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