6 The Discovery of the Higgs Boson at the LHC
273
Access for maintenance is very difficult, time consuming, and highly restricted.
Hence, a high degree of long-term operational reliability had to be attained,
comparable to that which is usually associated with instruments flying on space
missions.
The event selection process (called the trigger) must select among the billion
interactions that occur each second since no more than a thousand events per second
can be stored for subsequent analysis. The short time between bunch crossings,
25 ns, has major implications for the design of the readout and trigger systems. It
takes a long time to make a trigger decision, yet new events occur in every crossing
and a trigger decision must be made for every crossing; the selection process is split
in several levels. The first of these is the Level-1 trigger decision, which takes about
3 μs and selects, on average, one crossing out of 400. During this time the data
must be stored in pipelines integrated into the front-end electronics. In CMS, the
data from these selected events are then moved into a commercial farm of CPUs to
select and store about one thousand/s of the most interesting events for subsequent
analysis.
It cannot be stressed enough how important were the many years of R&D and
prototyping that preceded the start of detector construction. Technologies had to
be developed far beyond what was the state-of-the-art in early 1990s, in terms of
granularity, speed of readout, radiation tolerance, reliability, and very importantly
cost. For many detector subsystems, there were initially several technologies
considered, as it was far from certain which technologies would be able to attain the
required performance. In many cases several variants were developed, prototyped
and tested, before choosing the one best able to fulfill the stringent requirements.
This involved building and testing increasingly more realistic and larger prototypes,
in a process that involved industry from the outset. This took place over a number
of years before construction commenced in the second half of the 1990s.
In the 1990’s the two collaborations, ATLAS and CMS, grew rapidly in terms
of people and institutes. Today each comprises over 3500 scientists and engineers,
from over 150 institutions in more than 40 countries. The talents and resources of
all these scientists were needed to build the experiments, which are now performing
extraordinarily well at the LHC.
The single most important aspect of the experiment design and layout is the
magnetic field configuration for the identification of muons and the measurement
of their momentum. Large bending power is needed to measure precisely the
momentum of charged particles. This forces a choice of superconducting technology
for the magnets. The design configurations chosen by ATLAS and CMS are
discussed below.
6.4.2 The ATLAS Detector
The design of the ATLAS detector [35], shown in Fig. 6.3 (top), is based on
a novel superconducting air-core toroid magnet system, containing ~80 km of
273
Access for maintenance is very difficult, time consuming, and highly restricted.
Hence, a high degree of long-term operational reliability had to be attained,
comparable to that which is usually associated with instruments flying on space
missions.
The event selection process (called the trigger) must select among the billion
interactions that occur each second since no more than a thousand events per second
can be stored for subsequent analysis. The short time between bunch crossings,
25 ns, has major implications for the design of the readout and trigger systems. It
takes a long time to make a trigger decision, yet new events occur in every crossing
and a trigger decision must be made for every crossing; the selection process is split
in several levels. The first of these is the Level-1 trigger decision, which takes about
3 μs and selects, on average, one crossing out of 400. During this time the data
must be stored in pipelines integrated into the front-end electronics. In CMS, the
data from these selected events are then moved into a commercial farm of CPUs to
select and store about one thousand/s of the most interesting events for subsequent
analysis.
It cannot be stressed enough how important were the many years of R&D and
prototyping that preceded the start of detector construction. Technologies had to
be developed far beyond what was the state-of-the-art in early 1990s, in terms of
granularity, speed of readout, radiation tolerance, reliability, and very importantly
cost. For many detector subsystems, there were initially several technologies
considered, as it was far from certain which technologies would be able to attain the
required performance. In many cases several variants were developed, prototyped
and tested, before choosing the one best able to fulfill the stringent requirements.
This involved building and testing increasingly more realistic and larger prototypes,
in a process that involved industry from the outset. This took place over a number
of years before construction commenced in the second half of the 1990s.
In the 1990’s the two collaborations, ATLAS and CMS, grew rapidly in terms
of people and institutes. Today each comprises over 3500 scientists and engineers,
from over 150 institutions in more than 40 countries. The talents and resources of
all these scientists were needed to build the experiments, which are now performing
extraordinarily well at the LHC.
The single most important aspect of the experiment design and layout is the
magnetic field configuration for the identification of muons and the measurement
of their momentum. Large bending power is needed to measure precisely the
momentum of charged particles. This forces a choice of superconducting technology
for the magnets. The design configurations chosen by ATLAS and CMS are
discussed below.
6.4.2 The ATLAS Detector
The design of the ATLAS detector [35], shown in Fig. 6.3 (top), is based on
a novel superconducting air-core toroid magnet system, containing ~80 km of