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

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;

Layout Generalities Analysis Design

Generalities

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;

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;

Analysis

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.

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;

Design

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;

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;

Geometry

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;

Processes

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;

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…

Tracking time

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

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