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P. Jenni and T. S. Virdee
lower end of the remaining open mass range was considered to be especially difficult
in hadron colliders. Hence the LHC experiments had to pay particular attention to
the performance requirements imposed by the search for the Higgs boson in this
low mass range. As a consequence much importance was placed on the tracking
(inner and muon), as well as the magnetic field strength, and the electromagnetic
calorimeters.
The search for the high-mass Higgs boson, particles predicted by SUSY, and
other exotic states mentioned above, required excellent resolution for jets and
missing transverse momentum (p T
miss ), requiring full solid angle calorimeter
coverage.
A saying prevalent in the late 1980’s and early 1990’s captured the challenge:
‘We think we know how to build a high energy, high luminosity hadron collider—
but we don’t have the technology to build a detector for it’. Making discoveries in
the unprecedented high collision rate environment, generated by around one billion
proton-proton interactions per second, with several tens of simultaneous collisions
per bunch crossing, would require extraordinary detectors. Many technical, financial, industrial and human challenges lay ahead, which were all overcome, to yield
experiments of unprecedented complexity and power. A flavour can be attained from
articles in reference [35].
At the Evian meeting in 1992 four experiment designs were presented: two
deploying toroids (one with a superconducting magnet in the barrel) and two
deploying superconducting high-field solenoids. The choice of the magnetic field
configuration determined the overall design of the experiments.
The collaborations deploying toroids merged to form the ATLAS Collaboration.
The ATLAS design [35] was based on a very large superconducting air-core toroid
for the measurement of muons, and supplemented by a superconducting 2 Tesla
solenoid to provide the magnetic field for inner tracking and by a liquid-argon/lead
electromagnetic calorimeter with a novel “accordion” geometry. The CMS design
[36] was based on a single large-bore, long, high-field solenoid for analyzing muons,
together with powerful microstrip-based inner tracking and an electromagnetic
calorimeter comprising scintillating crystals.
On top of the selected event of interest, an average of up to around 40 other
proton-proton events are superimposed. These superposed events are referred to as
minimum-bias events, because no selection is made. Thus thousands of particles
emerge from the interaction region every 25 ns where one nanosecond (ns) = 10 −9 s.
Hence the products of an interaction under study can be confused with those from
other interactions in the same bunch crossing. This problem, known as pileup,
clearly becomes more severe if the response time of a detector element and its
electronic signal is longer than 25 ns. The effect of pileup can be reduced by using
highly granular detectors with fast, short duration, signals, giving low occupancy
(i.e., a low probability that a detector element will give a signal) at the expense
of having large numbers of detector channels. The resulting millions of electronic
channels require very good time synchronization.
The large flux of particles emanating from the interaction region creates a highradiation environment requiring radiation-hard detectors and front-end electronics.
P. Jenni and T. S. Virdee
lower end of the remaining open mass range was considered to be especially difficult
in hadron colliders. Hence the LHC experiments had to pay particular attention to
the performance requirements imposed by the search for the Higgs boson in this
low mass range. As a consequence much importance was placed on the tracking
(inner and muon), as well as the magnetic field strength, and the electromagnetic
calorimeters.
The search for the high-mass Higgs boson, particles predicted by SUSY, and
other exotic states mentioned above, required excellent resolution for jets and
missing transverse momentum (p T
miss ), requiring full solid angle calorimeter
coverage.
A saying prevalent in the late 1980’s and early 1990’s captured the challenge:
‘We think we know how to build a high energy, high luminosity hadron collider—
but we don’t have the technology to build a detector for it’. Making discoveries in
the unprecedented high collision rate environment, generated by around one billion
proton-proton interactions per second, with several tens of simultaneous collisions
per bunch crossing, would require extraordinary detectors. Many technical, financial, industrial and human challenges lay ahead, which were all overcome, to yield
experiments of unprecedented complexity and power. A flavour can be attained from
articles in reference [35].
At the Evian meeting in 1992 four experiment designs were presented: two
deploying toroids (one with a superconducting magnet in the barrel) and two
deploying superconducting high-field solenoids. The choice of the magnetic field
configuration determined the overall design of the experiments.
The collaborations deploying toroids merged to form the ATLAS Collaboration.
The ATLAS design [35] was based on a very large superconducting air-core toroid
for the measurement of muons, and supplemented by a superconducting 2 Tesla
solenoid to provide the magnetic field for inner tracking and by a liquid-argon/lead
electromagnetic calorimeter with a novel “accordion” geometry. The CMS design
[36] was based on a single large-bore, long, high-field solenoid for analyzing muons,
together with powerful microstrip-based inner tracking and an electromagnetic
calorimeter comprising scintillating crystals.
On top of the selected event of interest, an average of up to around 40 other
proton-proton events are superimposed. These superposed events are referred to as
minimum-bias events, because no selection is made. Thus thousands of particles
emerge from the interaction region every 25 ns where one nanosecond (ns) = 10 −9 s.
Hence the products of an interaction under study can be confused with those from
other interactions in the same bunch crossing. This problem, known as pileup,
clearly becomes more severe if the response time of a detector element and its
electronic signal is longer than 25 ns. The effect of pileup can be reduced by using
highly granular detectors with fast, short duration, signals, giving low occupancy
(i.e., a low probability that a detector element will give a signal) at the expense
of having large numbers of detector channels. The resulting millions of electronic
channels require very good time synchronization.
The large flux of particles emanating from the interaction region creates a highradiation environment requiring radiation-hard detectors and front-end electronics.