Using BgsTransportation with the Geant4 Toolkit

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Introduction

BgsTransportation is a specialized stepper developed for use in BaBar by David Williams. Unlike the default, Runge-Kutta stepper provided by Geant4, BgsTransportation uses perfect helices to track particles through magnetic fields. As a result, BgsTransportation is faster and more accurate than the default stepper, provided the field does not vary too rapidly.

It has been proposed to make BgsTransportation an official part of the Geant4 toolkit, but substantial changes in the Geant4 navigator must first be made. For the time being, BgsTransportation is being made available as part of the SLAC patch of the Geant4 toolkit and is included with the standard Geant4 libraries.

Building an Executable which uses BgsTransportation

Example A01 provides the ability to run simulations with either the default stepper or BgsTransportation. Instructions for building this example are found here .

Once the example is built, it may be run by executing

../bin/$G4SYSTEM/A01app novis.mac bgstrans

which uses BgsTransportation, or

../bin/$G4SYSTEM/A01app novis.mac

which uses the default stepper.

How BgsTransportation is Invoked

Like the default Geant4 stepper, BgsTransportation is a process which must be instantiated and registered to all particles requiring transportation. This is done in the PhysicsList. The implementation of this option, from the command line to the addition of the process, is outlined here:

• the option "bgstrans" is passed to the physics list in main() :
  • runManager->SetUserInitialization(new A01PhysicsList(UseBgsTrans));

• A01PhysicsList derives from G4VModularPhysicsList, so the constructor for A01PhysicsList looks like :
  • A01PhysicsList::A01PhysicsList(G4bool UseBgsTrans) :G4VModularPhysicsList(UseBgsTrans)
  In standard Geant4 the class G4VModularPhysicsList has no argument, so it was modified in the SLAC patch

• In G4VModularPhysicsList::ConstructProcess() either
  • AddBgsTransportation() or
  • AddTransportation()
  is called, depending on the value of the boolean UseBgsTrans

• G4VModularPhysicsList is derived from G4VUserPhysicsList. The method AddBgsTransportation() was added to the standard G4VUserPhysicsList in order to register the BgsTransportation process with all the particles and to set the order of process invokation.

Restrictions

• As mentioned above, BgsTransportation should not be used in magnetic fields which vary rapidly in space. In that case the routine will continue to reduce the step size until the variation of the field over that step is small relative to some tolerance. This will cause the tracking to slow down significantly.

• BgsTransportation does not use the G4Navigator. This means that certain classes and methods cannot be used. Amoung them are:
  • G4PVParameterised, used in generating parameterized volumes
  • G4TouchableHistory::GetHistory()