Refactoring of Barrel Time-of-Flight (BTOF) Detector Geometry

Constructing BTOF Geometry with ROOT's TGeo

Dmitri Smirnov

February 28, 2018

Updated

Introduction

  • Position of BTOF modules is used in official StBTofMatchMaker to match tracks to BTOF cells
  • BTOF library creates an internal set of volumes based on ROOT's TVolume using full Geant3 geometry as input
  • This makes real data reconstruction jobs depend on Geant3 simulation library
    • There is no other need for Geant3 in reco jobs. Therefore, it is desirable to remove such dependency
  • Changes to BTOF geometry in StBTofUtil have been proposed by S&C and reviewed by Frank (TOF expert)

Tests for new BTOF Geometry

  • Verified the BTOF volume positions and dimensions before and after switching to TGeo
    • Observed small difference in the 4-5th decimal digits. This converts to ~10 micron variations, negligible for vast majority of physics analyses
    • Numerical differences also expected as Geant3 uses single precision for volume position whereas TGeo uses double. Spacial transformations are also different
  • With high level tests verified the number of BTOF matched tracks
    • Reconstructed 100 events from Runs 15, 16, and 17
    • Small difference in the number of total matches. In Run 16, 3 out of 100 events had 1 mismatch in ~1000 tracks per event
    • For this statistics no difference found in number of "good" track matches
  • After integration the nightly tests did not report any issues
  • More precise tests are left to the BTOF users and experts...

Summary

  • BTOF geometry in StBTofUtil has been refactored to use TGeo. Deployed on Feb 26, 2018
  • Removal of Geant3 from reconstruction jobs has a positive impact
    • Opens path to 64-bit builds of STAR libraries. From this alone we already expect speed up of reco jobs by ~15%
    • Possible switch to vanilla root in place of root4star for jobs where interface to Fortran Geant3 libraries and dependency on cernlib libraries are not required
    • Allowed to avoid problems with optimized fortran code (See Gene's report on missing half of BTOF detector)
  • There is an ongoing work on ETOF software (Daniel B., Florian S.) for which a similar approach to geometry construction should be taken