 218 | Tunes made by experimental collaborations typically use the LHAPDF |
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| 219 | package to obtain their PDF values, and so PYTHIA has to be linked |
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| 220 | accordingly. For PDFs implemanted natively in PYTHIA it is possible |
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| 221 | to use the respective tunes, without having to link to LHAPDF, if you |
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| 222 | set <code>Tune:preferLHAPDF = off</code> <i>before</i> the |
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| 223 | <code>Tune:pp</code> choice. |
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| 224 | |
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| 225 | |
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| 226 | <p/><code>mode </code><strong> Tune:pp </strong> |
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| 227 | (<code>default = <strong>5</strong></code>; <code>minimum = -1</code>; <code>maximum = 11</code>)<br/> |
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| 228 | Choice of tune to <i>pp/ppbar</i> data, mainly for the |
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| 229 | initial-state-radiation, multiparton-interactions and beam-remnants |
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| 230 | aspects of PYTHIA. Note that the previous crude (non-)tunes |
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| 231 | 3C and 3M are removed as of 8.145, superseded by the 4C tune. |
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| 232 | You should study the <code>Settings::initTunePP(...)</code> method |
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| 233 | to find exactly which are the settings for the respective tune. |
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| 234 | <br/><code>option </code><strong> -1</strong> : reset all values that are affected by any of the |
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| 235 | <i>pp/ppbar</i> tunes to the default values. This option can be used |
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| 236 | on its own, but is also automatically used as a first step for either |
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| 237 | of the positive tune values below, to undo the effect of previous tune |
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| 238 | settings. |
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| 239 | |
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| 240 | <br/><code>option </code><strong> 0</strong> : no values are overwritten during the initial setup, |
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| 241 | step 2 above. Note that changing to <code>0</code> in the user code |
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| 242 | has no effect; if you want to restore the individual settings you |
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| 243 | should instead use <code>-1</code>. |
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| 244 | |
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| 245 | <br/><code>option </code><strong> 1</strong> : default used up to version 8.126, based on |
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| 246 | some early and primitive comparisons with data. |
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| 247 | |
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| 248 | <br/><code>option </code><strong> 2</strong> : "Tune 1", default in 8.127 - 8.139, based on some |
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| 249 | data comparisons by Peter Skands. Largely but not wholly overlaps |
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| 250 | with the default option 0. |
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| 251 | |
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| 252 | <br/><code>option </code><strong> 3</strong> : "Tune 2C", introduced with 8.140 [<a href="Bibliography.html" target="page">Cor10a</a>]. |
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| 253 | It uses the CTEQ 6L1 PDF, and is intended to give good agreement with |
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| 254 | much of the published CDF data. |
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| 255 | |
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| 256 | <br/><code>option </code><strong> 4</strong> : "Tune 2M", introduced with 8.140 [<a href="Bibliography.html" target="page">Cor10a</a>]. |
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| 257 | It is uses the MRST LO** PDF, which has a momentum sum somewhat above |
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| 258 | unity, which is compensated by a smaller <i>alpha_s</i> than in the |
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| 259 | previous tune. Again it is intended to give good agreement with much of |
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| 260 | the published CDF data. |
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| 261 | |
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| 262 | <br/><code>option </code><strong> 5</strong> : "Tune 4C", new tune, introduced with 8.145 |
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| 263 | [<a href="Bibliography.html" target="page">Cor10a</a>]. Starts out from tune 2C, but with a reduced cross |
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| 264 | section for diffraction, plus modified multiparton interactions parameters |
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| 265 | to give a higher and more rapidly increasing charged pseudorapidity |
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| 266 | plateau, for better agreement with some early key LHC numbers. |
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| 267 | See also the comparative study in [<a href="Bibliography.html" target="page">Buc11</a>]. |
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| 268 | |
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| 269 | <br/><code>option </code><strong> 6</strong> : "Tune 4Cx", based on tune 4C, but using the x-dependent |
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| 270 | matter profile, <code>MultipartonInteractions:bProfile = 4</code> and an |
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| 271 | increased <code>MultipartonInteractions:pT0Ref</code> [<a href="Bibliography.html" target="page">Cor11</a>]. |
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| 272 | |
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| 273 | <br/><code>option </code><strong> 7</strong> : "ATLAS MB Tune A2-CTEQ6L1", a minimum-bias tune based |
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| 274 | on tune 4Cx, but without rapidity-ordered spacelike emissions |
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| 275 | [<a href="Bibliography.html" target="page">ATL12</a>]. Uses CTEQ 6L1, by default from LHAPDF. |
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| 276 | |
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| 277 | <br/><code>option </code><strong> 8</strong> : "ATLAS MB Tune A2-MSTW2008LO", as above, |
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| 278 | but uses MSTW 2008 LO, by default from LHAPDF. |
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| 279 | |
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| 280 | <br/><code>option </code><strong> 9</strong> : "ATLAS UE Tune AU2-CTEQ6L1", an underlying-event tune |
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| 281 | based on tune 4Cx, but without rapidity-ordered spacelike emissions |
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| 282 | [<a href="Bibliography.html" target="page">ATL12</a>]. Uses CTEQ 6L1, by default from LHAPDF. |
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| 283 | |
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| 284 | <br/><code>option </code><strong> 10</strong> : "ATLAS UE Tune AU2-MSTW2008LO", as above, |
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| 285 | but uses MSTW 2008 LO, by default from LHAPDF. |
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| 286 | |
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| 287 | <br/><code>option </code><strong> 11</strong> : "ATLAS UE Tune AU2-CT10", as above, |
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| 288 | but uses CT 10, which is not currently implemented in PYTHIA, |
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| 289 | so you must link LHAPDF. |
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| 290 | |
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| 291 | <br/><code>option </code><strong> 12</strong> : "ATLAS UE Tune AU2-MRST2007LO*", as above, |
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| 292 | but uses MRST 2007 LO*, by default from LHAPDF. |
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| 293 | |
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| 294 | <br/><code>option </code><strong> 13</strong> : "ATLAS UE Tune AU2-MRST2007LO**", as above, |
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| 295 | but uses MRST 2007 LO**, by default from LHAPDF. |
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| 296 | |
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| 297 | |
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| 298 | |
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| 299 | </body> |
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| 300 | </html> |
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| 301 | |
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| 302 | <!-- Copyright (C) 2012 Torbjorn Sjostrand --> |
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