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C. W. Fabjan and D. Fournier
(i) Energy Calibration and Reconstruction
Many physics programmes at the modern colliders (HERA, Fermilab, LHC) require
energy measurements at the limit of the instrumental resolution and with ~1%
accuracy. The calorimeters are frequently composed of different electromagnetic
and hadronic instruments, made from different materials and sampling topologies.
Establishing the absolute energy scale in the reconstruction of particles (and jets)
needs a major effort to understand the detector, from an instrumental and technical
point. It requires a tight interplay between measurements and simulations. Energy
calibration and reconstruction, proceeds in several steps. Customarily, a calorimeter
(segment) is exposed to electrons, setting the ‘electromagnetic’ energy scale. For
hadrons a ‘weighing’ has to be applied to each cell, such that.
E i (true) = w i E i (reconstructed) with w i = E i (true) /E i (reconstructed) .
E i (true) expresses the total energy deposited. This can be a rather large correction, particularly in non-compensating calorimeters. In a further step, details of the
energy reconstruction algorithm (‘clustering’) are simulated to evaluate the energy
outside the cluster, usually chosen smaller than the true shower extent. In practical
calorimeters, non-sensitive regions (‘dead material’, DM) are unavoidable leading
to frequently sizeable corrections evaluated by MC.
Establishing the energy scale for jets is the most complex calibration task. Jets
are calibrated with a series of simulation-based corrections and in situ techniques.
In situ techniques exploit the transverse momentum balance between a jet and
a reference object such as a photon, Z boson or multijet system for jets with
20 < p T < 2000 GeV, using both data and simulation. In this way an uncertainty in the jet energy scale approaching 1% is obtained for high-p T -jets with
100 < p T < 500 GeV/ c. An uncertainty of about 4.5% is found for low-p T jets
(p T < 20 GeV/ c), dominated by uncertainties in the corrections for multiple protonproton interactions (pile-up), see Fig. 6.28 [61].
(ii) Particle Flow Analysis in Calorimeter Systems at Present and Future
Colliders
An important recent development is an ambitious analysis strategy for reconstructing the jet energy in calorimeters, the “Particle Flow” concept. It aims at
identifying and reconstructing individually each particle arising from the collision
(proton-proton, electron-positron, . . .) by combining the information from all the
subdetectors. The resulting particle-flow event reconstruction leads to an improved
performance for the reconstruction of jets and “Missing Transverse Energy” (MET).
The algorithm also improves the identification of electrons, muons, and taus. While
the concept has first been applied in the physics analysis at the LEP collider, it is
presently heavly used by the LHC collaborations [62, 63]. The improvement can be
dramatic, as shown in Fig. 6.29.
The benchmark performance for calorimeter systems (Sect. 6.7.6.2) for future
colliders (International Linear Collider, ILC; Future Circular Collider, FCC) aims
at a jet energy resolution of σ(jet) ~ 0.3/
√
E(GeV). This is motivated by the need
to measure, e.g. W- and Z-decays into two jets with a mass resolution approaching
C. W. Fabjan and D. Fournier
(i) Energy Calibration and Reconstruction
Many physics programmes at the modern colliders (HERA, Fermilab, LHC) require
energy measurements at the limit of the instrumental resolution and with ~1%
accuracy. The calorimeters are frequently composed of different electromagnetic
and hadronic instruments, made from different materials and sampling topologies.
Establishing the absolute energy scale in the reconstruction of particles (and jets)
needs a major effort to understand the detector, from an instrumental and technical
point. It requires a tight interplay between measurements and simulations. Energy
calibration and reconstruction, proceeds in several steps. Customarily, a calorimeter
(segment) is exposed to electrons, setting the ‘electromagnetic’ energy scale. For
hadrons a ‘weighing’ has to be applied to each cell, such that.
E i (true) = w i E i (reconstructed) with w i = E i (true) /E i (reconstructed) .
E i (true) expresses the total energy deposited. This can be a rather large correction, particularly in non-compensating calorimeters. In a further step, details of the
energy reconstruction algorithm (‘clustering’) are simulated to evaluate the energy
outside the cluster, usually chosen smaller than the true shower extent. In practical
calorimeters, non-sensitive regions (‘dead material’, DM) are unavoidable leading
to frequently sizeable corrections evaluated by MC.
Establishing the energy scale for jets is the most complex calibration task. Jets
are calibrated with a series of simulation-based corrections and in situ techniques.
In situ techniques exploit the transverse momentum balance between a jet and
a reference object such as a photon, Z boson or multijet system for jets with
20 < p T < 2000 GeV, using both data and simulation. In this way an uncertainty in the jet energy scale approaching 1% is obtained for high-p T -jets with
100 < p T < 500 GeV/ c. An uncertainty of about 4.5% is found for low-p T jets
(p T < 20 GeV/ c), dominated by uncertainties in the corrections for multiple protonproton interactions (pile-up), see Fig. 6.28 [61].
(ii) Particle Flow Analysis in Calorimeter Systems at Present and Future
Colliders
An important recent development is an ambitious analysis strategy for reconstructing the jet energy in calorimeters, the “Particle Flow” concept. It aims at
identifying and reconstructing individually each particle arising from the collision
(proton-proton, electron-positron, . . .) by combining the information from all the
subdetectors. The resulting particle-flow event reconstruction leads to an improved
performance for the reconstruction of jets and “Missing Transverse Energy” (MET).
The algorithm also improves the identification of electrons, muons, and taus. While
the concept has first been applied in the physics analysis at the LEP collider, it is
presently heavly used by the LHC collaborations [62, 63]. The improvement can be
dramatic, as shown in Fig. 6.29.
The benchmark performance for calorimeter systems (Sect. 6.7.6.2) for future
colliders (International Linear Collider, ILC; Future Circular Collider, FCC) aims
at a jet energy resolution of σ(jet) ~ 0.3/
√
E(GeV). This is motivated by the need
to measure, e.g. W- and Z-decays into two jets with a mass resolution approaching