1. Entering the production phase
Geant4 in Atlas
Entering the production phase
See also:
A.D. Beijing
A.Rimoldi’s users meeting, SLAC ‘02
C.Leggett’s users meeting, SLAC ‘02
Andrea Dell’Acqua CERN EP/SFT

2. Atlas simulation in a nutshell
The most complicated detector ever conceived
The most politicized collaboration ever gathered in HEP
The most ambitious detector simulation program ever written
At the last count we had ~27M volumes in our G3 simulation
Need to accommodate (under the same umbrella)
Testbeam studies
Physics studies
Fast/semi-fast simulation
A. Dell’Acqua – CERN EP/SFT
25/02/2019

3. A. Dell’Acqua – CERN EP/SFT
The main actors
Athena
Atlas’ chosen software framework
Based on Gaudi
Provides SW environment, services, access to persistency…
GOOFY
Simulation framework
Completely based on G4
G4Svc
Gaudi service: provides the link
A. Dell’Acqua – CERN EP/SFT
25/02/2019

4. Goofy’s Design principles
Scalability
The simulation package must be able to accept
the simplest testbeam simulation
the complete detector simulation
Clear separation between “core” software and user implementation
the “framework”
does not depend on what the users want to do
provides maximum flexibility and freedom to the user who wants to implement his/her own code
must be light
Implement some middleware on top of G4 for extending its functionality
Dynamic loading + plug-in techniques for connecting user’s code
Action on demand to avoid wasting memory
A. Dell’Acqua – CERN EP/SFT
25/02/2019

5. The simulation program
has no geometry
Users build it interactively at initialization time
has no physics
Users choose from a catalog, depending on what they want to do
..or implement their own, and add it to the catalog
has no kinematics
See above…
is an empty box
A. Dell’Acqua – CERN EP/SFT
25/02/2019

6. A. Dell’Acqua – CERN EP/SFT
Steps
The “standard” G4 framework is too static for our purpose: we try and get rid of it by maintaining full compatibility with G4
no UserDetectorConstruction, no UserPhysicsList…
Provide a set of services that users can refer to
MaterialManager
DetectorFacility
PhysicsListCatalog
VisCenter…
Use the Geant4 (G)UI facilities for re-creating all connections
Make sure that users can build their application (factorization)
Save memory, as much as one can, use proxies and lightweight objects (memory has a price, after all).
A. Dell’Acqua – CERN EP/SFT
25/02/2019

7. Geometry organization
Geometry is naturally split into Detectors
A DetectorFacility class is provided for this
Users must inherit from DF and (in principle) just implement a BuildGeometry() method
There is no predefined detector organization, users build their experiment at run time
Atlas logically contains the Atlas IDET, but you can make it such that the IDET contains Atlas
Users register their detectors into a catalog by just adding one line of code (plug-in)
static DetectorFacilityEntry<TileSection> calo("TileSection");
A. Dell’Acqua – CERN EP/SFT
25/02/2019

8. Geometry Organization
DetectorFacilities are accessible at run time through the catalog
and define their own UI directory
and can be combined together in the way you want
and then moved around
OK, *eventually* you have to decide who’s the boss
/control/ReadXML standard_materials.xml
/control/ReadXML pixel_materials.xml
/load DetectorEnvelopes
/load Pixels
/load BeamPipe
/Geometry/GetFacility AtlasDetector Atlas
/Geometry/GetFacility InnerDetector Idet
/Geometry/GetFacility BeamPipe BPipe
/Geometry/GetFacility PixelDetector Pixel
/Atlas/AddDetector Bpipe
/Idet/AddDetector Pixel
/Atlas/AddDetector Idet
/Atlas/SetAsWorld
/Geometry/GetFacility PixelDetector Pixel
/TileTBeam/GetDescription TILE
/Atlas/AddDetector TileTBeam
/TileTBeam/MoveTo m
/Atlas/SetAsWorld
A. Dell’Acqua – CERN EP/SFT
25/02/2019

9. A. Dell’Acqua – CERN EP/SFT
The Geometry wrapper
A set of classes (FadsVolume, FadsPhysicalVolume, FadsRotationMatrix etc..) wrap up the corresponding G4 classes, extend their functionality and self-register on creation to a
GeometryManager which keeps a handle to all geometry objects as organized by detector
At this point one can:
change the visualisation attributes of a volume interactively
change positions and rotations interactively
change physics cuts for a certain volume
assign sensitive detector objects to volumes interactively
remove complete detectors/detector trees
by removing the plug-in, for instance
… by means of the standard G4 (G)UI
A. Dell’Acqua – CERN EP/SFT
25/02/2019

10. A. Dell’Acqua – CERN EP/SFT
Sensitive detectors
Abstract interface
They are organized as plug-ins, that an user attaches when they are needed
Assigned interactively to the volumes they must act upon
A volume can become sensitive at run time
General purpose SDs can be implemented for specialized actions
to calculate the thickness in R.L. of a sub-detector
A. Dell’Acqua – CERN EP/SFT
25/02/2019

11. A. Dell’Acqua – CERN EP/SFT
Physics
Geant4 physics lists are great for physics customization but generate quite some confusion in the “normal” user
hence:
Physics list are proxied
a Physics list catalog provides a list of the most common lists to choose from (EM, EM+Had…)
New physics lists can always be added (dynamic loading + plug-in)
Normal users choose the most appropriate
Advanced users can define their own...
A. Dell’Acqua – CERN EP/SFT
25/02/2019

12. A. Dell’Acqua – CERN EP/SFT
Event Generators
Abstract interface based on the HepMC format
Generator  HepMC  Geant4
Plug-ins
Abstract interface for editing/filtering particles and vertices (plug-ins)
Vertex displacement
/ filtering
energy/particle type filtering
Primary event (signal)
Secondary events from a different generator
Pile-up at the generator level
A. Dell’Acqua – CERN EP/SFT
25/02/2019

13. A. Dell’Acqua – CERN EP/SFT
User Actions
Abstract interface which combines all G4 user actions (and more) into a single class that users must inherit from
Plug-ins
Actions must now be registered interactively
Several UAs can now be run concurrently
Priority mechanism in place to decide which comes first (if needed)
A. Dell’Acqua – CERN EP/SFT
25/02/2019

14. Frame Facilities and Analysis
Abstract interface to analysis systems for implementing some histogramming capabilities
currently implemented
Hbook
Root
HTL
A. Dell’Acqua – CERN EP/SFT
25/02/2019

15. A. Dell’Acqua – CERN EP/SFT
Other goodies
Automatic access to XML
For materials, colours, detector parameters, “datacards”
User-defined XML “handlers” to be plugged in on-the-fly
Access to the Atlas “NOVA” service (MySQL)
Detector parameters from Geant 3
Some automatic code generation facilities
For e.g. DetectorFacility’s or UserAction’s
Native persistency scheme built in
Objectivity early on, now Root
A. Dell’Acqua – CERN EP/SFT
25/02/2019

16. A. Dell’Acqua – CERN EP/SFT
Status
The three main “actors” are working happily together (modulo cmt)
Can use facilities from both systems (e.g. event generators from Athena and physics from Goofy) seamlessly
Integrating detector geometries for production at the end of this year
80% complete
Going to a higher level of abstraction for certain components (e.g. SensitiveDetectors) to ensure better compatibility
Athena mostly batch-oriented, Goofy mostly interactive
Some funny batch-interactive mixture possible
A. Dell’Acqua – CERN EP/SFT
25/02/2019

17. On a less optimistic note…
We are very much based on the G4 UI..
…and that does not fit so well with the Atlas “strategy” (if any)
We would certainly be very much behind a full “pythonization” of the G4 UI (besides, Messengers are a real PITA sometimes)
We have not grown a Graphics/Analysis tool of ours…
In the integration phase we are suffering because of lack of functionality (e.g. 2-D graphics, cut views, tree inspection)
Borrow CMS’ Iguana? Bother G4 to death?
A. Dell’Acqua – CERN EP/SFT
25/02/2019

18. A. Dell’Acqua – CERN EP/SFT
Conclusions
We are right in the middle of a big, “Big Bang” style move to Geant4 as the main simulation engine
The detector will come together at once
Shifting emphasis from physics to computing for a while
Are we in for some nasty surprise?
Memory, performance, initialization time etc. ARE a concern
We are working hard on parameterising our calorimeters
“GFLASH++” becoming available, looking forward to collaborate with other experiments
A. Dell’Acqua – CERN EP/SFT
25/02/2019