Abstract
VecGeom is a geometry modeller library with hit-detection features as needed by particle detector simulation at the LHC and beyond. It was incubated by a Geant-R&D initiative and the motivation to combine the code of Geant4 and ROOT/TGeo into a single, better maintainable piece of software within the EU-AIDA program. So far, VecGeom is mainly used by LHC experiments as a geometry primitive library called from Geant4, where it was shown to provide 7–12% reduction in CPU time due to its faster algorithms for complex primitives [1]. In this contribution, we discuss how VecGeom can be used as the navigating library in Geant4 in order to benefit from both its fast geometry primitives as well as its vectorised navigation module. We investigate whether this integration provides the speed improvements expected, in addition to the gain obtained from geometry primitives. We discuss and benchmark the application of a VecGeomnavigator plugin to Geant4 for a simplified geometry and show paths towards production usage.
Highlights
VecGeom [2] is a C++ geometry library for use in particle detector simulation
Its development started as part of the GeantV R&D initiative [3, 4] with the original goal to provide geometry kernels that are able to process multiple tracks/particles concurrently by utilizing vector micro-parallelism, implementing the single instruction - multiple data (SIMD) principle [2, 5]
It was realised that providing such a new library provides a chance to review the existing geometry code used in HEP such as those provided by Geant4 [6] and ROOT/TGeo [7]
Summary
VecGeom [2] is a C++ geometry library for use in particle detector simulation. Its development started as part of the GeantV R&D initiative [3, 4] with the original goal to provide geometry kernels that are able to process multiple tracks/particles concurrently by utilizing vector micro-parallelism, implementing the single instruction - multiple data (SIMD) principle [2, 5]. As visualised in Fig. 1(left), the integration of VecGeom solids is provided via special bridge/facade classes This change is transparent to the Geant geometry transport engine (G4Navigator) and user code. Improvements provided in the VecGeom navigation system, such as SIMD accelerated hit-detection [9], are not yet utilised This leaves room for further speedup of Geant4-based detector simulation which is one of the key motivations of the present work. Some HEP experiments perform their Geant detector simulation through the Virtual Monte Carlo (VMC) system [13, 14], and in this case typically the ROOT/TGeo geometry is called by the Geant transport (see Fig. 1(right)) For this scenario there is currently no VecGeom integration available at all and overcoming this limitation constitutes a secondary goal of our investigations
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