PhysicsConstructor enabling G4HepEm vectorised EM tracking for e-/e+/gamma. More...

Detailed Description

PhysicsConstructor enabling G4HepEm vectorised EM tracking for e-/e+/gamma.

This plugin replaces the Geant4 stepping loop for electrons, positrons and photons with the G4HepEmTrackingManager, which provides vectorised EM physics using SIMD intrinsics (AVX2/AVX-512). Hadronic physics from the base physics list (e.g. FTFP_BERT) is unaffected: G4VTrackingManager::SetTrackingManager() takes priority over the standard process manager for the three EM particles, while all other particles continue to use the default stepping loop.

HepEm can be restricted to selected G4Region names via the HepEmRegions property. If HepEmRegions is empty (default), HepEm applies in all regions; if non-empty but no regions are resolved, HepEm tracking is disabled (with a warning). Woodcock tracking for photons can be enabled per G4Region via the WoodcockRegions property. Woodcock tracking allows photons to bypass fine-grained geometry navigation (e.g. individual fibres in a ScFi calorimeter) by sampling interactions against a majorant cross-section. When HepEmRegions is non-empty, WoodcockRegions is applied only to the intersection with HepEmRegions; non-overlapping Woodcock regions are ignored with a warning.

Usage in a ddsim/npsim steering file:

SIM.physics.list = "FTFP_BERT"
 
def setupHepEm(kernel):
  from DDG4 import PhysicsList
  seq = kernel.physicsList()
  hepem = PhysicsList(kernel, 'Geant4HepEmTrackingPhysics/HepEmPhysics')
  hepem.HepEmRegions = ['EcalBarrelScFiLayerRegion']
  hepem.VerboseLevel = 1
  hepem.WoodcockRegions = ['EcalBarrelScFiLayerRegion']
  hepem.enableUI()
  seq.adopt(hepem)
 
SIM.physics.setupUserPhysics(setupHepEm)

The DDG4HepEm plugin library must be in LD_LIBRARY_PATH.