16 Integration of Detectors into a Large Experiment: Examples from ATLAS. . .
735
The accuracy of the stand-alone muon momentum measurement necessitates a
precision of 30 μm on the relative alignment of chambers both within each projective tower and between consecutive layers in immediately adjacent towers. The
internal deformations and relative positions of the MDT chambers are monitored by
approximately 12,000 precision-mounted alignment sensors, all based on the optical
monitoring of deviations from straight lines. Because of geometrical constraints,
the reconstruction and/or monitoring of the chamber positions rely on somewhat
different strategies and sensor types in the end-cap and barrel regions, respectively.
The accuracy required for the relative positioning of non-adjacent towers to
obtain adequate mass resolution for multi-muon final states, lies in the few
millimetre range. This initial positioning accuracy is approximately established
during the installation of the chambers. Ultimately, the relative alignment of the
barrel and forward regions of the muon spectrometer, of the calorimeters and of the
tracker will rely on high-momentum muon trajectories.
For magnetic field reconstruction, the goal is to determine the bending power
along the muon trajectory to a few parts in a thousand. The field is continuously
monitored by a total of approximately 1800 Hall sensors distributed throughout the
spectrometer volume. Their readings are compared with magnetic-field simulations
and used for reconstructing the position of the toroid coils in space, as well as to
account for magnetic perturbations induced by the tile calorimeter and other nearby
metallic structures.
The muon system consists of three large superconducting air-core toroid magnets, which are instrumented with different types of chambers to provide the two
needed functions, namely high-precision tracking and triggering. The central (or
barrel) region, |η| < 1.0, is covered by a large barrel magnet consisting of eight
coils which surround the hadron calorimeter. In this region, tracks are measured in
chambers arranged in three cylindrical layers (stations) around the beam axis. In
the end-cap region, 1.4 < |η| < 2.7, muon tracks are bent in two smaller end-cap
magnets inserted into both ends of the barrel toroid. The intermediate (transition)
region, 1.0 < |η| < 1.4, is less straightforward, since here the barrel and endcap fields overlap, thus partially reducing the bending power. To keep a uniform
resolution in this region, tracking chambers are place in strategic places to improve
the quality and accuracy of the measurement. Due to financial constraints, one out
of three sets of chambers in this region has been staged, thus leading to an inferior
performance in the transition region for the first years of data-taking.
The layout of the ATLAS muon spectrometer system is shown in Fig. 16.15.
A total of four types of detectors are used, the choice of technology being driven
by the very large surface to be covered, by trigger and precision measurement
requirements, and by the different radiation environments. Resistive Plate Chambers
(RPC) in the barrel region (|η| < 1.05) and Thin Gap Chambers (TGC) in the endcap regions (1.05 < |η| < 2.4) are used for triggering purposes. These chambers
provide a fast response with good time resolution but rather coarse position
resolution. The precision measurements are performed by Monitored Drift Tubes
(MDT) over most of the coverage. In the regions at large |η|, where background
735
The accuracy of the stand-alone muon momentum measurement necessitates a
precision of 30 μm on the relative alignment of chambers both within each projective tower and between consecutive layers in immediately adjacent towers. The
internal deformations and relative positions of the MDT chambers are monitored by
approximately 12,000 precision-mounted alignment sensors, all based on the optical
monitoring of deviations from straight lines. Because of geometrical constraints,
the reconstruction and/or monitoring of the chamber positions rely on somewhat
different strategies and sensor types in the end-cap and barrel regions, respectively.
The accuracy required for the relative positioning of non-adjacent towers to
obtain adequate mass resolution for multi-muon final states, lies in the few
millimetre range. This initial positioning accuracy is approximately established
during the installation of the chambers. Ultimately, the relative alignment of the
barrel and forward regions of the muon spectrometer, of the calorimeters and of the
tracker will rely on high-momentum muon trajectories.
For magnetic field reconstruction, the goal is to determine the bending power
along the muon trajectory to a few parts in a thousand. The field is continuously
monitored by a total of approximately 1800 Hall sensors distributed throughout the
spectrometer volume. Their readings are compared with magnetic-field simulations
and used for reconstructing the position of the toroid coils in space, as well as to
account for magnetic perturbations induced by the tile calorimeter and other nearby
metallic structures.
The muon system consists of three large superconducting air-core toroid magnets, which are instrumented with different types of chambers to provide the two
needed functions, namely high-precision tracking and triggering. The central (or
barrel) region, |η| < 1.0, is covered by a large barrel magnet consisting of eight
coils which surround the hadron calorimeter. In this region, tracks are measured in
chambers arranged in three cylindrical layers (stations) around the beam axis. In
the end-cap region, 1.4 < |η| < 2.7, muon tracks are bent in two smaller end-cap
magnets inserted into both ends of the barrel toroid. The intermediate (transition)
region, 1.0 < |η| < 1.4, is less straightforward, since here the barrel and endcap fields overlap, thus partially reducing the bending power. To keep a uniform
resolution in this region, tracking chambers are place in strategic places to improve
the quality and accuracy of the measurement. Due to financial constraints, one out
of three sets of chambers in this region has been staged, thus leading to an inferior
performance in the transition region for the first years of data-taking.
The layout of the ATLAS muon spectrometer system is shown in Fig. 16.15.
A total of four types of detectors are used, the choice of technology being driven
by the very large surface to be covered, by trigger and precision measurement
requirements, and by the different radiation environments. Resistive Plate Chambers
(RPC) in the barrel region (|η| < 1.05) and Thin Gap Chambers (TGC) in the endcap regions (1.05 < |η| < 2.4) are used for triggering purposes. These chambers
provide a fast response with good time resolution but rather coarse position
resolution. The precision measurements are performed by Monitored Drift Tubes
(MDT) over most of the coverage. In the regions at large |η|, where background