64
3 Jet Substructure at the LHC
energy density; hadronic showers tend to occur deeper in the calorimeter and be
less dense [409]. Charged and neutral pions are used to derive this classification
and calibration, called the Local Cell Weighting (LCW). In CMS, dedicated ECAL
(based on photons) and HCAL (based on neutral kaons) calibrations are combined
to account for energy and |η|-dependent non-linearities in the hadron calorimeter
response [407]. Both ATLAS and CMS validate the performance of these calibrations with single particle studies in data [407, 412].
Different strategies are used by ATLAS and CMS to reconstruct tracks from
their inner detectors. ATLAS focuses first on maintaining a high efficiency with
a rather inclusive first pass through inner detector hits. A second step known as
ambiguity solving reduces the fake rate. In contrast, CMS uses a sequential approach
with multiple passes through the remaining inner detector hits. With each pass, the
efficiency increases while maintaining a low fake rate. Both procedures are effective
at identifying about 90% of charged pions above 1 GeV with a percent-level (or
smaller) fake rate. Lower momentum particles can be reconstructed, at the cost of
a higher fake rate and lower efficiency. Due to its weaker magnetic field, ATLAS is
able to reach low track momentum of 100 MeV for physics analysis [413], although
most jet substructure measurements and searches use a threshold of 500 MeV. In
contrast, the momentum resolution in CMS is excellent up to higher momenta than
in ATLAS. The TRT can be used to improve the momentum resolution of high p T
tracks [414], but the weaker magnetic field despite a comparable inner detector radius
is a fundamental limitation.
Both experiments have implemented dedicated strategies for track reconstruction
in high density environments such as the core of high p T jets. In such environments, pixel and strip clusters can merge resulting in a loss in tracking efficiency
and degraded resolution. ATLAS has implemented a stacked neural network (NN)
approach to examine pixel clusters to identify multi-particle clusters, estimate the
position of the particles passing through the clusters, and also predict the residual
resolution of the position estimates [415–419]. CMS has introduced a dedicated
tracking step in which a cluster splitting procedure attempts to split merged clusters
exploiting the information of the jet direction, predicting the expected cluster shape
and charge.
For particle flow in CMS, tracks and calibrated clusters are combined taking the
tracking and calorimeter resolutions into account. First, a link is created between
tracks in the central tracker and calorimeter clusters. Links are also created between
clusters in the ECAL and HCAL, if the cluster position in the ECAL is within the
cluster envelope in the less granular HCAL. Tracks with a p T uncertainty in excess of
the calorimetric energy resolution expected for charged hadrons are masked, which
allows the rate of misreconstructed tracks at large p T to be reduced.
Charged hadrons are created from ECAL and HCAL clusters, linked to tracks.
If the calibrated calorimetric energy is compatible with the corresponding track
momenta under the charged-pion hypothesis, no neutral particles are created. Otherwise, the excess energy is interpreted to originate from photons and neutral hadrons
for deposits in the ECAL and HCAL, respectively. The remaining ECAL and HCAL
3 Jet Substructure at the LHC
energy density; hadronic showers tend to occur deeper in the calorimeter and be
less dense [409]. Charged and neutral pions are used to derive this classification
and calibration, called the Local Cell Weighting (LCW). In CMS, dedicated ECAL
(based on photons) and HCAL (based on neutral kaons) calibrations are combined
to account for energy and |η|-dependent non-linearities in the hadron calorimeter
response [407]. Both ATLAS and CMS validate the performance of these calibrations with single particle studies in data [407, 412].
Different strategies are used by ATLAS and CMS to reconstruct tracks from
their inner detectors. ATLAS focuses first on maintaining a high efficiency with
a rather inclusive first pass through inner detector hits. A second step known as
ambiguity solving reduces the fake rate. In contrast, CMS uses a sequential approach
with multiple passes through the remaining inner detector hits. With each pass, the
efficiency increases while maintaining a low fake rate. Both procedures are effective
at identifying about 90% of charged pions above 1 GeV with a percent-level (or
smaller) fake rate. Lower momentum particles can be reconstructed, at the cost of
a higher fake rate and lower efficiency. Due to its weaker magnetic field, ATLAS is
able to reach low track momentum of 100 MeV for physics analysis [413], although
most jet substructure measurements and searches use a threshold of 500 MeV. In
contrast, the momentum resolution in CMS is excellent up to higher momenta than
in ATLAS. The TRT can be used to improve the momentum resolution of high p T
tracks [414], but the weaker magnetic field despite a comparable inner detector radius
is a fundamental limitation.
Both experiments have implemented dedicated strategies for track reconstruction
in high density environments such as the core of high p T jets. In such environments, pixel and strip clusters can merge resulting in a loss in tracking efficiency
and degraded resolution. ATLAS has implemented a stacked neural network (NN)
approach to examine pixel clusters to identify multi-particle clusters, estimate the
position of the particles passing through the clusters, and also predict the residual
resolution of the position estimates [415–419]. CMS has introduced a dedicated
tracking step in which a cluster splitting procedure attempts to split merged clusters
exploiting the information of the jet direction, predicting the expected cluster shape
and charge.
For particle flow in CMS, tracks and calibrated clusters are combined taking the
tracking and calorimeter resolutions into account. First, a link is created between
tracks in the central tracker and calorimeter clusters. Links are also created between
clusters in the ECAL and HCAL, if the cluster position in the ECAL is within the
cluster envelope in the less granular HCAL. Tracks with a p T uncertainty in excess of
the calorimetric energy resolution expected for charged hadrons are masked, which
allows the rate of misreconstructed tracks at large p T to be reduced.
Charged hadrons are created from ECAL and HCAL clusters, linked to tracks.
If the calibrated calorimetric energy is compatible with the corresponding track
momenta under the charged-pion hypothesis, no neutral particles are created. Otherwise, the excess energy is interpreted to originate from photons and neutral hadrons
for deposits in the ECAL and HCAL, respectively. The remaining ECAL and HCAL