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30 <h2>Les Houches Accord</h2>
32 The Les Houches Accord (LHA) for user processes [<a href="Bibliography.php" target="page">Boo01</a>] is the
33 standard way to input parton-level information from a
34 matrix-elements-based generator into PYTHIA. The conventions for
35 which information should be stored has been defined in a Fortran context,
36 as two commonblocks. Here a C++ equivalent is defined, as a single class.
39 The <code>LHAup</code> class is a base class, containing reading and
40 printout functions, plus two pure virtual functions, one to set
41 initialization information and one to set information on each new event.
42 Derived classes have to provide these two virtual functions to do
43 the actual work. The existing derived classes are for reading information
44 from a Les Houches Event File (LHEF), from the respective Fortran
45 commonblocks, or from PYTHIA 8 itself.
48 You are free to write your own derived classes, using the rules and
49 methods to be described below. Normally, pointers to objects of such
50 derived classes should be handed in with the
51 <code><?php $filepath = $_GET["filepath"];
52 echo "<a href='ProgramFlow.php?filepath=".$filepath."' target='page'>";?>Pythia::init( LHAup*)</a></code>
53 method. However, with the LHEF format a filename can replace the
54 pointer, see further below.
57 Let us now describe the methods at your disposal to do the job.
59 <a name="method1"></a>
60 <p/><strong>LHAup::LHAup( int strategy = 3) </strong> <br/>
61 the base class constructor takes the choice of mixing/weighting
62 strategy as optional input argument, and calls <code>setStrategy</code>,
63 see below. It also reserves some space for processes and particles.
66 <a name="method2"></a>
67 <p/><strong>virtual LHAup::~LHAup() </strong> <br/>
68 the destructor does not need to do anything.
71 <a name="method3"></a>
72 <p/><strong>void LHAup::setPtr(Info* infoPtr) </strong> <br/>
73 this method only sets the pointer that allows some information
74 to be accessed, and is automatically called by
75 <code>Pythia::init(...)</code>.
78 <h3>Initialization</h3>
80 The <code>LHAup</code> class stores information equivalent to the
81 <code>/HEPRUP/</code> commonblock, as required to initialize the event
82 generation chain. The main difference is that the vector container
83 now allows a flexible number of subprocesses to be defined. For the
84 rest, names have been modified, since the 6-character-limit does not
85 apply, and variables have been regrouped for clarity, but nothing
86 fundamental is changed.
88 <a name="method4"></a>
89 <p/><strong>virtual bool LHAup::setInit() </strong> <br/>
90 this pure virtual method has to be implemented in the derived class,
91 to set relevant information when called. It should return false if it
92 fails to set the info.
96 Inside <code>setInit()</code>, such information can be set by the following
98 <a name="method5"></a>
99 <p/><strong>void LHAup::setBeamA( int identity, double energy, int pdfGroup, int pdfSet) </strong> <br/>
101 <strong>void LHAup::setBeamB( int identity, double energy, int pdfGroup, int pdfSet) </strong> <br/>
102 sets the properties of the first and second incoming beam, respectively
103 (cf. the Fortran <code>IDBMUP(1), EBMUP(i), PDFGUP(i), PDFSUP(i)</code>,
104 with <code>i</code> 1 or 2). The parton distribution information
105 defaults to zero. These numbers can be used to tell which PDF sets were
106 used when the hard process was generated, while the normal
107 <?php $filepath = $_GET["filepath"];
108 echo "<a href='PDFSelection.php?filepath=".$filepath."' target='page'>";?>PDF Selection</a> is used for the further
109 event generation in PYTHIA.
112 <a name="method6"></a>
113 <p/><strong>void LHAup::setStrategy( int strategy) </strong> <br/>
114 sets the event weighting and cross section strategy. The default,
115 provided in the class constructor, is 3, which is the natural value
117 <br/><code>argument</code><strong> strategy </strong> :
118 chosen strategy (cf. <code>IDWTUP</code>; see [<a href="Bibliography.php" target="page">Sjo06</a>]
119 section 9.9.1 for extensive comments).
120 <br/><code>argumentoption </code><strong> 1</strong> : events come with non-negative weight, given in units
121 of pb, with an average that converges towards the cross section of the
122 process. PYTHIA is in charge of the event mixing, i.e. for each new
123 try decides which process should be generated, and then decides whether
124 is should be kept, based on a comparison with <code>xMax</code>.
125 Accepted events therefore have unit weight.
126 <br/><code>argumentoption </code><strong> -1</strong> : as option 1, except that cross sections can now be
127 negative and events after unweighting have weight +-1. You can use
128 <code><?php $filepath = $_GET["filepath"];
129 echo "<a href='EventInformation.php?filepath=".$filepath."' target='page'>";?>Info::weight()</a></code>
130 to find the weight of the current event. A correct event mixing requires
131 that a process that can take both signs should be split in two, one limited
132 to positive or zero and the other to negative or zero values, with
133 <code>xMax</code> chosen appropriately for the two.
134 <br/><code>argumentoption </code><strong> 2</strong> : events come with non-negative weight, in unspecified
135 units, but such that <code>xMax</code> can be used to unweight the events
136 to unit weight. Again PYTHIA is in charge of the event mixing.
137 The total cross section of a process is stored in
139 <br/><code>argumentoption </code><strong> -2</strong> : as option 2, except that cross sections can now be
140 negative and events after unweighting have weight +-1. As for option -1
141 processes with indeterminate sign should be split in two.
142 <br/><code>argumentoption </code><strong> 3</strong> : events come with unit weight, and are thus accepted
143 as is. The total cross section of the process is stored in
145 <br/><code>argumentoption </code><strong> -3</strong> : as option 3, except that events now come with weight
146 +-1. Unlike options -1 and -2 processes with indeterminate sign need not be
147 split in two, unless you intend to mix with internal PYTHIA processes
149 <br/><code>argumentoption </code><strong> 4</strong> : events come with non-negative weight, given in units
150 of pb, with an average that converges towards the cross section of the
151 process, like for option 1. No attempt is made to unweight the events,
152 however, but all are generated in full, and retain their original weight.
153 For consistency with normal PYTHIA units, the weight stored in
154 <code>Info::weight()</code> has been converted to mb, however.
156 <br/><code>argumentoption </code><strong> -4</strong> : as option 4, except that events now can come
157 either with positive or negative weights.
158 <br/><b>Note 1</b>: if several processes have already been mixed and
159 stored in a common event file, either LHEF or some private format, it
160 would be problematical to read back events in a different order. Since it
161 is then not feasible to let PYTHIA pick the next process type, strategies
162 +-1 and +-2 would not work. Instead strategy 3 would be the recommended
163 choice, or -3 if negative-weight events are required.
164 <br/><b>Note 2</b>: it is possible to switch on internally implemented
165 processes and have PYTHIA mix these with LHA ones according to their relative
166 cross sections for strategies +-1, +-2 and 3. It does not work for strategy
167 -3 unless the positive and negative sectors of the cross sections are in
168 separate subprocesses (as must always be the case for -1 and -2), since
169 otherwise the overall mixture of PYTHIA and LHA processes will be off.
170 Mixing is not possible for strategies +-4, since the weighting procedure
171 is not specified by the standard. (For instance, the intention may be to
172 have events biased towards larger <i>pT</i> values in some particular
177 <a name="method7"></a>
178 <p/><strong>void LHAup::addProcess( int idProcess, double xSec, double xErr, double xMax) </strong> <br/>
179 sets info on an allowed process (cf. <code>LPRUP, XSECUP, XERRUP,
181 Each new call will append one more entry to the list of processes.
182 The choice of strategy determines which quantities are mandatory:
183 <code>xSec</code> for strategies +-2 and +-3,
184 <code>xErr</code> never, and
185 <code>xMax</code> for strategies +-1 and +-2.
188 <br/><b>Note</b>: PYTHIA does not make active use of the (optional)
189 <code>xErr</code> values, but calculates a statistical cross section
190 error based on the spread of event-to-event weights. This should work
191 fine for strategy options +-1, but not for the others. Specifically,
192 for options +-2 and +-3 the weight spread may well vanish, and anyway
193 is likely to be an underestimate of the true error. If the author of the
194 LHA input information does provide error information you may use that -
195 this information is displayed at initialization. If not, then a relative
196 error decreasing like <i>1/sqrt(n_acc)</i>, where <i>n_acc</i>
197 is the number of accepted events, should offer a reasonable estimate.
199 <a name="method8"></a>
200 <p/><strong>void LHAup::setXSec( int i, double xSec) </strong> <br/>
201 update the <code>xSec</code> value of the <code>i</code>'th process
202 added with <code>addProcess</code> method (i.e. <code>i</code> runs
203 from 0 through <code>sizeProc() - 1</code>, see below).
206 <a name="method9"></a>
207 <p/><strong>void LHAup::setXErr( int i, double xErr) </strong> <br/>
208 update the <code>xErr</code> value of the <code>i</code>'th process
209 added with <code>addProcess</code> method.
212 <a name="method10"></a>
213 <p/><strong>void LHAup::setXMax( int i, double xMax) </strong> <br/>
214 update the <code>xMax</code> value of the <code>i</code>'th process
215 added with <code>addProcess</code> method.
219 Information is handed back by the following methods
220 (that normally you would not need to touch):
221 <a name="method11"></a>
222 <p/><strong>int LHAup::idBeamA() </strong> <br/>
224 <strong>int LHAup::idBeamB() </strong> <br/>
226 <strong>double LHAup::eBeamA() </strong> <br/>
228 <strong>double LHAup::eBeamB() </strong> <br/>
230 <strong>int LHAup::pdfGroupBeamA() </strong> <br/>
232 <strong>int LHAup::pdfGroupBeamB() </strong> <br/>
234 <strong>int LHAup::pdfSetBeamA() </strong> <br/>
236 <strong>int LHAup::pdfSetBeamB() </strong> <br/>
237 for the beam properties.
239 <a name="method12"></a>
240 <p/><strong>int LHAup::strategy() </strong> <br/>
241 for the strategy choice.
243 <a name="method13"></a>
244 <p/><strong>int LHAup::sizeProc() </strong> <br/>
245 for the number of subprocesses.
247 <a name="method14"></a>
248 <p/><strong>int LHAup::idProcess(i) </strong> <br/>
250 <strong>double LHAup::xSec(i) </strong> <br/>
252 <strong>double LHAup::xErr(i) </strong> <br/>
254 <strong>double LHAup::xMax(i) </strong> <br/>
255 for process <code>i</code> in the range <code>0 <= i <
259 <a name="method15"></a>
260 <p/><strong>void LHAup::listInit(ostream& os = cout) </strong> <br/>
261 prints the above initialization information. This method is
262 automatically called from <code>Pythia::init(...)</code>,
263 so would normally not need to be called directly by the user.
268 The <code>LHAup</code> class also stores information equivalent to the
269 <code>/HEPEUP/</code> commonblock, as required to hand in the next
270 parton-level configuration for complete event generation. The main
271 difference is that the vector container now allows a flexible number
272 of partons to be defined. For the rest, names have been modified,
273 since the 6-character-limit does not apply, and variables have been
274 regrouped for clarity, but nothing fundamental is changed.
277 The LHA standard is based on Fortran arrays beginning with
278 index 1, and mother information is defined accordingly. In order to
279 be compatible with this convention, the zeroth line of the C++ particle
280 array is kept empty, so that index 1 also here corresponds to the first
281 particle. One small incompatibility is that the <code>sizePart()</code>
282 method returns the full size of the particle array, including the
283 empty zeroth line, and thus is one larger than the true number of
284 particles (<code>NUP</code>).
286 <a name="method16"></a>
287 <p/><strong>virtual bool LHAup::setEvent(int idProcess = 0) </strong> <br/>
288 this pure virtual method has to be implemented in the derived class,
289 to set relevant information when called. For strategy options +-1
290 and +-2 the input <code>idProcess</code> value specifies which process
291 that should be generated, while <code>idProcess</code> is irrelevant
292 for strategies +-3 and +-4. The method should return false if it fails
293 to set the info, i.e. normally that the supply of events in a file is
294 exhausted. If so, no event is generated, and <code>Pythia::next()</code>
295 returns false. You can then interrogate
296 <code><?php $filepath = $_GET["filepath"];
297 echo "<a href='EventInformation.php?filepath=".$filepath."' target='page'>";?>Info::atEndOfFile()</a></code>
298 to confirm that indeed the failure is caused in this method, and decide
299 to break out of the event generation loop.
302 Inside a normal <code>setEvent(...)</code> call, information can be set
303 by the following methods:
304 <a name="method17"></a>
305 <p/><strong>void LHAup::setProcess( int idProcess, double weight, double scale, double alphaQED, double alphaQCD) </strong> <br/>
306 tells which kind of process occured, with what weight, at what scale,
307 and which <i>alpha_EM</i> and <i>alpha_strong</i> were used
308 (cf. <code>IDPRUP, XWTGUP, SCALUP, AQEDUP, AQCDUP</code>). This method
309 also resets the size of the particle list, and adds the empty zeroth
310 line, so it has to be called before the <code>addParticle</code> method below.
312 <a name="method18"></a>
313 <p/><strong>void LHAup::addParticle( int id, int status, int mother1, int mother2, int colourTag1, int colourTag2, double p_x, double p_y, double p_z, double e, double m, double tau, double spin) </strong> <br/>
314 gives the properties of the next particle handed in (cf. <code>IDUP, ISTUP,
315 MOTHUP(1,..), MOTHUP(2,..), ICOLUP(1,..), ICOLUP(2,..), PUP(J,..),
316 VTIMUP, SPINUP</code>) .
320 Information is handed back by the following methods:
321 <a name="method19"></a>
322 <p/><strong>int LHAup::idProcess() </strong> <br/>
326 <a name="method20"></a>
327 <p/><strong>double LHAup::weight() </strong> <br/>
328 Note that the weight stored in <code>Info::weight()</code> as a rule
329 is not the same as the above <code>weight()</code>: the method here gives
330 the value before unweighting while the one in <code>info</code> gives
331 the one after unweighting and thus normally is 1 or -1. Only with strategy
332 options +-3 and +-4 would the value in <code>info</code> be the same as
333 here, except for a conversion from pb to mb for +-4.
336 <a name="method21"></a>
337 <p/><strong>double LHAup::scale() </strong> <br/>
339 <strong>double LHAup::alphaQED() </strong> <br/>
341 <strong>double LHAup::alphaQCD() </strong> <br/>
342 scale and couplings at that scale.
345 <a name="method22"></a>
346 <p/><strong>int LHAup::sizePart() </strong> <br/>
347 the size of the particle array, which is one larger than the number
348 of particles in the event, since the zeroth entry is kept empty
352 <a name="method23"></a>
353 <p/><strong>int LHAup::id(int i) </strong> <br/>
355 <strong>int LHAup::status(int i) </strong> <br/>
357 <strong>int LHAup::mother1(int i) </strong> <br/>
359 <strong>int LHAup::mother2(int i) </strong> <br/>
361 <strong>int LHAup::col1(int i) </strong> <br/>
363 <strong>int LHAup::col2(int i) </strong> <br/>
365 <strong>double LHAup::px(int i) </strong> <br/>
367 <strong>double LHAup::py(int i) </strong> <br/>
369 <strong>double LHAup::pz(int i) </strong> <br/>
371 <strong>double LHAup::e(int i) </strong> <br/>
373 <strong>double LHAup::m(int i) </strong> <br/>
375 <strong>double LHAup::tau(int i) </strong> <br/>
377 <strong>double LHAup::spin(int i) </strong> <br/>
378 for particle <code>i</code> in the range
379 <code>0 <= i < sizePart()</code>. (But again note that
380 <code>i = 0</code> is an empty line, so the true range begins at 1.)
384 In the LHEF description [<a href="Bibliography.php" target="page">Alw06</a>] an extension to
385 include information on the parton densities of the colliding partons
386 is suggested. This optional further information can be set by
387 <a name="method24"></a>
388 <p/><strong>void LHAup::setPdf( int id1, int id2, double x1, double x2, double scalePDF, double xpdf1, double xpdf2) </strong> <br/>
389 which gives the flavours , the <i>x</i> and the <ie>Q</i> scale
390 (in GeV) at which the parton densities <i>x*f_i(x, Q)</i> have been
395 This information is returned by the methods
396 <a name="method25"></a>
397 <p/><strong>bool LHAup::pdfIsSet() </strong> <br/>
399 <strong>int LHAup::id1() </strong> <br/>
401 <strong>int LHAup::id2() </strong> <br/>
403 <strong>double LHAup::x1() </strong> <br/>
405 <strong>double LHAup::x2() </strong> <br/>
407 <strong>double LHAup::scalePDF() </strong> <br/>
409 <strong>double LHAup::xpdf1() </strong> <br/>
411 <strong>double LHAup::xpdf2() </strong> <br/>
412 where the first one tells whether this optional information has been set
413 for the current event. (<code>setPdf(...)</code> must be called after the
414 <code>setProcess(...)</code> call of the event for this to work.)
418 <a name="method26"></a>
419 <p/><strong>void LHAup::listEvent(ostream& os = cout) </strong> <br/>
420 prints the above information for the current event. In cases where the
421 <code>LHAup</code> object is not available to the user, the
422 <code>Pythia::LHAeventList(ostream& os = cout)</code> method can
423 be used, which is a wrapper for the above.
426 <a name="method27"></a>
427 <p/><strong>virtual bool LHAup::skipEvent(int nSkip) </strong> <br/>
428 skip ahead <code>nSkip</code> events in the Les Houches generation
429 sequence, without doing anything further with them. Mainly
430 intended for debug purposes, e.g. when an event at a known
431 location in a Les Houches Event File is causing problems.
432 Will return false if operation fails, specifically if the
433 end of an LHEF has been reached. The implementation in the base class
434 simply executes <code>setEvent()</code> the requested number of times.
435 The derived <code>LHAupLHEF</code> class (see below) only uses the
436 <code>setNewEventLHEF(...)</code> part of its <code>setEvent()</code>
437 method, and other derived classes could choose other shortcuts.
441 The LHA expects the decay of resonances to be included as part of the
442 hard process, i.e. if unstable particles are produced in a process then
443 their decays are also described. This includes <i>Z^0, W^+-, H^0</i>
444 and other short-lived particles in models beyond the Standard Model.
445 Should this not be the case then PYTHIA will perform the decays of all
446 resonances it knows how to do, in the same way as for internal processes.
447 Note that you will be on slippery ground if you then restrict the decay of
448 these resonances to specific allowed channels since, if this is not what
449 was intended, you will obtain the wrong cross section and potentially the
450 wrong mix of different event types. (Since the original intention is
451 unknown, the cross section will not be corrected for the fraction of
452 open channels, i.e. the procedure used for internal processes is not
453 applied in this case.)
455 <h3>An interface to Les Houches Event Files</h3>
457 The LHEF standard [<a href="Bibliography.php" target="page">Alw06</a>] specifies a format where a single file
458 packs initialization and event information. This has become the most
459 frequently used procedure to process external parton-level events in
460 Pythia. Therefore a special
461 <code><?php $filepath = $_GET["filepath"];
462 echo "<a href='ProgramFlow.php?filepath=".$filepath."' target='page'>";?>Pythia::init(fileName)</a></code>
463 initialization option exists, where the LHEF name is provided as input.
464 Internally this name is then used to create an instance of the derived
465 class <code>LHAupLHEF</code>, which can do the job of reading an LHEF.
468 An example how to generate events from an LHEF is found in
469 <code>main12.cc</code>. Note the use of
470 <code>Info::atEndOfFile()</code> to find out when the whole
471 LHEF has been processed.
474 To allow the sequential use of several event files the
475 <code>Pythia::init(...)</code> method has an optional second argument:
476 <code>Pythia::init(fileName, bool skipInit = false)</code>.
477 If called with this argument <code>true</code> then there will be no
478 initialization, except that the existing <code>LHAupLHEF</code> class
479 instance will be deleted and replaced by ones pointing to the new file.
480 It is assumed (but never checked) that the initialization information is
481 identical, and that the new file simply contains further events of
482 exactly the same kind as the previous one. An example of this possibility,
483 and the option to mix with internal processes, is found in
484 <code>main13.cc</code>.
487 The workhorses of the <code>LHAupLHEF</code> class are three methods
488 found in the base class, so as to allow them to be reused in other
489 contexts. Specifically, it allows derived classes where one parton-level
490 configuration can be reused several times, e.g. in the context of
491 matrix-element-to-parton-shower matching (example in preparation).
492 To begin with also a small utility routine.
494 <a name="method28"></a>
495 <p/><strong>bool LHAup::fileFound() </strong> <br/>
496 always returns true in the base class, but in <code>LHAupLHEF</code>
497 it returns false if the LHEF provided in the constructor is not
498 found and opened correctly.
501 <a name="method29"></a>
502 <p/><strong>bool LHAup::setInitLHEF(ifstream& is) </strong> <br/>
503 read in and set all required initialization information from the
504 specified stream. Return false if it fails.
507 <a name="method30"></a>
508 <p/><strong>bool LHAup::setNewEventLHEF(ifstream& is) </strong> <br/>
509 read in event information from the specified stream into a staging area
510 where it can be reused by <code>setOldEventLHEF</code>.
513 <a name="method31"></a>
514 <p/><strong>bool LHAup::setOldEventLHEF() </strong> <br/>
515 store the event information from the staging area into the normal
516 location. Thus a single <code>setNewEventLHEF</code> call can be
517 followed by several <code>setOldEventLHEF</code> ones, so as to
518 process the same configuration several times. This method currently
519 only returns true, i.e. any errors should be caught by the preceding
520 <code>setNewEventLHEF</code> call.
524 <h3>A runtime Fortran interface</h3>
526 The runtime Fortran interface requires linking to an external Fortran
527 code. In order to avoid problems with unresolved external references
528 when this interface is not used, the code has been put in a separate
529 <code>LHAFortran.h</code> file, that is not included in any of the
530 other library files. Instead it should be included in the
531 user-supplied main program, together with the implementation of two
532 methods below that call the Fortran program to do its part of the job.
535 The <code>LHAupFortran</code> class derives from <code>LHAup</code>.
536 It reads initialization and event information from the LHA standard
537 Fortran commonblocks, assuming these commonblocks behave like two
538 <code>extern "C" struct</code> named <code>heprup_</code> and
539 <code>hepeup_</code>. (Note the final underscore, to match how the
540 gcc compiler internally names Fortran files.)
543 The instantiation does not require any arguments.
546 The user has to supply implementations of the <code>fillHepRup()</code>
547 and <code>fillHepEup()</code> methods, that is to do the actual calling
548 of the external Fortran routines that fill the <code>HEPRUP</code> and
549 <code>HEPEUP</code> commonblocks. The translation of this information to
550 the C++ structure is provided by the existing <code>setInit()</code> and
551 <code>setEvent()</code> code.
554 Up to and including version 8.125 the <code>LHAupFortran</code> class
555 was used to construct a runtime interface to PYTHIA 6.4. This was
556 convenient in the early days of PYTHIA 8 evolution, when this program
557 did not yet contain hard-process generation, and the LHEF standard
558 did not yet exist. Nowadays it is more of a bother, since a full
559 cross-platform support leads to many possible combinations. Therefore
560 the support has been reduced in the current version. Only the
561 <code>main51.cc</code> example remains as an illustration, where the
562 previously separate interface code
563 (<code>include/Pythia6Interface.h</code>) has been inserted in the
564 beginning. You also need to modify the <code>examples/Makefile</code>
565 to link <code>main51.cc</code> properly also to a PYTHIA 6.4 library
566 version, see commented-out section for ideas how to to this.
568 <h3>Methods for LHEF output</h3>
570 The main objective of the <code>LHAup</code> class is to feed information
571 from an external program into PYTHIA. It can be used to export information
572 as well, however. Specifically, there are four routines in the base class
573 that can be called to write a Les Houches Event File. These should be
574 called in sequence in order to build up the proper file structure.
576 <a name="method32"></a>
577 <p/><strong>bool LHAup::openLHEF(string filename) </strong> <br/>
578 Opens a file with the filename indicated, and writes a header plus a brief
579 comment with date and time information.
582 <a name="method33"></a>
583 <p/><strong>bool LHAup::initLHEF() </strong> <br/>
584 Writes initialization information to the file above. Such information should
585 already have been set with the methods described in the "Initialization"
589 <a name="method34"></a>
590 <p/><strong>bool LHAup::eventLHEF() </strong> <br/>
591 Writes event information to the file above. Such information should
592 already have been set with the methods described in the "Event input"
593 section above. This call should be repeated once for each event to be
597 <a name="method35"></a>
598 <p/><strong>bool LHAup::closeLHEF(bool updateInit = false) </strong> <br/>
599 Writes the closing tag and closes the file. Optionally, if
600 <code>updateInit = true</code>, this routine will reopen the file from
601 the beginning, rewrite the same header as <code>openLHEF()</code> did,
602 and then call <code>initLHEF()</code> again to overwrite the old
603 information. This is especially geared towards programs, such as PYTHIA
604 itself, where the cross section information is not available at the
605 beginning of the run, but only is obtained by Monte Carlo integration
606 in parallel with the event generation itself. Then the
607 <code>setXSec( i, xSec)</code>, <code>setXErr( i, xSec)</code> and
608 <code>setXMax( i, xSec)</code> can be used to update the relevant
609 information before <code>closeLHEF</code> is called.
610 <br/><b>Warning:</b> overwriting the beginning of a file without
611 upsetting anything is a delicate operation. It only works when the new
612 lines require exactly as much space as the old ones did. Thus, if you add
613 another process in between, the file will be corrupted.
616 <h3>PYTHIA 8 output to an LHEF</h3>
618 The above methods could be used by any program to write an LHEF.
619 For PYTHIA 8 to do this, a derived class already exists,
620 <code>LHAupFromPYTHIA8</code>. In order for it to do its job,
621 it must gain access to the information produced by PYTHIA,
622 specifically the <code>process</code> event record and the
623 generic information stored in <code>info</code>. Therefore, if you
624 are working with an instance <code>pythia</code> of the
625 <code>Pythia</code> class, you have to instantiate
626 <code>LHAupFromPYTHIA8</code> with pointers to the
627 <code>process</code> and <code>info</code> objects of
629 <br/><code>LHAupFromPYTHIA8 myLHA(&pythia.process, &pythia.info);</code>
632 The method <code>setInit()</code> should be called to store the
633 <code>pythia</code> initialization information in the LHA object,
634 and <code>setEvent()</code> to store event information.
635 Furthermore, <code>updateSigma()</code> can be used at the end
636 of the run to update cross-section information, cf.
637 <code>closeLHEF(true)</code> above. An example how the
638 generation, translation and writing methods should be ordered is
639 found in <code>main20.cc</code>.
642 Currently there are some limitations, that could be overcome if
643 necessary. Firstly, you may mix many processes in the same run,
644 but the cross-section information stored in <code>info</code> only
645 refers to the sum of them all, and therefore they are all classified
646 as a common process 9999. Secondly, you should generate your events
647 in the CM frame of the collision, since this is the assumed frame of
648 stored Les Houches events, and no boosts have been implemented
649 for the case that <code>Pythia::process</code> is not in this frame.
654 <!-- Copyright (C) 2010 Torbjorn Sjostrand -->