Parameterisation of EM showers in the ATLAS LAr Calorimeter Tom Atkinson - The University of Melbourne On behalf of Elisabetta Barberio - The University of Melbourne Anthony Waugh - The University of Sydney

Introduction Fast Simulation of ATLAS LAr calorimeter implemented in package: LArCalorimeter/LArG4/LArG4FastSimulation Class LArFastShower derived from G4 class FastSimModel Depends on LArCalorimeter/LArG4/LArG4Barrel EMBParticleBounds EMBShowerParameters Simulation testing done with ATHENA 10.3.0 G4AtlasApps-00-00-31 New tag LArG4FastSimulation-00-00-05 Hacked version of LArG4Barrel/src/EMBParticleBounds.cc NB: New tag produced LArG4Barrel-00-00-56 but not backwards compatible against 10.3.0

Original simulation scheme Timing performance previously significantly worse in full ATLAS simulation vs stand-alone reconstruction. Original simulation scheme: Energy range e+ / e- Photons 0.0GeV < E < 0.1GeV Full sim 0.1GeV < E < 0.5GeV 0.5GeV < E < 100GeV Kill + Fast param This scheme was fine for stand-alone testing of the calorimeter - no showering!

Original simulation scheme - Timing results But showering occurs in ATLAS when particles generated at nominal interaction point. Large numbers of low energy shower particles also entering the calorimeter Result is a large increase in execution time. ATLAS scan range -0.8 <  < 0.8 (NB: Full simulation in crack) 100 events per sample Energy Fast Param (CALO) Fast Param (ATLAS) 10GeV 0.13 sec 2.2 sec 50GeV 0.64 sec 5.3 sec 100GeV 1.08 sec 9.4 sec Average time / event

Timing vs.  - original scheme

New simulation scheme This scheme is not final. Cuts on “kill” limits will change as we optimise performance vs. timing… Energy range e+ / e- Photons 0.0GeV < E < 0.1GeV Kill 0.1GeV < E < 0.5GeV Full sim 0.5GeV < E < 100GeV Kill + Fast param

New simulation scheme - Timing results Electrons generated at nominal vertex -0.8 <  < 0.8 (NB: Full simulation in crack) 100 events per sample Energy Full Simulation Original scheme ( CALO Only) New scheme 10GeV 3.7 sec 2.2 sec 0.13 sec 0.6 sec 50GeV 16.9 sec 5.3 sec 0.64 sec 1.3 sec 100GeV 34.5 sec 9.4 sec 1.08 sec 2.1 sec Average time / event

Number of Parameterisation method calls Energy IsApplicable() ModelTrigger() DoIt() 10GeV 7400 200 48000 1100 4 100GeV 120000 1500 470000 8500 22 Original scheme / new scheme

Timing tests in the ATLAS endcap Electrons generated at nominal vertex 1.5 <  < 2.5 New particle simulation scheme used Energy Full Simulation Fast Parameterisation 10GeV 7.8 sec 3.3 sec 50GeV 38.2 sec 14.5 sec 100GeV  95 sec 23 sec Average time / event Under investigation - more complex geometry!

For discussion… A word on the direction of electrons… LArFastShower::ModelTrigger() Get Momentum Direction:(0.576413,-0.816949,-0.01849) LArFastShower::ElectronDoIt() Get Momentum Direction: (0.788559,-0.609188,-0.0840549) Change of direction between ModelTrigger and DoIt seen for electrons ONLY (Positrons and photons seem to be ok) Consistent with effects seen by CMS Does anybody know what causes this?

Conclusion Significant improvements in timing when low energy electrons and photons from the shower are “killed”. Impact on physics performance of new scheme is currently beginning. “Kill” cuts will be set to optimise performance vs. timing The End…