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30<h2>Program Files</h2>
31
32The code is subdivided into a set of files, mainly by physics
33task. Each file typically contains one main class, but often
34with a few related helper classes that are not used elsewhere in
35the program. Normally the files come in pairs.
36<ul>
37<li>A header file, <code>.h</code> in the <code>include</code>
38subdirectory, where the public interface of the class is declared,
39and inline methods are defined.</li>
40<li>A source code file, <code>.cc</code> in the <code>src</code>
41subdirectory, where most of the methods are implemented.</li>
42</ul>
43During compilation, related dependency files, <code>.d</code>, and
44compiled code, <code>.o</code> are created in the <code>tmp</code>
45subdirectory.
46
47<p/>
48In part the <code>.xml</code> documentation files in the
49<code>xmldoc</code> subdirectory have matching names, but the match
50is broken by the desire to group topics more by user interaction than
51internal operation. On these pages the function of the different code
52files is summarized. Currently, each <code>.xml</code> file is also
53translated into an <code>.html</code> one in the
54<code>htmldoc</code> subdirectory, to allow easy viewing of the
55contents in a web browser, and an <code>.php</code> one in
56<code>phpdoc</code>, for more sophisticated interactivity
57if that subdirectory is installed on a web server.
58
59<p/>
60Here is the current list of files, ordered alphabetically, with a brief
61description of contents.
62
63<p/><code>file&nbsp; </code><strong> Analysis &nbsp;</strong> <br/>
64contains routines to analyze events. Currently it can do sphericity,
65thrust, Lund/Jade/Durham jet clustering, cone-jet finding, and the
66<i>kT</i>, anti-<i>kT</i> and Cambridge/Aachen algorithms for
67hadron colliders.
68 
69
70<p/><code>file&nbsp; </code><strong> Basics &nbsp;</strong> <br/>
71contains some basic facilities of general use: a random number
72generator <code>Rndm</code>, a four-vector class <code>Vec4</code>, and a
73histogram class <code>Hist</code>. 
74 
75
76<p/><code>file&nbsp; </code><strong> BeamParticle &nbsp;</strong> <br/>
77contains information on all partons extracted from one of the two
78beams. Defines modified parton distributions accordingly during the
79showering and multiparton interactions processing, thereby extending on
80the one-particle-inclusive distributions defined by the previous class.
81Finds the internal structure for a beam remnant.
82 
83
84<p/><code>file&nbsp; </code><strong> BeamRemnants &nbsp;</strong> <br/>
85adds primordial <i>kT</i> to the set of hard subsystems,
86and combines these subsystems with the two beam remnants to provide
87the overall energy-momentum picture. Also ties together all the
88colour lines into consistent singlets.
89 
90
91<p/><code>file&nbsp; </code><strong> BeamShape &nbsp;</strong> <br/>
92generates momentum spread of beams, and spread of collision vertex
93position.
94 
95
96<p/><code>file&nbsp; </code><strong> BoseEinstein &nbsp;</strong> <br/>
97provides a simple method to impose Bose-Einstein correlations on
98pairs of identical mesons.
99 
100
101<p/><code>file&nbsp; </code><strong> Event &nbsp;</strong> <br/>
102contains the event record, which basically is a vector of particles.
103This file also contains the <code>Particle</code> class, used by
104<code>Event</code>. <code>Pythia</code> uses two <code>Event</code>
105objects, one for the process-level record (<code>process</code>) and
106one for the complete event (<code>event</code>).
107 
108
109<p/><code>file&nbsp; </code><strong> FragmentationFlavZpT &nbsp;</strong> <br/>
110contains the classes for describing the fragmentation steps in
111flavour and in longitudinal and transverse momentum.
112 
113
114<p/><code>file&nbsp; </code><strong> FragmentationSystems &nbsp;</strong> <br/>
115defines some containers of parton systems, for use in
116the fragmentation routines.
117 
118
119<p/><code>file&nbsp; </code><strong> HadronLevel &nbsp;</strong> <br/>
120turns the parton-level event above into a set of outgoing particles,
121by applying string fragmentation (with special treatment for low-mass
122systems) and secondary decays, and optionally Bose-Einstein corrections.
123 
124
125<p/><code>file&nbsp; </code><strong> HelicityBasics &nbsp;</strong> <br/>
126basic classes for the handling of helicities in tau lepton decays.
127 
128
129<p/><code>file&nbsp; </code><strong> HelicityMatrixElements &nbsp;</strong> <br/>
130helicity-dependent decay matrix elements for the tau lepton.
131 
132
133<p/><code>file&nbsp; </code><strong> HepMCInterface &nbsp;</strong> <br/>
134contains an interface to convert the PYTHIA 8 event record into the
135HepMC format. The <code>HepMCInterface.cc</code> file is located in
136the subdirectory <code>hepmcinterface</code> and is used to build a
137separate <code>libhepmcinterface</code> library.
138 
139
140<p/><code>file&nbsp; </code><strong> HiddenValleyFragmentation &nbsp;</strong> <br/>
141hadronization in models with a hidden sector that contains an unbroken
142<i>SU(N)</i>, which gives confinement. The model and code is largely
143derived from the normal fragmentation classes.
144 
145
146<p/><code>file&nbsp; </code><strong> History &nbsp;</strong> <br/>
147methods to reconstruct the imagined shower history of a
148matrix-element-generated multiparton configuration, as part of
149the CKKW-L matrix element merging.
150 
151
152<p/><code>file&nbsp; </code><strong> Info &nbsp;</strong> <br/>
153is a simple container that gives access to some information on the
154nature of the current process, such as Mandelstam variables.
155Also contains a small database for errors and warnings encountered
156during program execution.
157 
158
159<p/><code>file&nbsp; </code><strong> LesHouches &nbsp;</strong> <br/>
160gives the possibility to feed in parton configurations for the
161subsequent event generation. One base class is defined, with containers
162for initialization and event information, that can be read from
163<code>Pythia</code>. Derived classes allow for a few different cases.
164 
165
166<p/><code>file&nbsp; </code><strong> LHAFortran &nbsp;</strong> <br/>
167is a header file only, for a class derived from the above LesHouches
168one, to be used for runtime interfacing to Fortran programs, such as
169PYTHIA 6.
170 
171
172<p/><code>file&nbsp; </code><strong> LHAPDFInterface &nbsp;</strong> <br/>
173is a header file only, with interfaces to the key LHAPDF routines,
174as needed for a runtime interface. There is a file
175<code>lhapdfdummy/LHAPDFdummy.cc</code> with matching dummy
176implementations, however. This file is used to build a separate
177<code>liblhapdfdummy</code> library, to be linked when the LHAPDF
178library is not used, so as to avoid problems with undefined references.
179 
180
181<p/><code>file&nbsp; </code><strong> MergingHooks &nbsp;</strong> <br/>
182intercede in the normal shower evolution to construct the relevant
183Sudakov form factor suppressions as part of the CKKW-L matrix element
184merging.
185 
186
187<p/><code>file&nbsp; </code><strong> MiniStringFragmentation &nbsp;</strong> <br/>
188performs string fragmentation in cases where the colour singlet
189subsystem mass is so small that one or at most two primary hadrons
190should be produced from it.
191 
192
193<p/><code>file&nbsp; </code><strong> MultipartonInteractions &nbsp;</strong> <br/>
194performs multiparton interactions.
195 
196
197<p/><code>file&nbsp; </code><strong> ParticleData &nbsp;</strong> <br/>
198contains a database of all necessary particle data (masses, names, ..)
199and decay channels.
200 
201
202<p/><code>file&nbsp; </code><strong> ParticleDecays &nbsp;</strong> <br/>
203performs the decays of all normal unstable hadrons and leptons, i.e.
204in mass up to and including <i>b bbar</i> systems. It is not
205intended for decays of electroweak resonances, like <i>Z^0</i>. 
206 
207
208<p/><code>file&nbsp; </code><strong> PartonDistributions &nbsp;</strong> <br/>
209contains parton distribution functions for the proton and electron.
210Currently very simple, with only two <i>p</i> parametrizations
211and one <i>e</i> ditto available, but it is possible to link in
212external sets.
213 
214
215<p/><code>file&nbsp; </code><strong> PartonLevel &nbsp;</strong> <br/>
216turns the (hard) process above into a complete set of partons, by
217adding initial- and final-state radiation, multiparton
218interactions, and beam remnants.
219 
220
221<p/><code>file&nbsp; </code><strong> PartonSystems &nbsp;</strong> <br/>
222keeps track of which partons belong to which partonic subsystem,
223i.e. one of the multiparton (semi)hard interactions with associated
224showers.
225 
226
227<p/><code>file&nbsp; </code><strong> PhaseSpace &nbsp;</strong> <br/>
228selects a point in phase space for the hard-process generation,
229optimized separately for each process to give improved Monte Carlo
230efficiency.
231 
232
233<p/><code>file&nbsp; </code><strong> ProcessContainer &nbsp;</strong> <br/>
234packages the information on a given subprocess, combining the
235phase-space selection and cross-section evaluation machineries
236with some statistics information. Also sets up the list of processes
237to be studied in a run. 
238 
239
240<p/><code>file&nbsp; </code><strong> ProcessLevel &nbsp;</strong> <br/>
241handles the generation of the (hard) process that sets the character
242of the event. This involves either using internally implemented
243processes or linking to Les Houches information. The latter can
244be by runtime interfaces or by reading in a file. This step also
245includes resonance decays.
246 
247
248<p/><code>file&nbsp; </code><strong> Pythia &nbsp;</strong> <br/>
249is the main class, that administrates the whole event generation
250process by making use of all the others classes. Objects of most
251other classes reside (directly or indirectly) inside <code>Pythia</code>,
252so only a <code>Pythia</code> object needs to be explicitly instantiated
253and addressed   by the user.
254 
255
256<p/><code>file&nbsp; </code><strong> PythiaComplex &nbsp;</strong> <br/>
257is only a <code>.h</code> file, containing a <code>typedef</code> for
258double precision complex numbers.
259 
260
261<p/><code>file&nbsp; </code><strong> PythiaStdlib &nbsp;</strong> <br/>
262contains most of the <code>Stdlib</code> headers used in PYTHIA 8,
263with <code>using</code> directives. It defines <code>M_PI</code> if
264this is not already done. Also a few simple inline methods:
265<code>pow2(x)</code>, <code>pow3(x)</code>, <code>pow4(x)</code>,
266<code>pow5(x)</code> and <code>pow6(x)</code> for small integer
267powers, and <code>sqrtpos(x)</code> where a <code>max(0., x)</code>
268ensures that one does not take the square root of a negative number.
269Also non-inlined <code>GammaReal(x)</code> for the <i>Gamma</i>
270function value of a real argument.
271 
272
273<p/><code>file&nbsp; </code><strong> ResonanceDecays &nbsp;</strong> <br/>
274decays resonances as part of the hard-process stage, in many cases
275(but not all) including angular correlations between the decay products.
276 
277
278<p/><code>file&nbsp; </code><strong> ResonanceWidths &nbsp;</strong> <br/>
279encodes some properties of resonances, in particular the dynamic
280calculation of widths.
281 
282
283<p/><code>file&nbsp; </code><strong> RHadrons &nbsp;</strong> <br/>
284handles the production and decay of hadrons formed by long-lived
285gluinos, stops or sbottoms.
286 
287
288<p/><code>file&nbsp; </code><strong> Settings &nbsp;</strong> <br/>
289contains a database of all flags, modes, parameters and words that
290determine the performance of the generator. Initial values are obtained
291from the contents of the <code>.xml</code> files, but these values can
292then be changed by the user.
293 
294
295<p/><code>file&nbsp; </code><strong> SigmaCompositeness &nbsp;</strong> <br/>
296contains the cross sections and matrix elements for production of
297some particles in compositeness scenarios, specifically excited
298fermions.
299 
300
301<p/><code>file&nbsp; </code><strong> SigmaEW &nbsp;</strong> <br/>
302contains the cross sections and matrix elements for electroweak
303processes involving photons, <i>Z^0</i>'s and <i>W^+-</i>'s.
304 
305
306<p/><code>file&nbsp; </code><strong> SigmaExtraDim &nbsp;</strong> <br/>
307contains the cross sections and matrix elements for processes in
308scenarios involving extra dimensions.
309 
310
311<p/><code>file&nbsp; </code><strong> SigmaGeneric &nbsp;</strong> <br/>
312contains the cross sections and matrix elements for some generic
313processes, to be used as building blocks for a few BSM scenarios.
314 
315
316<p/><code>file&nbsp; </code><strong> SigmaHiggs &nbsp;</strong> <br/>
317contains the cross sections and matrix elements for Higgs production.
318 
319
320<p/><code>file&nbsp; </code><strong> SigmaLeftRightSym &nbsp;</strong> <br/>
321contains the cross sections and matrix elements for particle production
322in left-right-symmetry scenarios, specifically righthanded <i>Z</i>
323and <i>W</i> bosons and doubly-charged Higgs bosons.
324 
325
326<p/><code>file&nbsp; </code><strong> SigmaLeptoquark &nbsp;</strong> <br/>
327contains the cross sections and matrix elements for leptoquark production.
328 
329
330<p/><code>file&nbsp; </code><strong> SigmaNewGaugeBosons &nbsp;</strong> <br/>
331contains the cross sections and matrix elements for a <i>Z'^0</i>,
332a <i>W^+-</i> and a horizontal gauge boson <i>R^0</i>.
333 
334
335<p/><code>file&nbsp; </code><strong> SigmaOnia &nbsp;</strong> <br/>
336contains the cross sections and matrix elements for charmonium and
337bottomonium production.
338 
339
340<p/><code>file&nbsp; </code><strong> SigmaProcess &nbsp;</strong> <br/>
341contains the base class and derived classes for the evaluation of
342different matrix elements. Also keeps track of allowed incoming
343parton configurations and their cross sections, including parton
344densities. In order to keep this file from becoming too big, actual
345cross sections are found in several separate files of derived classes:
346<code>SigmaQCD</code>, <code>SigmaEW</code>, <code>SigmaOnia</code>,
347<code>SigmaHiggs</code>, <code>SigmaSUSY</code>, 
348<code>SigmaNewGaugeBosons</code>, <code>SigmaLeftRightSym</code>,
349<code>SigmaLeptoquark</code>, <code>SigmaCompositeness</code>,
350<code>SigmaExtraDim</code> and <code>SigmaGeneric</code>.
351 
352
353<p/><code>file&nbsp; </code><strong> SigmaQCD &nbsp;</strong> <br/>
354contains the cross sections and matrix elements for soft and hard
355QCD processes.
356 
357
358<p/><code>file&nbsp; </code><strong> SigmaSUSY &nbsp;</strong> <br/>
359contains the cross sections and matrix elements for Supersymmetric
360processes.
361 
362
363<p/><code>file&nbsp; </code><strong> SigmaTotal &nbsp;</strong> <br/>
364contains parametrizations of total, elastic and diffractive hadronic
365cross sections.
366 
367
368<p/><code>file&nbsp; </code><strong> SpaceShower &nbsp;</strong> <br/>
369performs spacelike initial-state transverse-momentum-ordered
370shower evolution.
371 
372
373<p/><code>file&nbsp; </code><strong> StandardModel &nbsp;</strong> <br/>
374contains the running <i>alpha_strong</i>, with <i>Lambda</i>
375matching at flavour thresholds, the running <i>alpha_em</i>,
376CKM mixing matrices, and a few other parameters such as
377<i>sin^2(theta_W)</i>.
378 
379
380<p/><code>file&nbsp; </code><strong> StringFragmentation &nbsp;</strong> <br/>
381performs string fragmentation of a given set of partons.
382 
383
384<p/><code>file&nbsp; </code><strong> SusyCouplings &nbsp;</strong> <br/>
385stores the various couplings used for SUSY cross sections and
386decays, as calculated from input e.g. based on the SUSY Les Houches
387Accord.
388 
389
390<p/><code>file&nbsp; </code><strong> SusyLesHouches &nbsp;</strong> <br/>
391contains information on SUSY parameters and particle data as specified
392by the SUSY Les Houches Accord.
393 
394
395<p/><code>file&nbsp; </code><strong> TauDecays &nbsp;</strong> <br/>
396the main routines for handling tau lepton decays with helicity
397information.
398 
399
400<p/><code>file&nbsp; </code><strong> TimeShower &nbsp;</strong> <br/>
401performs timelike final-state transverse-momentum-ordered
402shower evolution.
403 
404
405<p/><code>file&nbsp; </code><strong> UserHooks &nbsp;</strong> <br/>
406Provides a way for a user to study the event at a few intermediate
407stages of evolution, to reject the event as a whole or to modify
408its cross-section weight.
409 
410
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