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1 -------------------------------------------------------------------
2
3 =========================================================
4 Geant4 - an Object-Oriented Toolkit for Simulation in HEP
5 =========================================================
6
7 Example Par02
8 -------------
9
10 This example is a simplified version of a Geant4-based fast simulation
11 program written by Anna Zaborowska for Future Circular Collider (FCC)
12 studies.
13
14 This example shows how to do "track and energy smearing" in Geant4,
15 in order to have a very fast simulation based on assumed detector
16 resolutions.
17
18 The geometry which is considered is a simplified collider detector set-up,
19 inspired by ALEPH/ATLAS/CMS detectors. Although it is much simpler than
20 a realistic detector, it is anyhow fairly complex and therefore build up
21 from a GDML file, Par02FullDetector.gdml .
22
23 In this example:
24 - Particles with transverse momentum less than 1 MeV or pseudorapidity
25 larger (in module) than 5.5 are neglected (i.e. the corresponding
26 Geant4 track is killed as soon as it is created).
27 - Any primary charged particle is smeared in the tracker as follows:
28 its momentum is smeared according to a gaussian, with mean equal to 1.0
29 and sigma taken from the momentum resolution of the CMS tracker
30 (with ALEPH or ATLAS tracker as a possible alternative), and then placed
31 at the end of the tracker, at the position that it would reach if
32 normally transported (i.e. without smearing).
33 - Any primary electron, or positron, or gamma is smeared in the
34 electromagnetic calorimeter as follows: it is killed at the entrance
35 of the electromagnetic calorimeter, with a deposited energy equal to
36 the gaussian smearing (with mean equal to 1.0 and sigma taken from the
37 energy resolution of the CMS electromagnetic calorimeter - with ALEPH or
38 ATLAS electromagnetic calorimeter as a possible alternative) of its
39 kinetic energy (at the entrance of the electromagnetic calorimeter).
40 - Any primary hadron is smeared in the hadronic calorimeter as follows:
41 it is killed at the entrance of the hadronic calorimeter, with a
42 deposited energy equal to the gaussian smearing (with mean equal to 1.0
43 and sigma taken from the energy resolution of the CMS hadronic
44 calorimeter - with ALEPH or ATLAS hadronic calorimeter as a possible
45 alternative) of its kinetic energy (at the entrance of the hadronic
46 calorimeter).
47 - The only competing physical processes with respect to the above physics
48 parametrisations are the decays.
49 Note: no electromagnetic processes;
50 no momentum smearing in the tracker for secondary particles;
51 secondary electrons, positrons, gammas in the electromagnetic
52 calorimeter are killed (at the entrance) but without any
53 energy deposition;
54 secondary hadrons in the hadronic calorimeter are killed (at the
55 entrance) but without any energy deposition.
56
57
58 Below some details.
59
60
61 1. Detector description
62 -----------------------
63
64 The geometry is read in from a GDML file, Par02FullDetector.gdml .
65
66 The geometry is a simplified collider detector set-up used for the
67 first FCC studies, inspired by ALEPH/ATLAS/CMS detectors.
68 It is made of 4 main parts:
69 - Tracker
70 - Electromagnetic calorimeter
71 - Hadronic calorimeter
72 - Muon subdetector
73
74 In this example, fast simulation parametrisation models exist for the
75 first three subdetectors, but not for the Muon subdetector.
76 The three parametrisation models:
77 - Par02FastSimModelTracker : in the tracker
78 - Par02FastSimModelEMCal : in the electromagnetic calorimeter
79 - Par02FastSimModelHCal : in the hadronic calorimeter
80 are build and bound to the respective subdetector (i.e. Geant4 regions)
81 in the method: Par02DetectorConstruction::Construct() .
82
83 Three configurations are possible for those parametrisation models:
84 CMS-like, ALEPH-like, ATLAS-like.
85 By default, the CMS configuration is used.
86
87
88 2. Primary generation
89 ---------------------
90
91 In this example we use a very simple primary generation action,
92 Par02PrimaryGeneratorAction, that uses the G4ParticleGun.
93 One single particle type, with a well defined energy, and in one fixed
94 direction is used for each run: the corresponding values can be set via
95 macro commands. See examplePar02.in as an example.
96
97 For the FCC studies, Pythia8 events in HepMC format were used for the
98 generation of the primary particles.
99
100
101 3. Physics List
102 ---------------
103
104 A special, ad-hoc physics list is used in this example, in order to have
105 an ultra-fast parametrised simulation: for all particles, the only two
106 physics processes that are assigned are the decay process and the fast
107 simulation process.
108 The following three fast simulation models are defined:
109
110 - Par02FastSimModelTracker :
111 - bound to the tracker
112 (see the method Par02DetectorConstruction::Construct )
113 - applicable to all charged particles
114 (see the method Par02FastSimModelTracker::IsApplicable )
115 - triggered in all cases (i.e. no kinematic constraints)
116 (see the method Par02FastSimModelTracker::ModelTrigger )
117 - does the following: place the particle at the tracking detector exit
118 (at the place the particle would reach without smearing), and, only
119 if the particle is a primary, it smears the momentum of the particle
120 according to a gaussian, with mean equal to 1.0 and sigma taken from
121 the momentum resolution of the CMS tracker (with ALEPH or ATLAS tracker
122 as a possible alternative)
123 (see the method Par02FastSimModelTracker::DoIt )
124
125 - Par02FastSimModelEMCal :
126 - bound to the electromagnetic calorimeter
127 (see the method Par02DetectorConstruction::Construct )
128 - applicable to electrons, positrons, gammas
129 (see the method Par02FastSimModelEMCal::IsApplicable )
130 - triggered in all cases (i.e. no kinematic constraints)
131 (see the method Par02FastSimModelEMCal::ModelTrigger )
132 - does the following: kill the particle at the entrance of the
133 electromagnetic calorimeter, and, only if the particle is a primary,
134 it deposits in the electromagnetic calorimeter an energy obtained
135 by a gaussian smearing (with mean equal to 1.0 and sigma taken from the
136 energy resolution of the CMS electromagnetic calorimeter - with ALEPH
137 or ATLAS electromagnetic calorimeter as a possible alternative) of the
138 particle kinetic energy (at the entrance of the electromagnetic
139 calorimeter)
140 (see the method Par02FastSimModelEMCal::DoIt )
141
142 - Par02FastSimModelHCal :
143 - bound to the hadronic calorimeter
144 (see the method Par02DetectorConstruction::Construct )
145 - applicable to all hadrons (i.e. particles made of quarks)
146 (see the method Par02FastSimModelHCal::IsApplicable )
147 - triggered in all cases (i.e. no kinematic constraints)
148 (see the method Par02FastSimModelHCal::ModelTrigger )
149 - does the following: kill the particle at the entrance of the
150 hadronic calorimeter, and, only if the particle is a primary,
151 it deposits in the hadronic calorimeter an energy obtained by a
152 gaussian smearing (with mean equal to 1.0 and sigma taken from the
153 energy resolution of the CMS hadronic calorimeter - with ALEPH
154 or ATLAS hadronic calorimeter as a possible alternative) of the
155 particle kinetic energy (at the entrance of the hadronic
156 calorimeter)
157 (see the method Par02FastSimModelHCal::DoIt )
158
159
160 4. User actions, user information and user utility classes
161 ----------------------------------------------------------
162
163 - Par02RunAction : run action used for initialization and termination
164 of the run.
165
166 - Par02EventAction : event action used for initialization and termination
167 of the event.
168
169 - Par02TrackingAction : tracking action used for killing particles with
170 transverse momentum less than 1 MeV or
171 pseudorapidity larger (in module) than 5.5
172 (see method Par02TrackingAction::PreUserTrackingAction )
173 and to store the information about the track at
174 the end of the simulation of such a track
175 (see method Par02TrackingAction::PostUserTrackingAction ).
176
177 - Par02ActionInitialization : initialization of the primary generator class
178 and all user-defined actions (i.e. the three
179 classes above).
180
181 - Par02PrimaryParticleInformation : utility class to store information
182 associated with a primary particle.
183
184 - Par02EventInformation : utility class to store information associated
185 with a Geant4 event.
186
187 - Par02DetectorParametrisation : a simple class used to provide the detector
188 resolution and efficiency, according to the
189 type of detector: tracker, electromagnetic
190 calorimeter, hadronic calorimeter.
191 There are 3 choices: CMS-like (default),
192 ALEPH-like and ATLAS-like.
193 The efficiency is currently set to 1.0 in
194 all cases and not used.
195
196 - Par02Smearer : a simple class that does the gaussian smearing, either
197 of the momentum (in the tracker detector) or in energy
198 (in the electromagnetic or hadronic calorimeter).
199
200
201 5. Output
202 ---------
203
204 The execution of the program (examplePar02) produces in output, at the end
205 of a run, a Root file, by default named DefaultOutput.root, which contains
206 3 histograms and one ntuple.
207
208 The macro file examplePar02.in specifies one run made of 1000 events
209 each consisting of one 50 GeV electron.
210 By editing the file, one could select alternatively a run made of 1000
211 events each consisting of one 100 GeV muon, or a run made of 1000 events
212 each consisting of one 20 GeV pion- .
213
214 See the class Par02Output for the definition of the 3 histograms and the
215 ntuples. Here is a quick summary:
216 - histogram of the ratio of the momentum smeared and the original momentum
217 in the tracker (for primary charged particles);
218 - histogram of the ratio of the smeared energy deposited and the original
219 energy at the entrance in the electromagnetic calorimeter (for primary
220 electrons, positrons and gammas);
221 - histogram of the ratio of the smeared energy deposited and the original
222 energy at the entrance in the hadronic calorimeter (for primary hadrons);
223 - ntuple containing the "Monte-Carlo true" information regarding the
224 primary, and the resolution, efficiency, smeared momentum (tracker),
225 smeared energy (calorimeter) and impact position (calorimeter) of each
226 subdetector (tracker, electromagnetic calorimeter, hadronic calorimeter)
227 where the primary is parametrised (tracker and electromagnetic calorimeter
228 in the case of primary electrons, positrons and gammas; tracker and
229 hadronic calorimeter in the case of primary hadrons; tracker only for
230 all other primary charged particles, e.g. muons).
231
232 Note:
233 - you do not need to have the Root package available to run this example,
234 but you need it if you want to look at the histograms and the ntuple
235 contained in the Root output file;
236 - sensitive detectors and hits are not used in this example.
237
238
239 6. How to build and run the example
240 -----------------------------------
241
242 - You need to have built the Geant4 persistency/gdml module by having set
243 the -DGEANT4_USE_GDML=ON flag during the CMAKE configuration step,
244 as well as the -DXERCESC_ROOT_DIR=<path_to_xercesc> flag pointing to
245 the path where the XercesC XML parser package is installed in your system.
246
247 - Compile and link to generate the executable (in your CMAKE build directory):
248 % make
249
250 - Execute the application:
251 % examplePar02 examplePar02.in
252
253 which produces one Root file: DefaultOutput.root .
254
