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