Discovery Magazine

CMS Hadron Calorimeter Project

Design, construction, commissioning, maintenance and operation of the Compact Muon Solenoid hadron calorimeter calibration system using Laser and LED light.

The Compact Muon Solenoid calorimeter system measures quark, gluon and neutrino directions and energies by measuring the energy and direction of particle jets and of the ETmiss flow. This determination of missing energy will also form a crucial signature for new particles and phenomena, such as will be encountered in the searches for the supersymmetric partners of quarks and gluons. The hadron calorimeter also helps to identify electrons, photons and muons in conjunction with the electromagnetic calorimeter and the muon system. Thus the Hadron Calorimeter is an essential subsystem of the Compact Muon Solenoid detector and will contribute to most if not all of Compact Muon Solenoid physics studies.

The central pseudorapidity range, |η|<3.0, is covered by the barrel and endcap calorimeter system consisting of a hermetic crystal electromagnetic calorimeter (ECAL) followed by the hadron calorimeter barrel (HB) and endcap (HE). Both the barrel and endcap calorimeters experience the 4 Tesla field of the Compact Muon Solenoid solenoid and hence are necessarily made of non-magnetic material. The central hadron calorimeter is a sampling calorimeter: it consists of active material inserted between brass absorber plates. The absorber plates are 5cm thick in the barrel and 8cm thick in the endcap. The active elements of the entire central hadron calorimeter are 4mm-thick plastic scintillator tiles read out using wavelength-shifting (WLS) plastic fibers. The barrel hadron calorimeter is about 79cm deep, which at η = 0 is 5.15 nuclear interaction lengths in thickness. This is somewhat thin, as is the transition region between barrel and endcap. To ensure adequate sampling depth for the entire |η|<3.0 region, the first muon absorber layer is instrumented with scintillator tiles to form an outer hadronic calorimeter (HO).

To extend the hermeticity of the central hadron calorimeter system to 5 units of pseudorapidity (as required for a good ETmiss measurement), Compact Muon Solenoid employs a separate forward hadron calorimeter (HF) located 6m downstream of the HE endcaps. The HF calorimeter covers the region 3.0 < |η| < 5.0. It uses quartz fibers as the active medium, embedded in an iron absorber matrix. The HF is located in a very high radiation and a very high rate environment. Because of the quartz fiber active element, it is predominantly sensitive to Cerenkov light from neutral pions. This leads to its having the unique and desirable feature of a very localized response to hadronic showers.