11 Detector Simulation
517
11.5.6 Fast Simulation in CMS During LHC Run I and II
The fast simulations of the LHC experiments are sophisticated programs, combining
the most important physics processes for electrons, gammas in the trackers and
muons in the muon systems, with sampling from parameterized distributions for
the showering of electrons, gammas and hadrons in the calorimeters. CMS’s fast
simulation FastSim [117] is a simplified geometrical description of the tracker,
refined to obtain percent-level agreement for photon conversion, using around 30
thin nested cylinders.
The fast simulation in the tracker reconstructs each track using only its own
generated hits, and cannot reproduce fake tracks that result from the incorrect
association of hits. Though good agreement is seen in comparisons with Run-I data,
the limitations of this approach are apparent in modeling the efficiency of track
reconstruction and the fake rate [97].
Electron and hadron showers in calorimeters are turned into energy spot hits
directly, distributed according to a -function with parameters which fluctuate
between showers using GLASH [118] or a similar approach.
Regular comparisons with the full detector simulation are used to monitor all the
quantities used in physics analyses [119]. Agreement is observed at a level of 10%.
Good agreement is particularly important for the missing transverse energy E T .
During Run-I the CMS FastSim was used for the parameter scans for SuperSymmetry searches and samples of events used to evaluate systematic uncertainties [97],
because the computing resources required for full simulation would have exceeded
the available ones.
Once in-situ data is available, they are used as the final yardstick of the quality
of both the fast and full simulation, and are used to address possible discrepancies
between fast and detailed simulation.
11.5.7 Future Detectors: Fine-Grained Calorimeters
of CALICE
Proposed detectors for next-generation collider experiments rely on particle flow
reconstruction methods to obtain the required energy resolution for their physics
programme. In order to obtain this performance, it is necessary to accurately model
the energy deposition of charged hadrons, in order to subtract them from the
observed signals.
Measurements with fine-grained calorimeters provide the most promising methods to validate the most important properties of detector simulation tools, and in
particular their hadronic modeling. The CALICE experiment has undertaken test
beam measurements of prototype calorimeters with many layers of scintillator tiles
of fine granularity.
517
11.5.6 Fast Simulation in CMS During LHC Run I and II
The fast simulations of the LHC experiments are sophisticated programs, combining
the most important physics processes for electrons, gammas in the trackers and
muons in the muon systems, with sampling from parameterized distributions for
the showering of electrons, gammas and hadrons in the calorimeters. CMS’s fast
simulation FastSim [117] is a simplified geometrical description of the tracker,
refined to obtain percent-level agreement for photon conversion, using around 30
thin nested cylinders.
The fast simulation in the tracker reconstructs each track using only its own
generated hits, and cannot reproduce fake tracks that result from the incorrect
association of hits. Though good agreement is seen in comparisons with Run-I data,
the limitations of this approach are apparent in modeling the efficiency of track
reconstruction and the fake rate [97].
Electron and hadron showers in calorimeters are turned into energy spot hits
directly, distributed according to a -function with parameters which fluctuate
between showers using GLASH [118] or a similar approach.
Regular comparisons with the full detector simulation are used to monitor all the
quantities used in physics analyses [119]. Agreement is observed at a level of 10%.
Good agreement is particularly important for the missing transverse energy E T .
During Run-I the CMS FastSim was used for the parameter scans for SuperSymmetry searches and samples of events used to evaluate systematic uncertainties [97],
because the computing resources required for full simulation would have exceeded
the available ones.
Once in-situ data is available, they are used as the final yardstick of the quality
of both the fast and full simulation, and are used to address possible discrepancies
between fast and detailed simulation.
11.5.7 Future Detectors: Fine-Grained Calorimeters
of CALICE
Proposed detectors for next-generation collider experiments rely on particle flow
reconstruction methods to obtain the required energy resolution for their physics
programme. In order to obtain this performance, it is necessary to accurately model
the energy deposition of charged hadrons, in order to subtract them from the
observed signals.
Measurements with fine-grained calorimeters provide the most promising methods to validate the most important properties of detector simulation tools, and in
particular their hadronic modeling. The CALICE experiment has undertaken test
beam measurements of prototype calorimeters with many layers of scintillator tiles
of fine granularity.