16 Integration of Detectors into a Large Experiment: Examples from ATLAS. . .
703
The first level of trigger (or L1 trigger) in the ATLAS and CMS experiments
is based on custom-built hardware extracting as quickly as possible the necessary information from the calorimeters and muon spectrometer and provides
a decision in 2.5 to 3 μs, during which most of the time is spent in signal
transmission from the detector (to make the trigger decision) and to the detector
(to propagate this decision back to the front-end electronics). This reduces the
event rate to about 100 kHz with a very high efficiency for most of the events
of interest for physics analysis. During this very long (for relativistic particles)
time, the hundreds of thousands of very sensitive and sophisticated radiation-hard
electronics chips situated throughout the detectors have to store the successive
waves of data produced every 25 ns in pipelines and keep track of the time stamps
of all the data so that the correct information can be retrieved when the decision
from the L1 trigger is received. The synchronisation of a vast number of frontend electronics channels over very large volumes has been a major challenge for
the design of the overall trigger and timing control of the experiments.
16.2.2 Magnet Systems
The magnet systems of the ATLAS and CMS experiments [14] were at the heart
of the conceptual design of the detector components and they have driven many
of the fundamental geometrical parameters and of the broad technology choices
for the components of the detectors. The large bending power required to measure
muons of 1 TeV momentum with a precision of 10% has led both collaborations
to choose superconducting technology for their magnets to limit the size of the
experimental caverns and the overall costs. The choice of magnet system for CMS
was based on the elegant idea of fulfilling at the same time with one magnet a high
magnetic field in the tracker volume for all precision momentum measurements,
including muons, and a high enough return flux in the iron outside the magnet
to provide a muon trigger and a second muon momentum measurement for the
experiment. This is achieved with a single solenoid of a large enough radius to
contain most of the CMS calorimeter system. In contrast, the choice of magnet
system for ATLAS was driven by the requirement to achieve a high-precision standalone momentum measurement of muons over as large an acceptance in momentum
and η-coverage as possible. This is achieved using an arrangement of a smallradius thin-walled solenoid, integrated into the cryostat of the barrel electromagnetic
calorimeter, surrounded by a system of three large air-core toroids, situated outside
the ATLAS calorimeter systems and generating the magnetic field for the muon
spectrometer. The main parameters of these magnet systems are listed in Table 16.3
and their stored energies are compared to those of previous large-scale magnets in
high-energy physics experiments in Fig. 16.7.
In CMS, the length of the solenoid was driven by the need to achieve excellent
momentum resolution over the required η-coverage and its diameter was chosen
such that most of the calorimetry is contained inside the coil. In ATLAS, the
703
The first level of trigger (or L1 trigger) in the ATLAS and CMS experiments
is based on custom-built hardware extracting as quickly as possible the necessary information from the calorimeters and muon spectrometer and provides
a decision in 2.5 to 3 μs, during which most of the time is spent in signal
transmission from the detector (to make the trigger decision) and to the detector
(to propagate this decision back to the front-end electronics). This reduces the
event rate to about 100 kHz with a very high efficiency for most of the events
of interest for physics analysis. During this very long (for relativistic particles)
time, the hundreds of thousands of very sensitive and sophisticated radiation-hard
electronics chips situated throughout the detectors have to store the successive
waves of data produced every 25 ns in pipelines and keep track of the time stamps
of all the data so that the correct information can be retrieved when the decision
from the L1 trigger is received. The synchronisation of a vast number of frontend electronics channels over very large volumes has been a major challenge for
the design of the overall trigger and timing control of the experiments.
16.2.2 Magnet Systems
The magnet systems of the ATLAS and CMS experiments [14] were at the heart
of the conceptual design of the detector components and they have driven many
of the fundamental geometrical parameters and of the broad technology choices
for the components of the detectors. The large bending power required to measure
muons of 1 TeV momentum with a precision of 10% has led both collaborations
to choose superconducting technology for their magnets to limit the size of the
experimental caverns and the overall costs. The choice of magnet system for CMS
was based on the elegant idea of fulfilling at the same time with one magnet a high
magnetic field in the tracker volume for all precision momentum measurements,
including muons, and a high enough return flux in the iron outside the magnet
to provide a muon trigger and a second muon momentum measurement for the
experiment. This is achieved with a single solenoid of a large enough radius to
contain most of the CMS calorimeter system. In contrast, the choice of magnet
system for ATLAS was driven by the requirement to achieve a high-precision standalone momentum measurement of muons over as large an acceptance in momentum
and η-coverage as possible. This is achieved using an arrangement of a smallradius thin-walled solenoid, integrated into the cryostat of the barrel electromagnetic
calorimeter, surrounded by a system of three large air-core toroids, situated outside
the ATLAS calorimeter systems and generating the magnetic field for the muon
spectrometer. The main parameters of these magnet systems are listed in Table 16.3
and their stored energies are compared to those of previous large-scale magnets in
high-energy physics experiments in Fig. 16.7.
In CMS, the length of the solenoid was driven by the need to achieve excellent
momentum resolution over the required η-coverage and its diameter was chosen
such that most of the calorimetry is contained inside the coil. In ATLAS, the