Simulating Events with Athena

This section describes how to simulate previously generated events with Athena. Simulation is the process whereby generated events are passed through a GEANT4 Simulation of the ATLAS Detector to produce Hits i.e. a record of where each particle traversed the detector and how much energy etc was deposited.

You first need to set up your account as described in WorkBookSetAccount and check out a package to work in such as UserAnalysis as described in WorkBookRunAthenaHelloWorld. The generation of events is described in WorkBookGeneration.

Running Simulation in Athena

Go to your run directory such as:

> cd testarea/%WBRELEASE%
> cd PhysicsAnalysis/AnalysisCommon/UserAnalysis/run

The Geant4 simulation is run using the job transform which is available from the command line after a release has been set up. The command can be run as follows:

> inputEvgenFile=pythia.pool.root outputHitsFile=g4hits.pool.root maxEvents=10 skipEvents=0 randomSeed=1234 geometryVersion=%WBDETECTORDESC% physicsList=QGSP_BERT jobConfig=NONE > athena_sim.out 2>&1

The arguments are: the evgen file you produced in WorkBookGeneration, the name of the output file, the number of events to process, the number of events at the beginning of the evgen file to skip, a random number seed offset to use during simulation, the detector layout to use for simulation, the physics list to use (should almost always be 'QGSP_BERT'), and any additional job configuration fragments you might wish to use.

The additional job configuration fragments for simulation are listed here. Those most commonly used in production include for a displaced beam spot, for turning on the Lucid forward detector (normally off), and as a part of running Atlfast-II.

Simulation takes a LONG time (~15 mins/event for Z0->e+e-). On Lxplus your command will be killed after 40 mins so it is better to use Lxbatch as described here.

Eventually you should have a file of Geant4 Hits (this was 10 Z0->e+e- events):

-rw-r--r--  1 lloyd lloyd  12996310 May 23 20:02 g4hits.pool.root
-rw-r--r--  1 lloyd lloyd    254185 May 23 16:48 pythia.pool.root

For more details, see the official simulation documentation or the documentation of the simulation job transforms.

Simulation Layout/Detector Description

The line geometryVersion=%WBDETECTORDESC% defines the version of the Simulation Layout (also known as the Detector Description) to use. Information on possible values can be found here.

Previous Releases

-- ZacharyMarshall - 20 Jul 2009

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Topic revision: r3 - 2009-07-20 - ZacharyMarshall
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