6 The Discovery of the Higgs Boson at the LHC
275
The electromagnetic calorimeter consists of a lead/liquid-argon sampling
calorimeter in a novel ‘accordion’ geometry. A plastic scintillator—iron sampling
hadron calorimeter, also with a novel geometry, is used in the barrel part of
the experiment. Liquid-argon hadronic calorimeters are employed in the endcap
regions near the beam axis. The electromagnetic and hadronic calorimeters have
almost 200,000 and 20,000 cells, respectively, and are in an almost field-free region
between the toroids and the solenoid.
The momentum of the muons is precisely measured after traversing the calorimeters in the air-core toroid field over a distance of ~5 m. About 1200 large muon
chambers of various shapes, with a total area of 5000 m 2 , measure the impact
position with an accuracy of better than 0.1 mm. Another set of about 4200 fast
chambers is used to provide the “trigger”.
The reconstruction of all charged particles, and that of displaced vertices, is
achieved in the inner detector, which combines highly granular pixel (50 × 400 μm 2
elements, leading to 80 million channels) and microstrip (13 cm × 80 μm elements,
leading to six million channels) silicon semiconductor sensors placed close to the
beam axis, and a ‘straw tube’ gaseous detector (350,000 channels) which provides
about 30–40 signal hits per track. The latter also helps in the identification of
electrons using information from the effects of transition radiation.
The air-core magnet system allows a relatively lightweight overall structure
leading to a detector weighing 7000 tons. The muon spectrometer defines the overall
diameter of 25 m and length of 44 m of the ATLAS detector.
6.4.3 The CMS Detector
The design of the CMS detector [36], shown in Fig. 6.3 (bottom), is based on a
state-of-the-art superconducting high-field solenoid, which first reached the design
field of 4 Tesla in 2006.
The solenoid generates a uniform magnetic field parallel to the direction of the
LHC beams. The field is produced by a current of 20 kA flowing through a reinforced Nb-Ti superconducting coil built in four layers. Economic and transportation
constraints limited the outer radius of the coil to 3 m and its length to 13 m. The
field is returned through a 1.5 m thick iron yoke, which houses four muon stations
to ensure robustness of identification and measurement and full geometric coverage.
The CMS design was first optimized to cleanly identify, trigger and measure
muons, e.g. arising from processes such as H → ZZ ( ∗ ) → 4 μ and few TeV mass
Z’ → 2 μ, over a wide range of momenta. The muons trace a spiral path in the
magnetic field and are identified and reconstructed in ~3000 m 2 of gas chambers
interleaved with the iron plates in the return yoke. Another ~500 fast chambers are
used to provide a second system of detectors for the Level-1 muon trigger.
The next design priority was driven by the search for the decay of the SM Higgs
boson into two photons. A new type of scintillating crystal was selected: leadtungstate (PbWO 4 ) crystal.
275
The electromagnetic calorimeter consists of a lead/liquid-argon sampling
calorimeter in a novel ‘accordion’ geometry. A plastic scintillator—iron sampling
hadron calorimeter, also with a novel geometry, is used in the barrel part of
the experiment. Liquid-argon hadronic calorimeters are employed in the endcap
regions near the beam axis. The electromagnetic and hadronic calorimeters have
almost 200,000 and 20,000 cells, respectively, and are in an almost field-free region
between the toroids and the solenoid.
The momentum of the muons is precisely measured after traversing the calorimeters in the air-core toroid field over a distance of ~5 m. About 1200 large muon
chambers of various shapes, with a total area of 5000 m 2 , measure the impact
position with an accuracy of better than 0.1 mm. Another set of about 4200 fast
chambers is used to provide the “trigger”.
The reconstruction of all charged particles, and that of displaced vertices, is
achieved in the inner detector, which combines highly granular pixel (50 × 400 μm 2
elements, leading to 80 million channels) and microstrip (13 cm × 80 μm elements,
leading to six million channels) silicon semiconductor sensors placed close to the
beam axis, and a ‘straw tube’ gaseous detector (350,000 channels) which provides
about 30–40 signal hits per track. The latter also helps in the identification of
electrons using information from the effects of transition radiation.
The air-core magnet system allows a relatively lightweight overall structure
leading to a detector weighing 7000 tons. The muon spectrometer defines the overall
diameter of 25 m and length of 44 m of the ATLAS detector.
6.4.3 The CMS Detector
The design of the CMS detector [36], shown in Fig. 6.3 (bottom), is based on a
state-of-the-art superconducting high-field solenoid, which first reached the design
field of 4 Tesla in 2006.
The solenoid generates a uniform magnetic field parallel to the direction of the
LHC beams. The field is produced by a current of 20 kA flowing through a reinforced Nb-Ti superconducting coil built in four layers. Economic and transportation
constraints limited the outer radius of the coil to 3 m and its length to 13 m. The
field is returned through a 1.5 m thick iron yoke, which houses four muon stations
to ensure robustness of identification and measurement and full geometric coverage.
The CMS design was first optimized to cleanly identify, trigger and measure
muons, e.g. arising from processes such as H → ZZ ( ∗ ) → 4 μ and few TeV mass
Z’ → 2 μ, over a wide range of momenta. The muons trace a spiral path in the
magnetic field and are identified and reconstructed in ~3000 m 2 of gas chambers
interleaved with the iron plates in the return yoke. Another ~500 fast chambers are
used to provide a second system of detectors for the Level-1 muon trigger.
The next design priority was driven by the search for the decay of the SM Higgs
boson into two photons. A new type of scintillating crystal was selected: leadtungstate (PbWO 4 ) crystal.