Cuts per region

1. Cuts per region Layout Introduction Generalities Analysis Design (skipped) Plans John Apostolakis, 15 Nov 2002 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: • 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. Design 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.