An object-oriented simulation program for CMS

Abstract

The CMS detector simulation package, OSCAR, is based on the Geant4 simulation toolkit and the CMS object-oriented framework for simulation and reconstruction. Geant4 provides a rich set of physics processes describing in detail electromagnetic and hadronic interactions. It also provides the tools for the implementation of the full CMS detector geometry and the interfaces required for recovering information from the particle tracking in the detectors. This functionality is interfaced to the CMS framework, which, via its "action on demand" mechanisms, allows the user to selectively load desired modules and to configure and tune the final application. The complete CMS detector is rather complex with more than 1 million geometrical volumes. OSCAR has been validated by comparing its results with test beam data and with results from simulation with a GEANT3-based program. It has been successfully deployed in the 2004 data challenge for CMS, where more than 35 million events for various LHC physics channels were simulated and analysed.

Full text

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An Object
An Object- -Oriented
Simulation Program
Simulation Program
for CMS
for CMS
India-CMS (October, 2004)
Sunanda Banerjee (TIFR)
Oriented
Authors:
S. Abdulline (FNAL), N. Amapane (U. Torino), V. Andreev (UCLA),
P. Arce (CIEMAT), S. Banerjee (TIFR), T. Boccali (SNS, Pisa),
M. Case (UC Davis), A. De Roeck (CERN), S. Dutta (U. Pisa),
G. Eulisse (NorthEastern U.), D. Elvira (FNAL), A. Fanfani (U. Bologna),
F. Ferro (INFN, Genova), M. Liendl (CERN), S. Muzaffar (NorthEastern U.),
A. Nikitenko (Imperial College), K. Lassila-Perini (HIP),
I. Osborne (NorthEastern U.), M. Stavrianakou (FNAL), T. Todorov (IReS),
L. Tuura (NorthEastern U.), H.P. Wellisch (CERN), J. Weng (Karlsruhe U.),
T. Wildish (Princeton U.), S. Wynhoff (Princeton U.), M. Zanetti (INFN),
A. Zhokin (ITEP), P. Zych (Warsaw U.)

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Outline
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Overview
Run and event management
Geometry and Sensitive Detector
Hits
Generator interface and Physics
CMS detector simulation and validation
– Tracker
– ECAL
– HCAL
– Muons
– Forward detectors
Magnetic Field
Parameterized simulations
Code Quality
Performance
Production
Summary
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OSCAR overview
Object Oriented Simulation for CMS Analysis and Reconstruction
Full CMS simulation based on the Geant4 toolkit
? Geant4: physics processes describing in detail electro-magnetic
and hadronic interactions; tools for the CMS detector geometry
implementation; interfaces for tuning and monitoring particle
tracking
? CMS framework: application control, persistency, common
services and tools (magnetic field, generator interfaces and
support for MC truth, infrastructure for hits and readout units,…),
“action on demand” to selectively load desired modules,
configure, tune application
? CMS changed from CMSIM/GEANT3 to OSCAR/GEANT4 end 2003;
? OSCAR used for substantial fraction of DC04 production; will be used
for physics TDR production
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CPU: OSCAR ≤ 1.5 x CMSIM - with lower production cuts!
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Memory: ~110 MB/evt for pp in OSCAR ≈ 100 MB in CMSIM
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Robustness: from ~1/10000 crashes in pp events (mostly in hadronic
physics) in DC04 production to 0 crashes in latest stress test
(800K single particles, 300K full QCD events)

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Relies on COBRA for program's main; COBRA takes over and
handles the application with a SimApplication and a SimEvent in
the core
SimApplication instantiates and launches an event source factory
the SimEventSource, which is an abstract reader of simulated events
from the original source
SimEventSource instantiates the Mantis RunManager
RunManager instantiates a G4RunManagerKernel and controls
standard components such as the selection and instantiation of
generator, magnetic field and physics lists and interfaces to the run,
event, stacking, tracking and stepping actions; it also handles
?Storage/retrieval of random number seeds for run and events
?Storage/retrieval of the cross-section tables built for a given
detector configuration and physics list. Reading pre-built
cross-section tables reduces initialization time by factor ~4
Run

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SimEvent, inheriting from the COBRA SimEvent, manages the
Monte Carlo truth – common for and sharable by all CMS
applications (reconstruction, visualization etc); it consists of an
interface to the transient event. (be it Geant3, Geant4, fast
simulation) and an interface to a Geant4 event
Monte-Carlo truth, assembled at the EventAction, consists of
? the main event, its assigned weight and its type
? all particles with their tracks, vertices and decay trees from
the original generator event
? all tracks produced during simulation and flagged for
saving
Monte-Carlo truth organized so as to allow navigation from hits to
their corresponding tracks and parent vertices
Event

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Detector description in CMS is handled in the CMS
Detector Description Database, DDD, a COBRA
subsystem
Mantis provides mechanisms to convert DDD solids
and materials to their Geant4 counterparts as well as the
logical and physical volumes needed to build the Geant4
geometry for the chosen description
DDD SpecPars mechanism allows the definition of
special parameter sets (extra attributes, field parameters,
range cuts etc) to be associated with selected detectors
More than one million volumes are used to define CMS
detector setup
Geometry

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inherits from G4VSensitiveDetector and its concrete
implementation implements the Geant4 ProcessHits
method
is registered to the Geant4 sensitive detector manager
but is instantiated and “attached” to the corresponding
geometrical volumes at run time according to the set-up
described in the configuration DDD/XML file
There is a possibility of “instrumenting” (making
sensitive) any volume, for prototyping purposes or in
order to facilitate studies of energy losses in dead
materials or specific parts of the detector
Sensitive Detector

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Hit processing and hit collection are handled by the
individual detectors, based on COBRA classes common
for and sharable by all CMS applications; these
common classes are managed by the COBRA Profound
package
COBRA read-out factories handle the hit formatting as
required for the digitization, which is handled outside
the simulation, by the CMS reconstruction program
ORCA
Hits

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Based on the COBRA GeneratorInterface packages and services. It
provides a trigger method to produce a RawHepEvent or
HepMC::GenEvent;
RunManager instantiates the chosen generator. All generators can
be run-time configured in terms of
? the maximum number of events they can return
? the first event to be read
? An event vertex generator to be used (none, flat,
Gaussian, test-beam specific)
Available generators:
EventGunReader, EventNtplReader , EventTxtReader,
EventPythiaReader, EventStdHepReader,
EventHepMCReader
Generator Interface

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Physics
? an abstract physics list factory allows run-time
selection and configuration of specific physics
list
? physics cuts (i.e. range cuts) are implemented
as cuts per region (set of volumes)
? the regions with the set of volumes (typical
scheme distinguishes between “sensitive” and
“dead” regions) and the cut values for electrons,
positrons and photons are read at run-time from
a DDD/XML file

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Tracker
TOB layer 1
sensitive
Detailed description
of all active and
passive components;
material budget
2001
2001
Critical requirements for physics
studies with tracker
Correct, navigable Monte Carlo truth
(particle, track, vertex, history) with
trace-ability of initial primary particle
Special treatment of hard brem with
the assignment of new track for
electron above threshold
(500 MeV)
⇒ Extensive validation in terms of
tracking and hit distributions
X0OSCAR/X0CMSIM vs η
X0vs η
X0
η
Geant4 5.2.p02

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PixelBarrel
PixelEndcap
SiBarrel
SiEndcap
Raw simulated hitsReconstructed hits
Tracker cont’d
Hits from minimum bias in Tracker
Pixel cut in G3 too high
⇒10% increase expected
±5% differences in Si not significant
# hits # hits
# hits OSCAR / # hits CMSIM
# evts
# evts
Geant4 5.2.p02

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Electromagnetic Calorimeter (ECAL)
◆ Energy and position
resolution
◆ Shower shape
◆ Hadronic showers
◆ Level-1 e/m trigger response
◆ Preshower response
◆ Performance studies
Position resolution
Comparisons with CMSIM/G3
and test beam data
Geant4 5.2.p02

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Single crystal containment: E1x1/E3x3versus position
Data
G4
ECAL cont’d
ECAL standalone resolution
for electrons, with
bremsstrahlung recovery
Energy resolution
Geant4 5.2.p02

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Hadron Calorimeter (HCAL)
Energy resolution
Non-linearity in energy response
Extensive validation program
with comparisons to G3 and
several test beam data sets, incl.
combined ECAL-HCAL runs;
also in context of LCG simulation
physics validation project
Geant4 5.2.p02