1. Based on a presentation of M.Verderi
Cuts per region
John Apostolakis, 15 Nov 2002
Layout
Introduction
Generalities
Analysis
Design (skipped)
Plans
Based on a presentation of M.Verderi

2. I. Introduction Cut in all G4 & 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;
Promotes consistency of physics simulated
A volume with high cuts (ie poor physics quality) will not «pollute» a neighbouring volume with low cuts;
But requests from ATLAS, BABAR, CMS, LHCb, …, to allow several cuts;
Globally or per particle;

3. II. General considerations

4. 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;

5. 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;

6. Analysis

7. 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.

8. Relaxing the unity of range cuts
Request to allow several cuts has been analysed 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;

9. Snapshot of detailed design (Skipping in the interest of time)

10. IV. Status

11. Anticipated limitations
Partition of logical volumes;
The same logical volume can not belong to two different regions
G4ProductionCut defines a set of cuts for all particles;
Some potential duplication exists
The same cut value can appears for, say electrons, in two different production-cut objects;
And some materials appear in the related regions;
In this case cross-section table will be calculated twice;
This is a considerable complication
The current design does not take it into account;
Consideration is left to a future design iteration.

12. Status Detailed design for cut per region has been made;
It does not imply severe design revision of the existing GEANT4;
About the plans:
Development release(s): 1Q 2003
Target scheduled release: June 2003.