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| 2 | <head> |
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| 3 | <title>PDF Selection</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>PDF Selection</h2> |
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| 10 | |
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| 11 | This page contains five subsections. The first deals with how to |
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| 12 | pick the parton distribution set for protons, including from LHAPDF, |
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| 13 | to be used for all proton and antiproton beams. The second is a special |
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| 14 | option that allows a separate PDF set to be used for the hard process |
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| 15 | only, while the first choice would still apply to everything else. |
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| 16 | The third and fourth give access to pion and Pomeron PDF's, respectively, |
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| 17 | the latter being used to describe diffractive systems. |
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| 18 | The fifth gives the possibility to switch off the lepton |
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| 19 | "parton density". More information on PDF classes is found |
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| 20 | <a href="PartonDistributions.html" target="page">here</a>. |
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| 21 | |
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| 22 | <h3>Parton densities for protons</h3> |
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| 23 | |
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| 24 | The selection of parton densities is made once and then is propagated |
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| 25 | through the program. It is essential to make an informed choice, |
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| 26 | for several reasons [<a href="Bibliography.html" target="page">Kas10</a>]: |
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| 27 | <br/><b>Warning 1:</b> the choice of PDF set affects a number of |
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| 28 | properties of events. A change of PDF therefore requires a complete |
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| 29 | retuning e.g. of the multiparton-interactions model for minimum-bias and |
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| 30 | underlying events. |
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| 31 | <br/><b>Warning 2:</b> People often underestimate the differences |
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| 32 | between different sets on the market. The sets for the same order are |
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| 33 | constructed to behave more or less similarly at large <i>x</i> and |
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| 34 | <i>Q^2</i>, while the multiparton interactions are dominated by the |
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| 35 | behaviour in the region of small <i>x</i> and <i>Q^2</i>. A good |
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| 36 | PDF parametrization ought to be sensible down to <i>x = 10^-6</i> |
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| 37 | (<i>x = 10^-7</i>) and <i>Q^2 = 1</i> GeV^2 for Tevatron (LHC) |
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| 38 | applications. Unfortunately there are distributions on the market that |
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| 39 | completely derail in that region. The <code>main51.cc</code> and |
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| 40 | <code>main52.cc</code> programs in the <code>examples</code> |
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| 41 | subdirectory provide some examples of absolutely minimal sanity checks |
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| 42 | before a new PDF set is put in production. |
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| 43 | <br/><b>Warning 3:</b> NLO and LO sets tend to have quite different |
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| 44 | behaviours, e.g. NLO ones have less gluons at small x, which then is |
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| 45 | compensated by positive corrections in the NLO matrix elements. |
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| 46 | Therefore do not blindly assume that an NLO tune has to be better than |
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| 47 | an LO one when combined with the LO matrix elements in PYTHIA. There are |
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| 48 | explicit examples where such thinking can lead you down the wrong alley, |
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| 49 | especially if you study low-<i>pT</i> physics. In the list below you |
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| 50 | should therefore be extra cautious when using set 6 or set 9. |
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| 51 | |
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| 52 | <p/> |
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| 53 | The simplest option is to pick one |
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| 54 | of the distributions available internally: |
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| 55 | |
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| 56 | <p/><code>mode </code><strong> PDF:pSet </strong> |
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| 57 | (<code>default = <strong>2</strong></code>; <code>minimum = 1</code>; <code>maximum = 12</code>)<br/> |
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| 58 | Parton densities to be used for proton beams (and, by implication, |
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| 59 | antiproton ones): |
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| 60 | <br/><code>option </code><strong> 1</strong> : GRV 94L, LO <i>alpha_s(M_Z) = 0.128</i> |
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| 61 | (this set is out of date, but retained for historical comparisons). |
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| 62 | <br/><code>option </code><strong> 2</strong> : CTEQ 5L, LO <i>alpha_s(M_Z) = 0.127</i> |
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| 63 | (this set is also out of date, but not badly so, and many tunes |
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| 64 | are based on it). |
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| 65 | <br/><code>option </code><strong> 3</strong> : MRST LO* (2007), |
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| 66 | NLO <i>alpha_s(M_Z) = 0.12032</i>. |
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| 67 | <br/><code>option </code><strong> 4</strong> : MRST LO** (2008), |
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| 68 | NLO <i>alpha_s(M_Z) = 0.11517</i>. |
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| 69 | <br/><code>option </code><strong> 5</strong> : MSTW 2008 LO (central member), |
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| 70 | LO <i>alpha_s(M_Z) = 0.13939</i>. |
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| 71 | <br/><code>option </code><strong> 6</strong> : MSTW 2008 NLO (central member), |
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| 72 | NLO <i>alpha_s(M_Z) = 0.12018</i> (NLO, see Warning 3 above). |
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| 73 | <br/><code>option </code><strong> 7</strong> : CTEQ6L, NLO <i>alpha_s(M_Z) = 0.1180</i>. |
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| 74 | <br/><code>option </code><strong> 8</strong> : CTEQ6L1, LO <i>alpha_s(M_Z) = 0.1298</i>. |
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| 75 | <br/><code>option </code><strong> 9</strong> : CTEQ66.00 (NLO, central member), |
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| 76 | NLO <i>alpha_s(M_Z) = 0.1180</i> (NLO, see Warning 3 above). |
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| 77 | <br/><code>option </code><strong> 10</strong> : CT09MC1, LO <i>alpha_s(M_Z) = 0.1300</i>. |
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| 78 | <br/><code>option </code><strong> 11</strong> : CT09MC2, NLO <i>alpha_s(M_Z) = 0.1180</i>. |
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| 79 | <br/><code>option </code><strong> 12</strong> : CT09MCS, NLO <i>alpha_s(M_Z) = 0.1180</i>. |
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| 80 | |
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| 81 | <br/><b>Note:</b> the <i>alpha_s(M_Z)</i> values and the order of the |
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| 82 | running in the description above is purely informative, and does not |
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| 83 | affect any other parts of the program. Instead you have the freedom to |
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| 84 | set <i>alpha_s(M_Z)</i> value and running separately for |
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| 85 | <a href="CouplingsAndScales.html" target="page">hard processes</a> |
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| 86 | (including resonance decays), |
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| 87 | <a href="MultipartonInteractions.html" target="page">multiparton interactions</a>, |
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| 88 | <a href="SpacelikeShowers.html" target="page">initial-state radiation</a>, and |
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| 89 | <a href="TimelikeShowers.html" target="page">final-state radiation</a>. |
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| 90 | |
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| 91 | <p/> |
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| 92 | This is a reasonably complete list of recent LO fits, both |
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| 93 | ones within the normal LO context and ones with modifications for better |
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| 94 | matching to event generators. In addition two older sets are |
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| 95 | included for backwards reference (most studies to date are based on |
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| 96 | CTEQ 5L). If you link to the |
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| 97 | <a href="http://projects.hepforge.org/lhapdf/" target="page">LHAPDF |
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| 98 | library</a> [<a href="Bibliography.html" target="page">Wha05</a>] you get access to a much wider selection. |
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| 99 | <br/><b>Warning 1:</b> owing to previous problems with the behaviour |
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| 100 | of PDF's beyond the <i>x</i> and <i>Q^2</i> boundaries of a set, |
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| 101 | you should only use LHAPDF <b>version 5.3.0 or later</b>. |
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| 102 | <br/><b>Warning 2:</b> the behaviour of the LHAPDF sets need not be |
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| 103 | identical with the implementation found in PYTHIA. Specifically we |
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| 104 | are aware of the following points that may influence a comparison. |
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| 105 | <br/>(a) CTEQ 5L in PYTHIA is the parametrization, in LHAPDF the grid |
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| 106 | interpolation. |
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| 107 | <br/>(b) MRST LO* and LO** in PYTHIA is based on an updated edition, |
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| 108 | where one makes use of the expanded MSTW grid format, while LHAPDF |
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| 109 | is based on the original smaller grid. |
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| 110 | <br/>(c) The CTEQ 6 and CT09MC sets in PYTHIA are frozen at the |
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| 111 | boundaries of the grid, by recommendation of the authors, while |
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| 112 | LHAPDF also offers an option with a smooth extrapolation outside |
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| 113 | the grid boundaries. |
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| 114 | |
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| 115 | <p/><code>flag </code><strong> PDF:useLHAPDF </strong> |
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| 116 | (<code>default = <strong>off</strong></code>)<br/> |
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| 117 | If off then the choice of proton PDF is based on <code>PDF:pSet</code> |
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| 118 | above. If on then it is instead based on the choice of |
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| 119 | <code>PDF:LHAPDFset</code> and <code>PDF:LHAPDFmember</code> below. |
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| 120 | <br/><b>Note:</b> in order for this option to work you must have |
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| 121 | compiled PYTHIA appropriately and have set the <code>LHAPATH</code> |
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| 122 | environment variable to provide the data-files directory of your local |
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| 123 | LHAPDF installation. See the README file in the <code>examples</code> |
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| 124 | directory for further instructions. |
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| 125 | |
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| 126 | |
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| 127 | <p/><code>word </code><strong> PDF:LHAPDFset </strong> |
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| 128 | (<code>default = <strong>MRST2004FF4lo.LHgrid</strong></code>)<br/> |
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| 129 | Name of proton PDF set from LHAPDF to be used. You have to choose |
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| 130 | from the |
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| 131 | <a href="http://projects.hepforge.org/lhapdf/pdfsets" target="page"> |
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| 132 | list of available sets</a>. Examples of some fairly recent ones |
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| 133 | (but still less recent than found above) would be |
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| 134 | cteq61.LHpdf, cteq61.LHgrid, cteq6l.LHpdf, cteq6ll.LHpdf, |
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| 135 | MRST2004nlo.LHpdf, MRST2004nlo.LHgrid, MRST2004nnlo.LHgrid and |
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| 136 | MRST2004FF3lo.LHgrid. If you pick a LHpdf set it will require some |
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| 137 | calculation the first time it is called. |
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| 138 | <br/><b>Technical note:</b> if you provide a name beginning with a |
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| 139 | slash (/) it is assumed you want to provide the full file path and then |
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| 140 | <code>initPDFsetM(name)</code> is called, else the correct path is assumed |
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| 141 | already set and <code>initPDFsetByNameM(name)</code> is called. |
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| 142 | |
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| 143 | |
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| 144 | <p/><code>mode </code><strong> PDF:LHAPDFmember </strong> |
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| 145 | (<code>default = <strong>0</strong></code>; <code>minimum = 0</code>)<br/> |
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| 146 | Further choice of a specific member from the set picked above. Member 0 |
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| 147 | should normally correspond to the central value, with higher values |
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| 148 | corresponding to different error PDF's somewhat off in different |
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| 149 | directions. You have to check from set to set which options are open. |
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| 150 | <br/><b>Note:</b> you can only use one member in a run, so if you |
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| 151 | want to sweep over many members you either have to do many separate |
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| 152 | runs or, as a simplification, save the |
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| 153 | <a href="EventInformation.html" target="page">pdf weights</a> at the hard scattering |
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| 154 | and do an offline reweighting of events. |
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| 155 | |
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| 156 | |
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| 157 | <p/><code>flag </code><strong> PDF:extrapolateLHAPDF </strong> |
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| 158 | (<code>default = <strong>off</strong></code>)<br/> |
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| 159 | Parton densities have a guaranteed range of validity in <i>x</i> |
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| 160 | and <i>Q^2</i>, and what should be done beyond that range usually is |
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| 161 | not explained by the authors of PDF sets. Nevertheless these boundaries |
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| 162 | very often are exceeded, e.g. minimum-bias studies at LHC may sample |
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| 163 | <i>x</i> values down to <i>10^-8</i>, while many PDF sets stop |
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| 164 | already at <i>10^-5</i>. The default behaviour is then that the |
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| 165 | PDF's are frozen at the boundary, i.e. <i>xf(x,Q^2)</i> is fixed at |
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| 166 | its value at <i>x_min</i> for all values <i>x < x_min</i>, |
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| 167 | and so on. This is a conservative approach. Alternatively, if you |
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| 168 | switch on extrapolation, then parametrizations will be extended beyond |
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| 169 | the boundaries, by some prescription. In some cases this will provide a |
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| 170 | more realistic answer, in others complete rubbish. Another problem is |
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| 171 | that some of the PDF-set codes will write a warning message anytime the |
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| 172 | limits are exceeded, thus swamping your output file. Therefore you should |
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| 173 | study a set seriously before you run it with this switch on. |
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| 174 | |
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| 175 | |
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| 176 | <p/> |
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| 177 | If you want to use PDF's not found in LHAPDF, or you want to interface |
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| 178 | LHAPDF another way, you have full freedom to use the more generic |
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| 179 | <a href="PartonDistributions.html" target="page">interface options</a>. |
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| 180 | |
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| 181 | <h3>Parton densities for protons in the hard process</h3> |
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| 182 | |
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| 183 | The above options provides a PDF set that will be used everywhere: |
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| 184 | for the hard process, the parton showers and the multiparton interactions |
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| 185 | alike. As already mentioned, therefore a change of PDF should be |
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| 186 | accompanied by a <b>complete</b> retuning of the whole MPI framework, |
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| 187 | and maybe more. There are cases where one may want to explore |
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| 188 | different PDF options for the hard process, but would not want to touch |
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| 189 | the rest. If several different sets are to be compared, a simple |
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| 190 | reweighting based on the <a href="EventInformation.html" target="page">originally |
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| 191 | used</a> flavour, <i>x</i>, <i>Q^2</i> and PDF values may offer the |
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| 192 | best route. The options in this section allow a choice of the PDF set |
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| 193 | for the hard process alone, while the choice made in the previous section |
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| 194 | would still be used for everything else. The hardest interaction |
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| 195 | of the minimum-bias process is part of the multiparton-interactions |
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| 196 | framework and so does not count as a hard process here. |
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| 197 | |
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| 198 | <p/> |
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| 199 | Of course it is inconsistent to use different PDF's in different parts |
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| 200 | of an event, but if the <i>x</i> and <i>Q^2</i> ranges mainly accessed |
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| 201 | by the components are rather different then the contradiction would not be |
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| 202 | too glaring. Furthermore, since standard PDF's are one-particle-inclusive |
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| 203 | we anyway have to 'invent' our own PDF modifications to handle configurations |
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| 204 | where more than one parton is kicked out of the proton [<a href="Bibliography.html" target="page">Sjo04</a>]. |
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| 205 | |
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| 206 | <p/> |
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| 207 | The PDF choices that can be made are the same as above, so we do not |
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| 208 | repeat the detailed discussion. |
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| 209 | |
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| 210 | <p/><code>flag </code><strong> PDF:useHard </strong> |
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| 211 | (<code>default = <strong>off</strong></code>)<br/> |
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| 212 | If on then select a separate PDF set for the hard process, using the |
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| 213 | variables below. If off then use the same PDF set for everything, |
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| 214 | as already chosen above. |
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| 215 | |
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| 216 | |
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| 217 | <p/><code>mode </code><strong> PDF:pHardSet </strong> |
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| 218 | (<code>default = <strong>2</strong></code>; <code>minimum = 1</code>; <code>maximum = 12</code>)<br/> |
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| 219 | Parton densities to be used for proton beams (and, by implication, |
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| 220 | antiproton ones): |
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| 221 | <br/><code>option </code><strong> 1</strong> : GRV 94L, LO <i>alpha_s(M_Z) = 0.128</i> |
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| 222 | (out of date). |
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| 223 | <br/><code>option </code><strong> 2</strong> : CTEQ 5L, LO <i>alpha_s(M_Z) = 0.127</i> |
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| 224 | (slightly out of date; many tunes are based on it). |
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| 225 | <br/><code>option </code><strong> 3</strong> : MRST LO* (2007), |
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| 226 | NLO <i>alpha_s(M_Z) = 0.12032</i>. |
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| 227 | <br/><code>option </code><strong> 4</strong> : MRST LO** (2008), |
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| 228 | NLO <i>alpha_s(M_Z) = 0.11517</i>. |
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| 229 | <br/><code>option </code><strong> 5</strong> : MSTW 2008 LO (central member), |
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| 230 | LO <i>alpha_s(M_Z) = 0.13939</i>. |
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| 231 | <br/><code>option </code><strong> 6</strong> : MSTW 2008 NLO (central member), |
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| 232 | LO <i>alpha_s(M_Z) = 0.12018</i>. |
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| 233 | <br/><code>option </code><strong> 7</strong> : CTEQ6L, NLO <i>alpha_s(M_Z) = 0.1180</i>. |
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| 234 | <br/><code>option </code><strong> 8</strong> : CTEQ6L1, LO <i>alpha_s(M_Z) = 0.1298</i>. |
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| 235 | <br/><code>option </code><strong> 9</strong> : CTEQ66.00 (NLO, central member), |
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| 236 | NLO <i>alpha_s(M_Z) = 0.1180</i>. |
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| 237 | <br/><code>option </code><strong> 10</strong> : CT09MC1, LO <i>alpha_s(M_Z) = 0.1300</i>. |
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| 238 | <br/><code>option </code><strong> 11</strong> : CT09MC2, NLO <i>alpha_s(M_Z) = 0.1180</i>. |
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| 239 | <br/><code>option </code><strong> 12</strong> : CT09MCS, NLO <i>alpha_s(M_Z) = 0.1180</i>. |
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| 240 | |
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| 241 | |
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| 242 | <p/><code>flag </code><strong> PDF:useHardLHAPDF </strong> |
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| 243 | (<code>default = <strong>off</strong></code>)<br/> |
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| 244 | If off then the choice of proton PDF is based on <code>hardpPDFset</code> |
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| 245 | above. If on then it is instead based on the choice of |
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| 246 | <code>hardLHAPDFset</code> and <code>hardLHAPDFmember</code> below. |
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| 247 | Note that if you want to use LHAPDF here, and you also use LHAPDF |
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| 248 | for the "normal" PDF set, then LHAPDF must have been compiled so as to |
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| 249 | handle (at least) two concurrent sets, with the configure statement |
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| 250 | <code>--with-max-num-pdfsets=2</code>. |
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| 251 | |
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| 252 | |
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| 253 | <p/><code>word </code><strong> PDF:hardLHAPDFset </strong> |
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| 254 | (<code>default = <strong>MRST2004FF4lo.LHgrid</strong></code>)<br/> |
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| 255 | Name of proton PDF set from LHAPDF to be used. |
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| 256 | |
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| 257 | |
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| 258 | <p/><code>mode </code><strong> PDF:hardLHAPDFmember </strong> |
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| 259 | (<code>default = <strong>0</strong></code>; <code>minimum = 0</code>)<br/> |
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| 260 | Further choice of a specific member from the set picked above. |
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| 261 | |
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| 262 | |
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| 263 | <p/> |
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| 264 | Note that there is no separate equivalent of the |
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| 265 | <code>PDF:extrapolateLHAPDF</code> flag specifically for the hard |
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| 266 | PDF. Since LHAPDF only has one global flag for extrapolation or not, |
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| 267 | the choice for the normal PDF's also applies to the hard ones. |
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| 268 | |
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| 269 | <h3>Parton densities for pions</h3> |
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| 270 | |
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| 271 | The parton densities of the pion are considerably less well known than |
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| 272 | those of the proton. There are only rather few sets on the market, |
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| 273 | and none particularly recent. Only one comes built-in, but others can |
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| 274 | be accessed from LHAPDF. Input parametrizations are for the <i>pi+</i>. |
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| 275 | >From this the <i>pi-</i> is obtained by charge conjugation and the |
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| 276 | <i>pi0</i> from averaging (half the pions have <i>d dbar</i> |
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| 277 | valence quark content, half <i>u ubar</i>. |
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| 278 | |
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| 279 | <p/> |
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| 280 | Much of the switches are taken over from the proton case, with obvious |
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| 281 | modifications; therefore the description is briefer. Currently we have |
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| 282 | not seen the need to allow separate parton densities for hard processes. |
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| 283 | When using LHAPDF the <code>PDF:extrapolateLHAPDF</code> switch of the |
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| 284 | proton also applies to pions. |
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| 285 | |
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| 286 | <p/><code>mode </code><strong> PDF:piSet </strong> |
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| 287 | (<code>default = <strong>1</strong></code>; <code>minimum = 1</code>; <code>maximum = 1</code>)<br/> |
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| 288 | Internal parton densities that can be used for pion beams, currently with |
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| 289 | only one choice. |
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| 290 | <br/><code>option </code><strong> 1</strong> : GRV 92 L. |
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| 291 | |
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| 292 | |
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| 293 | <p/><code>flag </code><strong> PDF:piUseLHAPDF </strong> |
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| 294 | (<code>default = <strong>off</strong></code>)<br/> |
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| 295 | If off then the choice of proton PDF is based on <code>PDF:piSet</code> |
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| 296 | above. If on then it is instead based on the choice of |
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| 297 | <code>PDF:piLHAPDFset</code> and <code>PDF:piLHAPDFmember</code> below. |
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| 298 | |
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| 299 | |
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| 300 | <p/><code>word </code><strong> PDF:piLHAPDFset </strong> |
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| 301 | (<code>default = <strong>OWPI.LHgrid</strong></code>)<br/> |
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| 302 | Name of pion PDF set from LHAPDF to be used. You have to choose from the |
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| 303 | <a href="http://projects.hepforge.org/lhapdf/pdfsets" target="page"> |
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| 304 | list of available sets</a>. |
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| 305 | |
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| 306 | |
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| 307 | <p/><code>mode </code><strong> PDF:piLHAPDFmember </strong> |
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| 308 | (<code>default = <strong>0</strong></code>; <code>minimum = 0</code>)<br/> |
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| 309 | Further choice of a specific member from the set picked above. |
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| 310 | |
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| 311 | |
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| 312 | <h3>Parton densities for Pomerons</h3> |
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| 313 | |
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| 314 | The Pomeron is introduced in the description of diffractive events, |
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| 315 | i.e. a diffractive system is viewed as a Pomeron-proton collision at a |
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| 316 | reduced CM energy. Here the PDF's are even less well known. |
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| 317 | Most experimental parametrizations are NLO, which makes them less |
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| 318 | well suited for Monte Carlo applications. Furthemore note that |
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| 319 | the momentum sum is arbitrarily normalized to a non-unity value. |
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| 320 | |
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| 321 | <p/><code>mode </code><strong> PDF:PomSet </strong> |
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| 322 | (<code>default = <strong>6</strong></code>; <code>minimum = 1</code>; <code>maximum = 6</code>)<br/> |
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| 323 | Parton densities that can be used for Pomeron beams. |
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| 324 | <br/><code>option </code><strong> 1</strong> : <i>Q^2</i>-independent parametrizations |
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| 325 | <i>xf(x) = N_ab x^a (1 - x)^b</i>, where <i>N_ab</i> ensures |
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| 326 | unit momentum sum. The <i>a</i> and <i>b</i> parameters can be |
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| 327 | set separately for the gluon and the quark distributions. The |
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| 328 | momentum fraction of gluons and quarks can be freely mixed, and |
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| 329 | production of <i>s</i> quarks can be suppressed relative to |
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| 330 | that of <i>d</i> and <i>u</i> ones, with antiquarks as likely |
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| 331 | as quarks. See further below how to set the six parameters of this |
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| 332 | approach. |
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| 333 | |
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| 334 | <br/><code>option </code><strong> 2</strong> : <i>pi0</i> distributions, as specified in the |
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| 335 | section above. |
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| 336 | |
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| 337 | <br/><code>option </code><strong> 3</strong> : the H1 2006 Fit A NLO <i>Q^2</i>-dependent |
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| 338 | parametrization, based on a tune to their data [<a href="Bibliography.html" target="page">H1P06</a>], |
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| 339 | rescaled by the factor <code>PomRescale</code> below. |
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| 340 | |
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| 341 | <br/><code>option </code><strong> 4</strong> : the H1 2006 Fit B NLO <i>Q^2</i>-dependent |
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| 342 | parametrization, based on a tune to their data [<a href="Bibliography.html" target="page">H1P06</a>], |
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| 343 | rescaled by the factor <code>PomRescale</code> below. |
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| 344 | |
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| 345 | <br/><code>option </code><strong> 5</strong> : the H1 2007 Jets NLO <i>Q^2</i>-dependent |
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| 346 | parametrization, based on a tune to their data [<a href="Bibliography.html" target="page">H1P07</a>], |
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| 347 | rescaled by the factor <code>PomRescale</code> below. |
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| 348 | |
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| 349 | <br/><code>option </code><strong> 6</strong> : the H1 2006 Fit B LO <i>Q^2</i>-dependent |
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| 350 | parametrization, based on a tune to their data [<a href="Bibliography.html" target="page">H1P06</a>], |
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| 351 | rescaled by the factor <code>PomRescale</code> below. |
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| 352 | |
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| 353 | |
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| 354 | |
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| 355 | <p/><code>parm </code><strong> PDF:PomGluonA </strong> |
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| 356 | (<code>default = <strong>0.</strong></code>; <code>minimum = -0.5</code>; <code>maximum = 2.</code>)<br/> |
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| 357 | the parameter <i>a</i> in the ansatz <i>xg(x) = N_ab x^a (1 - x)^b</i> |
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| 358 | for option 1 above. |
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| 359 | |
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| 360 | |
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| 361 | <p/><code>parm </code><strong> PDF:PomGluonB </strong> |
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| 362 | (<code>default = <strong>3.</strong></code>; <code>minimum = 0.</code>; <code>maximum = 10.</code>)<br/> |
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| 363 | the parameter <i>b</i> in the ansatz <i>xg(x) = N_ab x^a (1 - x)^b</i> |
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| 364 | for option 1 above. |
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| 365 | |
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| 366 | |
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| 367 | <p/><code>parm </code><strong> PDF:PomQuarkA </strong> |
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| 368 | (<code>default = <strong>0.</strong></code>; <code>minimum = -0.5</code>; <code>maximum = 2.</code>)<br/> |
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| 369 | the parameter <i>a</i> in the ansatz <i>xq(x) = N_ab x^a (1 - x)^b</i> |
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| 370 | for option 1 above. |
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| 371 | |
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| 372 | |
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| 373 | <p/><code>parm </code><strong> PDF:PomQuarkB </strong> |
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| 374 | (<code>default = <strong>3.</strong></code>; <code>minimum = 0.</code>; <code>maximum = 10.</code>)<br/> |
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| 375 | the parameter <i>b</i> in the ansatz <i>xq(x) = N_ab x^a (1 - x)^b</i> |
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| 376 | for option 1 above. |
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| 377 | |
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| 378 | |
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| 379 | <p/><code>parm </code><strong> PDF:PomQuarkFrac </strong> |
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| 380 | (<code>default = <strong>0.2</strong></code>; <code>minimum = 0.</code>; <code>maximum = 1.</code>)<br/> |
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| 381 | the fraction of the Pomeron momentum carried by quarks |
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| 382 | for option 1 above, with the rest carried by gluons. |
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| 383 | |
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| 384 | |
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| 385 | <p/><code>parm </code><strong> PDF:PomStrangeSupp </strong> |
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| 386 | (<code>default = <strong>0.5</strong></code>; <code>minimum = 0.</code>; <code>maximum = 1.</code>)<br/> |
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| 387 | the suppression of the <i>s</i> quark density relative to that of the |
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| 388 | <i>d</i> and <i>u</i> ones for option 1 above. |
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| 389 | |
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| 390 | |
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| 391 | <p/><code>parm </code><strong> PDF:PomRescale </strong> |
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| 392 | (<code>default = <strong>1.0</strong></code>; <code>minimum = 0.5</code>; <code>maximum = 5.0</code>)<br/> |
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| 393 | Rescale the four H1 fits above by this uniform factor, e.g. to bring |
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| 394 | up their momentum sum to around unity. By default all three have |
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| 395 | a momentum sum of order 0.5, suggesting that a factor around 2.0 |
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| 396 | should be used. You can use <code>examples/main51.cc</code> to get |
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| 397 | a more precise value. Note that also other parameters in the |
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| 398 | <a href="Diffraction.html" target="page">diffraction</a> framework may need to |
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| 399 | be retuned when this parameter is changed. |
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| 400 | |
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| 401 | |
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| 402 | <h3>Parton densities for leptons</h3> |
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| 403 | |
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| 404 | For electrons/muons/taus there is no need to choose between different |
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| 405 | parametrizations, since only one implementation is available, and |
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| 406 | should be rather uncontroversial (apart from some technical details). |
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| 407 | However, insofar as e.g. <i>e^+ e^-</i> data often are corrected |
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| 408 | back to a world without any initial-state photon radiation, it is |
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| 409 | useful to have a corresponding option available here. |
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| 410 | |
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| 411 | <p/><code>flag </code><strong> PDF:lepton </strong> |
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| 412 | (<code>default = <strong>on</strong></code>)<br/> |
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| 413 | Use parton densities for lepton beams or not. If off the colliding |
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| 414 | leptons carry the full beam energy, if on part of the energy is |
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| 415 | radiated away by initial-state photons. In the latter case the |
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| 416 | initial-state showers will generate the angles and energies of the |
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| 417 | set of photons that go with the collision. In addition one collinear |
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| 418 | photon per beam carries any leftover amount of energy not described |
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| 419 | by shower emissions. If the initial-state showers are switched off |
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| 420 | these collinear photons will carry the full radiated energy. |
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| 421 | |
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| 422 | |
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| 423 | <p/> |
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| 424 | Neutrinos are always taken pointlike. Do note that the phase space |
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| 425 | selection machinery currently does not allow one resolved and one |
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| 426 | unresolved beam. For lepton-neutrino collisions to work you must |
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| 427 | therefore set <code>PDF:lepton = off</code>. |
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| 428 | |
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| 429 | <h3>Incoming parton selection</h3> |
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| 430 | |
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| 431 | There is one useful degree of freedom to restrict the set of incoming |
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| 432 | quark flavours for hard processes. It does not change the PDF's as such, |
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| 433 | only which quarks are allowed to contribute to the hard-process cross |
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| 434 | sections. Note that separate but similarly named modes are available |
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| 435 | for multiparton interactions and spacelike showers. |
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| 436 | |
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| 437 | <p/><code>mode </code><strong> PDFinProcess:nQuarkIn </strong> |
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| 438 | (<code>default = <strong>5</strong></code>; <code>minimum = 0</code>; <code>maximum = 5</code>)<br/> |
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| 439 | Number of allowed incoming quark flavours in the beams; a change |
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| 440 | to 4 would thus exclude <i>b</i> and <i>bbar</i> as incoming |
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| 441 | partons, etc. |
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| 442 | |
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| 443 | |
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| 444 | </body> |
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| 445 | </html> |
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| 446 | |
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| 447 | <!-- Copyright (C) 2012 Torbjorn Sjostrand --> |
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