1 <chapter name="PDF Selection">
5 This page contains three subsections. The first deals with how to
6 pick the parton distribution set for protons, including from LHAPDF,
7 to be used for all proton and antiproton beams. The second is a special
8 option that allows a separate PDF set to be used for the hard process
9 only, while the first choice would still apply to everything else.
10 The third gives the possibility to switch off the lepton
13 <h3>Parton densities for protons</h3>
15 The selection of parton densities is made once and then is propagated
16 through the program. It is essential to make an informed choice,
18 <note>Warning 1:</note> the choice of PDF set affects a number of
19 properties of events. A change of PDF therefore requires a complete
20 retuning e.g. of the multiple-interactions model for minimum-bias and
22 <note>Warning 2:</note> People often underestimate the differences
23 between different sets on the market. The sets for the same order are
24 constructed to behave more or less similarly at large <ei>x</ei> and
25 <ei>Q^2</ei>, while the multiple interactions are dominated by the
26 behaviour in the region of small <ei>x</ei> and <ei>Q^2</ei>. A good
27 PDF parametrization ought to be sensible down to <ei>x = 10^-6</ei>
28 (<ei>x = 10^-7</ei>) and <ei>Q^2 = 1</ei> GeV^2 for Tevatron (LHC)
29 applications. Unfortunately there are distributions on the market that
30 completely derail in that region. The <code>main41.cc</code> and
31 <code>main42.cc</code> programs in the <code>examples</code>
32 subdirectory provide some examples of absolutely minimal sanity checks
33 before a new PDF set is put in production.
34 <note>Warning 3:</note> NLO and LO sets tend to have quite different
35 behaviours, e.g. NLO ones have less gluons at small x, which then is
36 compensated by positive corrections in the NLO matrix elements.
37 Therefore do not blindly assume that an NLO tune has to be better than
38 an LO one when combined with the LO matrix elements in PYTHIA. There are
39 explicit examples where such thinking can lead you down the wrong alley.
42 The simplest option is to pick one
43 of the few distributions available internally:
45 <modepick name="PDF:pSet" default="2" min="1" max="2">
46 Parton densities to be used for proton beams (and, by implication,
48 <option value="1">GRV 94 L;</option>
49 <option value="2">CTEQ 5 L.</option>
53 Obviously this choice is mainly intended to get going, and if you link to
54 the <a href="http://projects.hepforge.org/lhapdf/" target="page">LHAPDF
55 library</a> <ref>Wha05</ref> you get access to a much wider selection.
56 <note>Warning:</note> owing to previous problems with the behaviour of PDF's
57 beyond the <ei>x</ei> and <ei>Q^2</ei> boundaries of a set, you should
58 only use LHAPDF <b>version 5.3.0 or later</b>.
60 <flag name="PDF:useLHAPDF" default="off">
61 If off then the choice of proton PDF is based on <code>pPDFset</code>
62 above. If on then it is instead based on the choice of
63 <code>LHAPDFset</code> and <code>LHAPDFmember</code> below.
64 <note>Note:</note> in order for this option to work you must have
65 compiled PYTHIA appropriately and have set the <code>LHAPATH</code>
66 environment variable to provide the data-files directory of your local
67 LHAPDF installation. See the README file in the <code>examples</code>
68 directory for further instructions.
71 <word name="PDF:LHAPDFset" default="MRST2004FF4lo.LHgrid">
72 Name of proton PDF set from LHAPDF to be used. You have to choose
74 <a href="http://projects.hepforge.org/lhapdf/pdfsets" target="page">
75 list of available sets</a>. Examples of some recent ones would be
76 cteq61.LHpdf, cteq61.LHgrid, cteq6l.LHpdf, cteq6ll.LHpdf,
77 MRST2004nlo.LHpdf, MRST2004nlo.LHgrid, MRST2004nnlo.LHgrid and
78 MRST2004FF3lo.LHgrid. If you pick a LHpdf set it will require some
79 calculation the first time it is called.
80 <note>Technical note:</note> if you provide a name beginning with a
81 slash (/) it is assumed you want to provide the full file path and then
82 <code>initPDFsetM(name)</code> is called, else the correct path is assumed
83 already set and <code>initPDFsetByNameM(name)</code> is called.
86 <modeopen name="PDF:LHAPDFmember" default="0" min="0">
87 Further choice of a specific member from the set picked above. Member 0
88 should normally correspond to the central value, with higher values
89 corresponding to different error PDF's somewhat off in different
90 directions. You have to check from set to set which options are open.
91 <note>Note:</note> you can only use one member in a run, so if you
92 want to sweep over many members you either have to do many separate
93 runs or, as a simplification, save the
94 <aloc href="EventInformation">pdf weights</aloc> at the hard scattering
95 and do an offline reweighting of events.
98 <flag name="PDF:extrapolateLHAPDF" default="off">
99 Parton densities have a guaranteed range of validity in <ei>x</ei>
100 and <ei>Q^2</ei>, and what should be done beyond that range usually is
101 not explained by the authors of PDF sets. Nevertheless these boundaries
102 very often are exceeded, e.g. minimum-bias studies at LHC may sample
103 <ei>x</ei> values down to <ei>10^-8</ei>, while many PDF sets stop
104 already at <ei>10^-5</ei>. The default behaviour is then that the
105 PDF's are frozen at the boundary, i.e. <ei>xf(x,Q^2)</ei> is fixed at
106 its value at <ei>x_min</ei> for all values <ei>x < x_min</ei>,
107 and so on. This is a conservative approach. Alternatively, if you
108 switch on extrapolation, then parametrizations will be extended beyond
109 the boundaries, by some prescription. In some cases this will provide a
110 more realistic answer, in others complete rubbish. Another problem is
111 that some of the PDF-set codes will write a warning message anytime the
112 limits are exceeded, thus swamping your output file. Therefore you should
113 study a set seriously before you run it with this switch on.
117 If you want to use PDF's not found in LHAPDF, or you want to interface
118 LHAPDF another way, you have full freedom to use the more generic
119 <aloc href="PartonDistributions">interface options</aloc>.
121 <h3>Parton densities for protons in the hard process</h3>
123 The above options provides a PDF set that will be used everywhere:
124 for the hard process, the parton showers and the multiple interactions
125 alike. As already mentioned, therefore a change of PDF should be
126 accompanied by a <b>complete</b> retuning of the whole MI framework,
127 and maybe more. There are cases where one may want to explore
128 different PDF options for the hard process, but would not want to touch
129 the rest. If several different sets are to be compared, a simple
130 reweighting based on the <aloc href="EventInformation">originally
131 used</aloc> flavour, <ei>x</ei>, <ei>Q^2</ei> and PDF values may offer the
132 best route. The options in this section allow a choice of the PDF set
133 for the hard process alone, while the choice made in the previous section
134 would still be used for everything else. The hardest interaction
135 of the minimum-bias process is part of the multiple-interactions
136 framework and so does not count as a hard process here.
139 Of course it is inconsistent to use different PDF's in different parts
140 of an event, but if the <ei>x</ei> and <ei>Q^2</ei> ranges mainly accessed
141 by the components are rather different then the contradiction would not be
142 too glaring. Furthermore, since standard PDF's are one-particle-inclusive
143 we anyway have to 'invent' our own PDF modifications to handle configurations
144 where more than one parton is kicked out of the proton <ref>Sjo04</ref>.
147 The PDF choices that can be made are the same as above, so we do not
148 repeat the detailed discussion.
150 <flag name="PDF:useHard" default="off">
151 If on then select a separate PDF set for the hard process, using the
152 variables below. If off then use the same PDF set for everything,
153 as already chosen above.
156 <modepick name="PDF:pHardSet" default="2" min="1" max="2">
157 Parton densities to be used for proton beams (and, by implication,
159 <option value="1">GRV 94 L;</option>
160 <option value="2">CTEQ 5 L.</option>
163 <flag name="PDF:useHardLHAPDF" default="off">
164 If off then the choice of proton PDF is based on <code>hardpPDFset</code>
165 above. If on then it is instead based on the choice of
166 <code>hardLHAPDFset</code> and <code>hardLHAPDFmember</code> below.
169 <word name="PDF:hardLHAPDFset" default="MRST2004FF4lo.LHgrid">
170 Name of proton PDF set from LHAPDF to be used.
173 <modeopen name="PDF:hardLHAPDFmember" default="0" min="0">
174 Further choice of a specific member from the set picked above.
178 Note that there is no separate equivalent of the
179 <code>PDF:extrapolateLHAPDF</code> flag specifically for the hard
180 PDF. Since LHAPDF only has one global flag for extrapolation or not,
181 the choice for the normal PDF's also applies to the hard ones.
183 <h3>Parton densities for leptons</h3>
185 For electrons/leptons there is no need to choose between different
186 parametrizations, since only one implementation is available, and
187 should be rather uncontroversial (apart from some technical details).
188 However, insofar as e.g. <ei>e^+ e^-</ei> data often are corrected
189 back to a world without any initial-state photon radiation, it is
190 useful to have a corresponding option available here.
192 <flag name="PDF:lepton" default="on">
193 Use parton densities for lepton beams or not. If off the colliding
194 leptons carry the full beam energy, if on part of the energy is
195 radiated away by initial-state photons. In the latter case the
196 initial-state showers will generate the angles and energies of the
197 set of photons that go with the collision. In addition one collinear
198 photon per beam carries any leftover amount of energy not described
199 by shower emissions. If the initial-state showers are switched off
200 these collinear photons will carry the full radiated energy.
203 <h3>Incoming parton selection</h3>
205 There is one useful degree of freedom to restrict the set of incoming
206 quark flavours for hard processes. It does not change the PDF's as such,
207 only which quarks are allowed to contribute to the hard-process cross
208 sections. Note that separate but similarly named modes are available
209 for multiple interactions and spacelike showers.
211 <modeopen name="PDFinProcess:nQuarkIn" default="5" min="0" max="5">
212 Number of allowed incoming quark flavours in the beams; a change
213 to 4 would thus exclude <ei>b</ei> and <ei>bbar</ei> as incoming
219 <!-- Copyright (C) 2008 Torbjorn Sjostrand -->