Stack management, Digitization and Pile-up

Stack Management, Digitization and Pile-up - M.Asai (SLAC) - Geant4 Users Workshop @ CERN (Nov/13/2002)
Introduction
This tutorial covers the features of
Stack management
Digitization
Multiple events handling and pile-up
These features are not necessary for naïve simulation. But by utilizing them, you can make your simulation program more flexible and efficient.

Stack Management, Digitization and Pile-up - M.Asai (SLAC) - Geant4 Users Workshop @ CERN (Nov/13/2002)
Stack management
By default, Geant4 has three track stacks.
“Urgent”, “Waiting”, “PostponeToNextEvent”
Each stack is a simple Last-in-First-out stack.
Tracks are popped out only from Urgent stack.
Once Urgent stack becomes empty, tracks in Waiting stack are moved to Urgent stack.
UserStackingAction class classifies each track which stack to be pushed in (or killed).
By utilizing stacks, you can manage priorities of the tracks without achieving “highest priority scan”.
You can add stacks for more intelligent stack management.
Proper stack management and selection/abortion of event gives you better performance of your simulation.

Stack Management, Digitization and Pile-up - M.Asai (SLAC) - Geant4 Users Workshop @ CERN (Nov/13/2002)
Stack management
Event Manager
Tracking
Manager
Stacking
Manager
User Stacking
Action
Urgent
Stack
Waiting
Stack
Postpone To
Next Event
Stack
Push
Pop
Push
Push
Push
Pop
Classify
secondary or suspended tracks
Process One Track
primary tracks

Stack Management, Digitization and Pile-up - M.Asai (SLAC) - Geant4 Users Workshop @ CERN (Nov/13/2002)
G4UserStackingAction
G4UserStackingAction class has three methods.
void NewStage()
Invoked when Urgent stack becomes empty
Invoked after tracks in Waiting stack had moved to Urgent stack
You can issue stackManager->ReClassify()thus all tracks in Urgent stack will be re-examined by ClassifyNewTrack method
G4ClassificationOfNewTrack
ClassifyNewTrack(const G4Track*)
Invoked every time a new track is pushed in
Classification
fUrgent // pushed into Urgent stack
fWaiting // pushed into Waiting stack
fPostpone // pushed into PostponeToNextEevnt
fKill // killed
void PrepareNewEvent()
Invoked at the beginning of new event

Stack Management, Digitization and Pile-up - M.Asai (SLAC) - Geant4 Users Workshop @ CERN (Nov/13/2002)
G4UserStackingAction
ExampleN04 gives you a good example of stack management.
This example has a simplified collider detector geometry and event sample of Higgs particle decays into muons via Zs.
At the beginning of an event, only primary muons are pushed into Urgent stack and others to Waiting stack.
Hits in muon counters are examined when Urgent stack becomes empty
Event is aborted if reasonable number of hits are NOT found in the muon counters, otherwise proceed to next stage.

Stack Management, Digitization and Pile-up - M.Asai (SLAC) - Geant4 Users Workshop @ CERN (Nov/13/2002)
G4UserStackingAction
Only primary charged tracks are pushed into Urgent stack
Each track is traced in the tracking region and then suspended and pushed back to Waiting Stack once it reaches to calorimeter.
Hits in tracking chambers are examined
Event is aborted if reasonable numbers of isolated muons are NOT found.
Only tracks (both primary and secondary) in “Region of Interest” are pushed into Urgent stack and traced.
I.e. Shower is simulated only inside of RoI.

Stack Management, Digitization and Pile-up - M.Asai (SLAC) - Geant4 Users Workshop @ CERN (Nov/13/2002)
Digitization
Digit represents a detector output (e.g. ADC/TDC count, trigger signal).
Digit is created with one or more hits and/or other digits by a concrete implementation derived from G4VDigitizerModule.
In contradiction to the Hit which is generated at tracking time automatically, the digitize() method of each G4VDigitizerModule must be explicitly invoked by the user’s code (typically at EndOfEventAction).

Stack Management, Digitization and Pile-up - M.Asai (SLAC) - Geant4 Users Workshop @ CERN (Nov/13/2002)
Digitization
The usages of methods in G4VDigitizerModule is quite similar to those of G4VSensitiveDetector.
Constructor
Registration of digits collection name(s)
Digitize() method
Get hits from proper hits collections and/or digits from proper digits collections
Generate digit(s) and store to dedicated digits collection(s)
Set created digits collection(s) to G4DCofThisEvent