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1<html>
2<head>
3<title>PDF Selection</title>
4<link rel="stylesheet" type="text/css" href="pythia.css"/>
5<link rel="shortcut icon" href="pythia32.gif"/>
6</head>
7<body>
8
9<h2>PDF Selection</h2>
10
11This page contains five subsections. The first deals with how to
12pick  the parton distribution set for protons, including from LHAPDF,
13to be used for all proton and antiproton beams. The second is a special
14option that allows a separate PDF set to be used for the hard process
15only, while the first choice would still apply to everything else.
16The third and fourth give access to pion and Pomeron PDF's, respectively,
17the latter being used to describe diffractive systems.
18The fifth gives the possibility to switch off the lepton
19"parton density". More information on PDF classes is found
20<a href="PartonDistributions.html" target="page">here</a>.
21
22<h3>Parton densities for protons</h3>
23
24The selection of parton densities is made once and then is propagated
25through the program. It is essential to make an informed choice,
26for several reasons [<a href="Bibliography.html" target="page">Kas10</a>]:
27<br/><b>Warning 1:</b> the choice of PDF set affects a number of
28properties of events. A change of PDF therefore requires a complete
29retuning e.g.  of the multiparton-interactions model for minimum-bias and
30underlying events.
31<br/><b>Warning 2:</b> People often underestimate the differences
32between different sets on the market. The sets for the same order are
33constructed to behave more or less similarly at large <i>x</i> and
34<i>Q^2</i>, while the multiparton interactions are dominated by the
35behaviour in the region of small <i>x</i> and <i>Q^2</i>. A good
36PDF parametrization ought to be sensible down to <i>x = 10^-6</i> 
37(<i>x = 10^-7</i>) and <i>Q^2 = 1</i> GeV^2 for Tevatron (LHC)
38applications. Unfortunately there are distributions on the market that
39completely derail in that region. The <code>main51.cc</code> and
40<code>main52.cc</code> programs in the <code>examples</code> 
41subdirectory provide some examples of absolutely minimal sanity checks
42before a new PDF set is put in production.
43<br/><b>Warning 3:</b> NLO and LO sets tend to have quite different
44behaviours, e.g. NLO ones have less gluons at small x, which then is
45compensated by positive corrections in the NLO matrix elements.
46Therefore do not blindly assume that an NLO tune has to be better than
47an LO one when combined with the LO matrix elements in PYTHIA. There are
48explicit examples where such thinking can lead you down the wrong alley,
49especially if you study low-<i>pT</i> physics. In the list below you
50should therefore be extra cautious when using set 6 or set 9.
51
52<p/>
53The simplest option is to pick one
54of the distributions available internally:
55
56<p/><code>mode&nbsp; </code><strong> PDF:pSet &nbsp;</strong> 
57 (<code>default = <strong>2</strong></code>; <code>minimum = 1</code>; <code>maximum = 12</code>)<br/>
58Parton densities to be used for proton beams (and, by implication,
59antiproton ones):
60<br/><code>option </code><strong> 1</strong> : GRV 94L, LO <i>alpha_s(M_Z) = 0.128</i>
61(this set is out of date, but retained for historical comparisons). 
62<br/><code>option </code><strong> 2</strong> : CTEQ 5L, LO <i>alpha_s(M_Z) = 0.127</i>
63(this set is also out of date, but not badly so, and many tunes
64are based on it). 
65<br/><code>option </code><strong> 3</strong> : MRST LO* (2007),
66NLO <i>alpha_s(M_Z) = 0.12032</i>. 
67<br/><code>option </code><strong> 4</strong> : MRST LO** (2008),
68NLO <i>alpha_s(M_Z) = 0.11517</i>. 
69<br/><code>option </code><strong> 5</strong> : MSTW 2008 LO (central member),
70LO <i>alpha_s(M_Z) = 0.13939</i>. 
71<br/><code>option </code><strong> 6</strong> : MSTW 2008 NLO (central member),
72NLO <i>alpha_s(M_Z) = 0.12018</i> (NLO, see Warning 3 above). 
73<br/><code>option </code><strong> 7</strong> : CTEQ6L, NLO <i>alpha_s(M_Z) = 0.1180</i>. 
74<br/><code>option </code><strong> 8</strong> : CTEQ6L1, LO <i>alpha_s(M_Z) = 0.1298</i>. 
75<br/><code>option </code><strong> 9</strong> : CTEQ66.00 (NLO, central member),
76NLO <i>alpha_s(M_Z) = 0.1180</i> (NLO, see Warning 3 above). 
77<br/><code>option </code><strong> 10</strong> : CT09MC1, LO <i>alpha_s(M_Z) = 0.1300</i>. 
78<br/><code>option </code><strong> 11</strong> : CT09MC2, NLO <i>alpha_s(M_Z) = 0.1180</i>. 
79<br/><code>option </code><strong> 12</strong> : CT09MCS, NLO <i>alpha_s(M_Z) = 0.1180</i>. 
80   
81<br/><b>Note:</b> the <i>alpha_s(M_Z)</i> values and the order of the
82running in the description above is purely informative, and does not
83affect any other parts of the program. Instead you have the freedom to
84set <i>alpha_s(M_Z)</i> value and running separately for
85<a href="CouplingsAndScales.html" target="page">hard processes</a> 
86(including resonance decays),
87<a href="MultipartonInteractions.html" target="page">multiparton interactions</a>,
88<a href="SpacelikeShowers.html" target="page">initial-state radiation</a>, and
89<a href="TimelikeShowers.html" target="page">final-state radiation</a>.
90
91<p/>
92This is a reasonably complete list of recent LO fits, both
93ones within the normal LO context and ones with modifications for better
94matching to event generators. In addition two older sets are
95included for backwards reference (most studies to date are based on
96CTEQ 5L). If you link to the
97<a href="http://projects.hepforge.org/lhapdf/" target="page">LHAPDF
98library</a> [<a href="Bibliography.html" target="page">Wha05</a>] you get access to a much wider selection.
99<br/><b>Warning 1:</b> owing to previous problems with the behaviour
100of PDF's beyond the <i>x</i> and <i>Q^2</i> boundaries of a set,
101you should only use LHAPDF <b>version 5.3.0 or later</b>.
102<br/><b>Warning 2:</b> the behaviour of the LHAPDF sets need not be
103identical with the implementation found in PYTHIA. Specifically we
104are aware of the following points that may influence a comparison.
105<br/>(a) CTEQ 5L in PYTHIA is the parametrization, in LHAPDF the grid
106interpolation.
107<br/>(b) MRST LO* and LO** in PYTHIA is based on an updated edition,
108where one makes use of the expanded MSTW grid format, while LHAPDF
109is based on the original smaller grid.
110<br/>(c) The CTEQ 6 and CT09MC sets in PYTHIA are frozen at the
111boundaries of the grid, by recommendation of the authors, while
112LHAPDF also offers an option with a smooth extrapolation outside
113the grid boundaries.
114
115<p/><code>flag&nbsp; </code><strong> PDF:useLHAPDF &nbsp;</strong> 
116 (<code>default = <strong>off</strong></code>)<br/>
117If off then the choice of proton PDF is based on <code>PDF:pSet</code>
118above. If on then it is instead based on the choice of
119<code>PDF:LHAPDFset</code> and <code>PDF:LHAPDFmember</code> below.
120<br/><b>Note:</b> in order for this option to work you must have
121compiled PYTHIA appropriately and have set the <code>LHAPATH</code> 
122environment variable to provide the data-files directory of your local
123LHAPDF installation. See the README file in the <code>examples</code> 
124directory for further instructions.
125 
126
127<p/><code>word&nbsp; </code><strong> PDF:LHAPDFset &nbsp;</strong> 
128 (<code>default = <strong>MRST2004FF4lo.LHgrid</strong></code>)<br/>
129Name of proton PDF set from LHAPDF to be used. You have to choose
130from the
131<a href="http://projects.hepforge.org/lhapdf/pdfsets" target="page">
132list of available sets</a>. Examples of some fairly recent ones
133(but still less recent than found above) would be
134cteq61.LHpdf, cteq61.LHgrid, cteq6l.LHpdf, cteq6ll.LHpdf,
135MRST2004nlo.LHpdf, MRST2004nlo.LHgrid, MRST2004nnlo.LHgrid and
136MRST2004FF3lo.LHgrid. If you pick a LHpdf set it will require some
137calculation the first time it is called.
138<br/><b>Technical note:</b> if you provide a name beginning with a
139slash (/) it is assumed you want to provide the full file path and then
140<code>initPDFsetM(name)</code> is called, else the correct path is assumed
141already set and <code>initPDFsetByNameM(name)</code> is called.
142   
143
144<p/><code>mode&nbsp; </code><strong> PDF:LHAPDFmember &nbsp;</strong> 
145 (<code>default = <strong>0</strong></code>; <code>minimum = 0</code>)<br/>
146Further choice of a specific member from the set picked above. Member 0
147should normally correspond to the central value, with higher values
148corresponding to different error PDF's somewhat off in different
149directions. You have to check from set to set which options are open.
150<br/><b>Note:</b> you can only use one member in a run, so if you
151want to sweep over many members you either have to do many separate
152runs or, as a simplification, save the
153<a href="EventInformation.html" target="page">pdf weights</a> at the hard scattering
154and do an offline reweighting of events.
155     
156
157<p/><code>flag&nbsp; </code><strong> PDF:extrapolateLHAPDF &nbsp;</strong> 
158 (<code>default = <strong>off</strong></code>)<br/>
159Parton densities have a guaranteed range of validity in <i>x</i>
160and <i>Q^2</i>, and what should be done beyond that range usually is
161not explained by the authors of PDF sets. Nevertheless these boundaries
162very often are exceeded, e.g. minimum-bias studies at LHC may sample
163<i>x</i> values down to <i>10^-8</i>, while many PDF sets stop
164already at <i>10^-5</i>. The default behaviour is then that the
165PDF's are frozen at the boundary, i.e. <i>xf(x,Q^2)</i> is fixed at
166its value at <i>x_min</i> for all values <i>x &lt; x_min</i>,
167and so on. This is a conservative approach. Alternatively, if you
168switch on extrapolation, then parametrizations will be extended beyond
169the boundaries, by some prescription. In some cases this will provide a
170more realistic answer, in others complete rubbish. Another problem is
171that some of the PDF-set codes will write a warning message anytime the
172limits are exceeded, thus swamping your output file. Therefore you should
173study a set seriously before you run it with this switch on.
174 
175
176<p/> 
177If you want to use PDF's not found in LHAPDF, or you want to interface
178LHAPDF another way, you have full freedom to use the more generic
179<a href="PartonDistributions.html" target="page">interface options</a>.
180
181<h3>Parton densities for protons in the hard process</h3>
182
183The above options provides a PDF set that will be used everywhere:
184for the hard process, the parton showers and the multiparton interactions
185alike. As already mentioned, therefore a change of PDF should be
186accompanied by a <b>complete</b> retuning of the whole MPI framework,
187and maybe more. There are cases where one may want to explore
188different PDF options for the hard process, but would not want to touch
189the rest. If several different sets are to be compared, a simple
190reweighting based on the <a href="EventInformation.html" target="page">originally
191used</a> flavour, <i>x</i>, <i>Q^2</i> and PDF values may offer the
192best route. The options in this section allow a choice of the PDF set
193for the hard process alone, while the choice made in the previous section
194would still be used for everything else. The hardest interaction
195of the minimum-bias process is part of the multiparton-interactions
196framework and so does not count as a hard process here.
197
198<p/>
199Of course it is inconsistent to use different PDF's in different parts
200of an event, but if the <i>x</i> and <i>Q^2</i> ranges mainly accessed
201by the components are rather different then the contradiction would not be
202too glaring. Furthermore, since standard PDF's are one-particle-inclusive
203we anyway have to 'invent' our own PDF modifications to handle configurations
204where more than one parton is kicked out of the proton [<a href="Bibliography.html" target="page">Sjo04</a>].
205
206<p/>
207The PDF choices that can be made are the same as above, so we do not
208repeat the detailed discussion.
209
210<p/><code>flag&nbsp; </code><strong> PDF:useHard &nbsp;</strong> 
211 (<code>default = <strong>off</strong></code>)<br/>
212If on then select a separate PDF set for the hard process, using the
213variables below. If off then use the same PDF set for everything,
214as already chosen above.   
215 
216
217<p/><code>mode&nbsp; </code><strong> PDF:pHardSet &nbsp;</strong> 
218 (<code>default = <strong>2</strong></code>; <code>minimum = 1</code>; <code>maximum = 12</code>)<br/>
219Parton densities to be used for proton beams (and, by implication,
220antiproton ones):
221<br/><code>option </code><strong> 1</strong> : GRV 94L, LO <i>alpha_s(M_Z) = 0.128</i>
222(out of date). 
223<br/><code>option </code><strong> 2</strong> : CTEQ 5L, LO <i>alpha_s(M_Z) = 0.127</i>
224(slightly out of date; many tunes are based on it). 
225<br/><code>option </code><strong> 3</strong> : MRST LO* (2007),
226NLO <i>alpha_s(M_Z) = 0.12032</i>. 
227<br/><code>option </code><strong> 4</strong> : MRST LO** (2008),
228NLO <i>alpha_s(M_Z) = 0.11517</i>. 
229<br/><code>option </code><strong> 5</strong> : MSTW 2008 LO (central member),
230LO <i>alpha_s(M_Z) = 0.13939</i>. 
231<br/><code>option </code><strong> 6</strong> : MSTW 2008 NLO (central member),
232LO <i>alpha_s(M_Z) = 0.12018</i>. 
233<br/><code>option </code><strong> 7</strong> : CTEQ6L, NLO <i>alpha_s(M_Z) = 0.1180</i>. 
234<br/><code>option </code><strong> 8</strong> : CTEQ6L1, LO <i>alpha_s(M_Z) = 0.1298</i>. 
235<br/><code>option </code><strong> 9</strong> : CTEQ66.00 (NLO, central member),
236NLO <i>alpha_s(M_Z) = 0.1180</i>. 
237<br/><code>option </code><strong> 10</strong> : CT09MC1, LO <i>alpha_s(M_Z) = 0.1300</i>. 
238<br/><code>option </code><strong> 11</strong> : CT09MC2, NLO <i>alpha_s(M_Z) = 0.1180</i>. 
239<br/><code>option </code><strong> 12</strong> : CT09MCS, NLO <i>alpha_s(M_Z) = 0.1180</i>. 
240   
241
242<p/><code>flag&nbsp; </code><strong> PDF:useHardLHAPDF &nbsp;</strong> 
243 (<code>default = <strong>off</strong></code>)<br/>
244If off then the choice of proton PDF is based on <code>hardpPDFset</code>
245above. If on then it is instead based on the choice of
246<code>hardLHAPDFset</code> and <code>hardLHAPDFmember</code> below.
247Note that if you want to use LHAPDF here, and you also use LHAPDF
248for the "normal" PDF set, then LHAPDF must have been compiled so as to
249handle (at least) two concurrent sets, with the configure statement
250<code>--with-max-num-pdfsets=2</code>.
251 
252
253<p/><code>word&nbsp; </code><strong> PDF:hardLHAPDFset &nbsp;</strong> 
254 (<code>default = <strong>MRST2004FF4lo.LHgrid</strong></code>)<br/>
255Name of proton PDF set from LHAPDF to be used.
256   
257
258<p/><code>mode&nbsp; </code><strong> PDF:hardLHAPDFmember &nbsp;</strong> 
259 (<code>default = <strong>0</strong></code>; <code>minimum = 0</code>)<br/>
260Further choice of a specific member from the set picked above.
261     
262
263<p/>
264Note that there is no separate equivalent of the
265<code>PDF:extrapolateLHAPDF</code> flag specifically for the hard
266PDF. Since LHAPDF only has one global flag for extrapolation or not,
267the choice for the normal PDF's also applies to the hard ones.
268
269<h3>Parton densities for pions</h3>
270
271The parton densities of the pion are considerably less well known than
272those of the proton. There are only rather few sets on the market,
273and none particularly recent. Only one comes built-in, but others can
274be accessed from LHAPDF. Input parametrizations are for the <i>pi+</i>.
275>From this the <i>pi-</i> is obtained by charge conjugation and the
276<i>pi0</i> from averaging (half the pions have <i>d dbar</i> 
277valence quark content, half <i>u ubar</i>.
278
279<p/>
280Much of the switches are taken over from the proton case, with obvious
281modifications; therefore the description is briefer. Currently we have
282not seen the need to allow separate parton densities for hard processes.
283When using LHAPDF the <code>PDF:extrapolateLHAPDF</code> switch of the
284proton also applies to pions.
285 
286<p/><code>mode&nbsp; </code><strong> PDF:piSet &nbsp;</strong> 
287 (<code>default = <strong>1</strong></code>; <code>minimum = 1</code>; <code>maximum = 1</code>)<br/>
288Internal parton densities that can be used for pion beams, currently with
289only one choice.
290<br/><code>option </code><strong> 1</strong> : GRV 92 L. 
291   
292
293<p/><code>flag&nbsp; </code><strong> PDF:piUseLHAPDF &nbsp;</strong> 
294 (<code>default = <strong>off</strong></code>)<br/>
295If off then the choice of proton PDF is based on <code>PDF:piSet</code>
296above. If on then it is instead based on the choice of
297<code>PDF:piLHAPDFset</code> and <code>PDF:piLHAPDFmember</code> below.
298 
299
300<p/><code>word&nbsp; </code><strong> PDF:piLHAPDFset &nbsp;</strong> 
301 (<code>default = <strong>OWPI.LHgrid</strong></code>)<br/>
302Name of pion PDF set from LHAPDF to be used. You have to choose from the
303<a href="http://projects.hepforge.org/lhapdf/pdfsets" target="page">
304list of available sets</a>.
305   
306
307<p/><code>mode&nbsp; </code><strong> PDF:piLHAPDFmember &nbsp;</strong> 
308 (<code>default = <strong>0</strong></code>; <code>minimum = 0</code>)<br/>
309Further choice of a specific member from the set picked above.
310     
311
312<h3>Parton densities for Pomerons</h3>
313
314The Pomeron is introduced in the description of diffractive events,
315i.e. a diffractive system is viewed as a Pomeron-proton collision at a
316reduced CM energy. Here the PDF's are even less well known.
317Most experimental parametrizations are NLO, which makes them less
318well suited for Monte Carlo applications. Furthemore note that
319the momentum sum is arbitrarily normalized to a non-unity value.
320
321<p/><code>mode&nbsp; </code><strong> PDF:PomSet &nbsp;</strong> 
322 (<code>default = <strong>6</strong></code>; <code>minimum = 1</code>; <code>maximum = 6</code>)<br/>
323Parton densities that can be used for Pomeron beams.
324<br/><code>option </code><strong> 1</strong> : <i>Q^2</i>-independent parametrizations
325<i>xf(x) = N_ab x^a (1 - x)^b</i>, where <i>N_ab</i> ensures
326unit momentum sum. The <i>a</i> and <i>b</i> parameters can be
327set separately for the gluon and the quark distributions. The
328momentum fraction of gluons and quarks can be freely mixed, and
329production of <i>s</i> quarks can be suppressed relative to
330that of <i>d</i> and <i>u</i> ones, with antiquarks as likely
331as quarks. See further below how to set the six parameters of this
332approach.
333 
334<br/><code>option </code><strong> 2</strong> : <i>pi0</i> distributions, as specified in the
335section above.
336 
337<br/><code>option </code><strong> 3</strong> : the H1 2006 Fit A NLO <i>Q^2</i>-dependent
338parametrization, based on a tune to their data [<a href="Bibliography.html" target="page">H1P06</a>],
339rescaled by the factor <code>PomRescale</code> below.
340 
341<br/><code>option </code><strong> 4</strong> : the H1 2006 Fit B NLO <i>Q^2</i>-dependent
342parametrization, based on a tune to their data [<a href="Bibliography.html" target="page">H1P06</a>],
343rescaled by the factor <code>PomRescale</code> below.
344 
345<br/><code>option </code><strong> 5</strong> : the H1 2007 Jets NLO <i>Q^2</i>-dependent
346parametrization, based on a tune to their data [<a href="Bibliography.html" target="page">H1P07</a>],
347rescaled by the factor <code>PomRescale</code> below.
348 
349<br/><code>option </code><strong> 6</strong> : the H1 2006 Fit B LO <i>Q^2</i>-dependent
350parametrization, based on a tune to their data [<a href="Bibliography.html" target="page">H1P06</a>],
351rescaled by the factor <code>PomRescale</code> below.
352 
353   
354
355<p/><code>parm&nbsp; </code><strong> PDF:PomGluonA &nbsp;</strong> 
356 (<code>default = <strong>0.</strong></code>; <code>minimum = -0.5</code>; <code>maximum = 2.</code>)<br/>
357the parameter <i>a</i> in the ansatz <i>xg(x) = N_ab x^a (1 - x)^b</i>
358for option 1 above.
359 
360
361<p/><code>parm&nbsp; </code><strong> PDF:PomGluonB &nbsp;</strong> 
362 (<code>default = <strong>3.</strong></code>; <code>minimum = 0.</code>; <code>maximum = 10.</code>)<br/>
363the parameter <i>b</i> in the ansatz <i>xg(x) = N_ab x^a (1 - x)^b</i>
364for option 1 above.
365 
366
367<p/><code>parm&nbsp; </code><strong> PDF:PomQuarkA &nbsp;</strong> 
368 (<code>default = <strong>0.</strong></code>; <code>minimum = -0.5</code>; <code>maximum = 2.</code>)<br/>
369the parameter <i>a</i> in the ansatz <i>xq(x) = N_ab x^a (1 - x)^b</i>
370for option 1 above.
371 
372
373<p/><code>parm&nbsp; </code><strong> PDF:PomQuarkB &nbsp;</strong> 
374 (<code>default = <strong>3.</strong></code>; <code>minimum = 0.</code>; <code>maximum = 10.</code>)<br/>
375the parameter <i>b</i> in the ansatz <i>xq(x) = N_ab x^a (1 - x)^b</i>
376for option 1 above.
377 
378
379<p/><code>parm&nbsp; </code><strong> PDF:PomQuarkFrac &nbsp;</strong> 
380 (<code>default = <strong>0.2</strong></code>; <code>minimum = 0.</code>; <code>maximum = 1.</code>)<br/>
381the fraction of the Pomeron momentum carried by quarks
382for option 1 above, with the rest carried by gluons.
383 
384
385<p/><code>parm&nbsp; </code><strong> PDF:PomStrangeSupp &nbsp;</strong> 
386 (<code>default = <strong>0.5</strong></code>; <code>minimum = 0.</code>; <code>maximum = 1.</code>)<br/>
387the suppression of the <i>s</i> quark density relative to that of the
388<i>d</i> and <i>u</i> ones for option 1 above.
389 
390
391<p/><code>parm&nbsp; </code><strong> PDF:PomRescale &nbsp;</strong> 
392 (<code>default = <strong>1.0</strong></code>; <code>minimum = 0.5</code>; <code>maximum = 5.0</code>)<br/>
393Rescale the four H1 fits above by this uniform factor, e.g. to bring
394up their momentum sum to around unity. By default all three have
395a momentum sum of order 0.5, suggesting that a factor around 2.0
396should be used. You can use <code>examples/main51.cc</code> to get
397a more precise value. Note that also other parameters in the
398<a href="Diffraction.html" target="page">diffraction</a> framework may need to
399be retuned when this parameter is changed.
400 
401
402<h3>Parton densities for leptons</h3>
403
404For electrons/muons/taus there is no need to choose between different
405parametrizations, since only one implementation is available, and
406should be rather uncontroversial (apart from some technical details).
407However, insofar as e.g. <i>e^+ e^-</i> data often are corrected
408back to a world without any initial-state photon radiation, it is
409useful to have a corresponding option available here.
410
411<p/><code>flag&nbsp; </code><strong> PDF:lepton &nbsp;</strong> 
412 (<code>default = <strong>on</strong></code>)<br/>
413Use parton densities for lepton beams or not. If off the colliding
414leptons carry the full beam energy, if on part of the energy is
415radiated away by initial-state photons. In the latter case the
416initial-state showers will generate the angles and energies of the
417set of photons that go with the collision. In addition one collinear
418photon per beam carries any leftover amount of energy not described
419by shower emissions. If the initial-state showers are switched off
420these collinear photons will carry the full radiated energy. 
421   
422
423<p/>
424Neutrinos are always taken pointlike. Do note that the phase space
425selection machinery currently does not allow one resolved and one
426unresolved beam. For lepton-neutrino collisions to work you must
427therefore set <code>PDF:lepton = off</code>.
428
429<h3>Incoming parton selection</h3>
430
431There is one useful degree of freedom to restrict the set of incoming
432quark flavours for hard processes. It does not change the PDF's as such,
433only which quarks are allowed to contribute to the hard-process cross
434sections. Note that separate but similarly named modes are available
435for multiparton interactions and spacelike showers.
436
437<p/><code>mode&nbsp; </code><strong> PDFinProcess:nQuarkIn &nbsp;</strong> 
438 (<code>default = <strong>5</strong></code>; <code>minimum = 0</code>; <code>maximum = 5</code>)<br/>
439Number of allowed incoming quark flavours in the beams; a change
440to 4 would thus exclude <i>b</i> and <i>bbar</i> as incoming
441partons, etc.
442 
443
444</body>
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446
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