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1
2 ------------------------------------------------------------
3
4 Example of the external decayer implementation with Pythia8
5 ------------------------------------------------------------
6
7 This example demonstrates how to outfit Pythia8-based decay features
8 to those resonances in Geant4 where decay tables are not implemented
9 by default. In addition, it showns how to replace existing
10 Geant4 decay tables to such resonances as tau+/- or B+/- with
11 the Pythia8-based ones.
12
13 This example is activated by setting up PYTHIA8 environment variable
14 to point to the area where Pythia8 is installed.
15
16 The complete Pythia8 information, including on download, and documentation
17 is available from the following site:
18 https://pythia.org
19
20 The original version of this example has been implemented by Julia Yarba
21 (FNAL, USA)
22
23 For the complete list of the classes that compose this example please
24 see later in this document.
25
26 Location of example:
27
28 examples/extended/eventgenerator/pythia/py8decayer
29
30
31 Installation of Pythia8:
32
33 NOTE: As of December 2023, pythia8.3.10 is the most current version,
34 thus it is used in this example.
35 In the future, please check updates at Pythia8 site: https://pythia.org
36
37 1. cd path/to/your/pythia8/area
38
39 2. Download desired version of Pythia8 and un-tar it, e.g.
40 wget https://pythia.org/download/pythia83/pythia8310.tgz
41 tar xzf pythia8310.tgz
42
43 3. Build/install Pythia8
44 cd pythia8310
45 export CXX=\`which g++\`
46 ./configure --prefix=$PWD --cxx=$CXX
47 make
48 NOTE: By default, Pythia8 (as of 8.3.10) builds with C++11 standards.
49 If one wants to turn to e.g. C++17 standard, one needs to override flags
50 via --cxx-common argument to configure script.
51 Alternatively, one can setup CXX_COMMON environment variable.
52 As of pythia8310, the default flags are the following:
53 -O2 -std=c++11 -pedantic -W -Wall -Wshadow -pthread
54 Please note use of -pthread which was not among default flags in earlier
55 relesaes of Pythia8.
56 Example of specifying C++17 standards by overriding the default flags via
57 use of --cxx_common argument to configure script:
58 ./configure --prefix=$PWD --cxx=$CXX \
59 --cxx-common='-O2 -std=c++17 -pedantic -W -Wall -Wshadow -fPIC -pthread'
60
61 4. Setup Pythia8_ROOT environment variable to point to the area where Pythia8
62 is built/installed:
63 export Pythia8_ROOT=$PWD
64
65
66 Building example:
67
68 Upon setup of PYTHIA8 environment variable to point to the area where
69 Pythia8 package is installed, the pythia/py8decayer example will be
70 compiled together with several other features of the eventgenerator example.
71
72
73 Description of classes:
74
75 Py8Decayer class provides implementation of the G4VExternalDecayer interface
76 with the use of PYTHIA8.
77 It is reasonably annotated, and demonstrates what features of Pythia8 need
78 to be activated and/or disactivated in order to make Pythia8 work only in
79 the decay mode.
80 It also illustrated how to control several other features of Pythia8, including
81 some reduction of Pythia8 verbosity (by default, Pythia8 produces quite a large
82 amount of printouts, thus reducing it could be useful in some cases).
83 It also shown how to deactivate decays of pi0's by Pythia8 as the idea is to handle
84 pi0's back to Geant4 for decays.
85 Last but not least, it shows how to outfit Pythia8 with a custom random engine,
86 i.e. Py8DecayerEngine.
87
88 Py8DecayerEngine class inherits from Pythia8::RndmEngine which in its turns is
89 provided by the Pythia8 package in order to allow replacing the generator's native
90 random engine with the one of user's choice.
91 In this specific case Py8DecayerEngine allows to replace Pythia8 native random engine
92 with the same engine that Geant4 uses (i.e. CLHEP::RanecuEngine as in this application).
93 The feature is important for running the example in the MT/Tasking mode.
94
95 Py8DecayerPhysics class implements a G4VPhysicsConstructor type of component
96 with the use of Py8Decayer; this component can later be used with a ddsired
97 physics list (see main program).
98 Specifically, in the Py8DecayerPhysics::ConstructProcess() the Py8Decayer is
99 instantiated and is used to
100 a) replace existing decay tables of such resonances as tau+/- and B+/-
101 b) supplement decay features to those resonances in Geant4 where the decay
102 tables are not implemnted by defaukt
103
104 In principle, classes Py8Decayer, Py8DecayerEngine, and Py8DecayerPhysics can be
105 directly reused with another user application.
106 Alternatively, they can be used as an inspiration to implement similar, or perhaps
107 even more extensive Pythia8-based functionalities of user's choice.
108
109 Class DetConstruction demostrates how to implement minimalistic detector geometry.
110
111 Class SingleParticleGun demonstrates how to implement generaton of the primary
112 particle.
113
114 Class ActionInitialization instantiates and registers to Geant4 kernel all user
115 action classes; in this case it is SingleParticleGun.
116
117 Main program:
118
119 pythia8_decayer.cc
120
121 This application currently uses the default RunManager which is Tasking and is
122 initialized with 5 threads.
123 It will then run 5 events, one per thread.
124 SerialOnly RunManager can also be employed should the user choose so.
125
126
127 Executable:
128
129 pythia8_decayer
130
131
132 Execution:
133
134 At present, the pythia8_decayer executable does not take any input arguments.
135 Everything, including the choine of primary particle, is hardcoded.
136 Although in the future some configurability may be added.
137
138 As mentioned earlier, it will run 5 single tau events using Pythia8 to decays them.
139
140 It should print some Pythia8 event information, including on decays.
141 Once again, please bear in mind that the decay of pi0's by Pythia8 is disabled
142 (see Py8Decayer constructor) since the idea is to hand the pi0's back to Geant4
143 and make Geant4 decay them.
144
145
146 Additional notes on the contents of Geant4 and Pythia8 Particle Data Tables (PDT) :
147
148 In their default form, PDT's in Geant4 and Pythia8 have a number of differences
149 that need to be kept in mind.
150
151 In the case of the Geant4 py8decayer example those differences are unlikely
152 to cause any major issues.
153
154 But Pythia8 can, in principle, be used within Geant4 in more ways that just
155 as an external decayer.
156 Thus, if one is potentially interested in more sophisticated use of Pythia8
157 in Geant4, one may want to consider whatever differences exist between (default)
158 Geant4 and Pythia8 PDT's.
159
160 Both Geant4 and Pythia8 codes are evolving, and specific numbers may be different
161 in earlier and/or in future releases.
162
163 To be more precise, by default Pythia8.3.10 PDT contains 677 entries, of which
164 531 particles have an antiparticle (it looks like antiparticles do not make
165 separate entries in Pythia8 PDT, but the total number of available species
166 should be considered as 1208).
167
168 Geant4 PDT contains 508 entries.
169
170 Of those, 239 particles/antiparticles match by Particle ID's (on the Geant4 side
171 it is explicitly called "PDG encoding" while on Pythia8 side it is just "id").
172
173 Many of Pythia8 PDT's entries are not available in Geant4 PDT, e.g. Z or W bosons
174 are not in Geant4.
175
176 Some of the species in the Geant4 PDT do not seem to be in the Pythia8 PDT
177 (e.g. excited nucleons do not seem to be in the Pythia8 PDT).
178
179 Also, there are entries in both PDT's that mean the same particles but are
180 marked with different ID's.
181 For example, excited Delta(s) are present in both PDT's but in Geant4 each one
182 is marked with a 4-digit number as an ID (PDG encoding) while in Pythia8 an ID
183 for such particle would be a 6-digit number starting with "20" and the last
184 4 digits would be the same as the Geant4 ID for such particle.
185
186 Speaking of the particles that match by ID (PDG ID), there may be further differences,
187 e.g. by mass, either central value or width, or both (there might be other aspects but
188 they have not been checked for).
189
190 Starting October 2022, checks have been made from time to time for differences
191 larger that 1 keV in either mass central value or width.
192
193 The largest differences have been observed for quarks/diquarks.
194 It appears that Geant4 sets (at least) masses of quarks as listed in PDG.
195 For details on default settings for the quark masses in Pythia8 please refer
196 to the Pythia8 manual:
197 https://pythia.org/manuals/pythia8310/Welcome.html
198 See Particles and Decays section, Particle Data subsection.
199
200 Beyond quarks/diquarks some differences in mass central values or width have also
201 been observed, mainly for resonances.
202 But even for such particles as proton or muon there may be differences on the order
203 of a few keV (e.g. central value of the proton mass is 938.27 MeV in Pythia8 and
204 938.272 MeV in Geant4)
