1. Marc Verderi GEANT4 collaboration meeting 01/10/2002
Cut per region
Marc Verderi
GEANT4 collaboration meeting
01/10/2002

2. Introduction Cut here = « production threshold »;
Not tracking cut;
GEANT4 originally designed to allow a unique cut in range;
Unique cut in range per particle;
Default being a same cut for all particles;
Consistency of the physics simulated:
Garanties that a volume with high cuts (ie poor physics quality) will not « pollute » the simulation of a neighbouring volume with low cuts;
But requests from ATLAS, BABAR, CMS, LHCb, …, to allow several cuts;
Globally or per particle;

3. Layout
Generalities
Analysis
Design

4. Generalities

5. Cuts for what ? Some physics processes involve infra-red divergences;
Bremsstrahlung;
Infinity of lower and lower energy photons;
Ionisation;
Huge number of low energy electrons;
Limited by the (low) ionisation potential;
Goal of cuts is to limit the discrete production of secondaries;
Corresponding energy is transfered to the continuous component;

6. Today’s picture On the user side: On the G4 kernel side:
User constructs a detector:
Volumes
Materials
(S)he defines the physics processes to be used;
And then sets the cut;
Cut in range for the all simulation;
Eventually the cut may depend on the particle type;
On the G4 kernel side:
For each particle, G4 triggers the conversion of the cut in range into the equivalent energy threshold;
For each material;
Processes can then use these thresholds to compute their cross-section tables;
One table per material;

7. Analysis

8. Motivation for several cuts
Having a unique cut can be the source of performance penalties;
Part of the detector with lower cut needs fixes the cut for the all simulation;
Can be far too low than necessary in other parts;
Silicon vertex detector: a few 10 mm;
Hadronic calorimeter: 1 cm;
Other parts being geometrically far, to.

9. Relaxing the unicity of cuts
Request to allow several cuts has been analyzed as follows:
A cut value is typically required at the level of a detector sub-system:
Silicon vertex detector: a few 10 mm;
Hadronic calorimeter: 1 cm;
Introduce the concept of « region »:
Large geometrical area,typically the root logical volume of a sub-system;
Or an group of root logical volumes;
Eg: barrel + end-caps of the calorimeter;
A cut in range is associated to a region;
Eventually a range cut per particle is allowed;

10. Design

11. The region and cut classes, from the user point of view
The concept of region is realized by a new class, G4Region:
The user can set one or several root logical volumes to a region with method:
void AddRootLogicalVolume(G4LogicalVolume *);
Cuts are implemented as a new class to, G4ProductionCut;
Allows to defines a « default cut »;
Allows to specify eventually cuts for e-, e+, g.
The user sets a G4ProductionCut pointer to each region (s)he defined;

12. The machinery, from the G4 kernel point of view (1)
Geometry:
Class G4Region implemented for the purpose of cuts, but could be of more general usage;
Could carry the magnetic field for example;
For performance reasons at tracking time, the region pointer is propagated recursively in the daughter volumes from the root logical volume;
Processes can interrogate directly the current volume at tracking time;
Same mechanism as in parameterisation;
Above mechanism requires a partition of the logical volumes;
A same logical volume can not belong to two different regions;
Understood as being a (very) weak limitation in practice;

13. Geometry

14. The machinery, from the G4 kernel point of view (2)
Processes:
Only regards processes dealing with cuts;
Main issue is to know which cross-section table to use in the current volume at tracking time;
In the current scheme, for a given process, there was a one-to-one relation between a material and a cross-section table:
This was used to retrieve the physics table using:
« index of material » == « index of physics table »
Now, since a same material may appear in several regions above relation is replaced by:
« index of {material, region} couple » == « index of physics table »
G4MaterialRegionCouple introduced for this management purpose;

15. Processes

16. The machinery (3) Initialisation time:
Unreadable  scenario diagrams exist;
Basic scheme is:
Geometry and cut set up:
The user builds the geometry, sets up the regions, assigns cuts to the regions;
When the run manager closes the geometry:
It triggers a loop on the regions, builds, if needed, and set to the logical volumes the proper G4MaterialRegionCouple pointers;
The couple is updated with the energies from range conversion;
Then, the run manager triggers a loop on the physics processes, which can find in the material-cut table all informations to build the needed cross-section tables.
A scheme for reinitialisation after changes in the geometry was also made;

17. The machinery, from the G4 kernel point of view (4)
Tracking time:
Basic scheme is:
At a given point, the process asks the G4Track for the current material-cut couple;
It gets the related index;
And attacks the related cross-section table;
Case of parametrised volume anticipated also:
Less unreadable scenario diagram after…

18. Tracking time

19. Anticipated limitations
Partition of logical volumes;
Told about before;
G4ProductionCut defines a set of cuts for all particles;
But it can be that the same cut value appears for, say electrons, in two different cut objects;
And that same materials appear in the related regions;
In this case cross-section table will be calculated twice;
Looked quite a complication to take into account this case;
Might not be hopeless

20. Conclusion Detailed design for cut per region has been made;
It does not imply severe design revision of the existing GEANT4;
About the status:
See Makoto’s presentation on Thursday