598
R. Frühwirth et al.
similar way, the tracks reaching the time-of-flight (TOF) detector are matched to
TOF clusters.
• At the final stage of the track reconstruction, all tracks in both ITS and TPC
subdetectors are propagated inwards and refitted one last time to determine the
final estimate of the track position, direction, inverse curvature, and its associated
covariance matrix.
The final interaction vertex is then re-determined using the all tracks reconstructed
in TPC and ITS. The precise vertex fit is performed using track weighting to
suppress the contribution of any remaining outliers. For data-taking conditions
where a high pileup rate is expected, a more robust version of vertex finding inspired
by the algorithm described in [132] is used. It is based on iterative vertex finding and
fitting using Tukey bisquare weights to suppress outliers. The algorithm stops when
no more vertices are identified in the scan along the beam direction. Once the tracks
and the interaction vertex have been found, a search for photon conversions and
decays of strange hadrons such as K 0
S and 0 concludes the central-barrel tracking
procedure.
13.3.2 ATLAS
ATLAS [145] is the largest of the four LHC experiments, measuring 25 m in
diameter and 44 m in length. Its magnet system is composed of a Central Solenoid
Magnet with a 2 T field, a Barrel Toroid and an Endcap Toroids with 4 T each.
The Inner Detector (ID) is very compact and highly sensitive in order to measure
accurately the decay products of each collision. It consists of three different
systems of sensors immersed in the solenoid magnetic field: the Pixel Detector,
the Semiconductor Tracker (SCT), and the Transition Radiation Tracker (TRT).
The Pixel Detector is situated closest to the interaction point and has the highest
granularity with about 80 million readout channels. The intrinsic spatial resolution
of the Pixel Detector sensors is 10 μm in r–φ and 115 μm in z. The SCT is a silicon
microstrip detector surrounding the Pixel Detector. It provides eight measurements
per track with an overall resolution of 16 μm in r–φ and 580 μm in z. In the
outermost region, the TRT is placed. It is a light-weight detector composed of
proportional gas counters (70% Xe, 27% CO 2 and 3% O 2 straws) embedded in a
radiator material and its operational drift radius accuracy is about 130 μm. The TRT
contributes both to the track pattern recognition stage, featuring typically around 30
hits per track, and to particle identification.
The basic concepts of the ATLAS track reconstruction are described in [146,
147]. The tracking in the ID consists of two principal sequences: an initial inside-out
tracking, and a subsequent outside-in tracking. Inside-out tracking starts with space
point formation in the silicon part of the ID. Using the space points, track seeds
are generated with or without a constraint on the longitudinal vertex position. The
seeds are then followed through the SCT by a combinatorial Kalman filter/smoother.
R. Frühwirth et al.
similar way, the tracks reaching the time-of-flight (TOF) detector are matched to
TOF clusters.
• At the final stage of the track reconstruction, all tracks in both ITS and TPC
subdetectors are propagated inwards and refitted one last time to determine the
final estimate of the track position, direction, inverse curvature, and its associated
covariance matrix.
The final interaction vertex is then re-determined using the all tracks reconstructed
in TPC and ITS. The precise vertex fit is performed using track weighting to
suppress the contribution of any remaining outliers. For data-taking conditions
where a high pileup rate is expected, a more robust version of vertex finding inspired
by the algorithm described in [132] is used. It is based on iterative vertex finding and
fitting using Tukey bisquare weights to suppress outliers. The algorithm stops when
no more vertices are identified in the scan along the beam direction. Once the tracks
and the interaction vertex have been found, a search for photon conversions and
decays of strange hadrons such as K 0
S and 0 concludes the central-barrel tracking
procedure.
13.3.2 ATLAS
ATLAS [145] is the largest of the four LHC experiments, measuring 25 m in
diameter and 44 m in length. Its magnet system is composed of a Central Solenoid
Magnet with a 2 T field, a Barrel Toroid and an Endcap Toroids with 4 T each.
The Inner Detector (ID) is very compact and highly sensitive in order to measure
accurately the decay products of each collision. It consists of three different
systems of sensors immersed in the solenoid magnetic field: the Pixel Detector,
the Semiconductor Tracker (SCT), and the Transition Radiation Tracker (TRT).
The Pixel Detector is situated closest to the interaction point and has the highest
granularity with about 80 million readout channels. The intrinsic spatial resolution
of the Pixel Detector sensors is 10 μm in r–φ and 115 μm in z. The SCT is a silicon
microstrip detector surrounding the Pixel Detector. It provides eight measurements
per track with an overall resolution of 16 μm in r–φ and 580 μm in z. In the
outermost region, the TRT is placed. It is a light-weight detector composed of
proportional gas counters (70% Xe, 27% CO 2 and 3% O 2 straws) embedded in a
radiator material and its operational drift radius accuracy is about 130 μm. The TRT
contributes both to the track pattern recognition stage, featuring typically around 30
hits per track, and to particle identification.
The basic concepts of the ATLAS track reconstruction are described in [146,
147]. The tracking in the ID consists of two principal sequences: an initial inside-out
tracking, and a subsequent outside-in tracking. Inside-out tracking starts with space
point formation in the silicon part of the ID. Using the space points, track seeds
are generated with or without a constraint on the longitudinal vertex position. The
seeds are then followed through the SCT by a combinatorial Kalman filter/smoother.