Theoretical and Experimental Approaches to Dark Energy and the Cosmological Constant Problem by Ahmad Borzou

Author:Ahmad Borzou
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

6.3 Electromagnetic Calorimeter

Photons and electrons deposit their energies in the electromagnetic calorimeter (ECAL), which is located directly outside of the tracking system. A thorough review of the CMS electromagnetic calorimeter is given in [4, 5] and its layout is shown in Fig. 6.4. The electromagnetic calorimeter has two major sections. The electromagnetic barrel (EB) calorimeter has a volume of 8.14 m3 and its closest point to the z axis is at r = 129  cm. The electromagnetic endcap (EE) calorimeter is located at z = ±315  cm. Both sections are built symmetrically and homogeneously over the x–y and r–z planes. The coverage of the ECAL is |η| < 3, which is a relatively large angle. Electromagnetic calorimeters measure the energy of electrons and photons as they interact with the charged particles in the dense material of the detector volume and make “electromagnetic” showers consisting of secondary electrons and photons. Here we use the fact that charged particles decelerating through matter emit photons via bremsstrahlung radiation. These radiated photons, as well as prompt photons from collisions, decay to an electron-positron pair when passing within the material and these particles also start to lose their energy through bremsstrahlung radiation. This cycle continues until the energy of the electrons and photons falls below a threshold and the loss of their energies by ionization dominates. The characteristic thickness of the material in the ECAL is the radiation length X 0. This X 0 is the mean distance over which a high energy electron loses e −1 of its energy by bremsstrahlung radiation, and 7/9 of the mean free path for pair production for a high energy photon. The whole volume of the CMS ECAL detector is used to initiate electromagnetic showers and also as a scintillator to detect the deposited energy by particles created in the showers. The material necessary for scintillation must have a quick response, be resistant to harsh radiation, and also be compact in volume. Therefore, lead tungstate (PbWO4) was chosen for building the scintillator crystals. The barrel ECAL is made of 61,200 22 × 22 mm2 crystals where each is 23 cm long, corresponding to 25.8X 0, while the endcaps contain 7324 crystals with an area of 28.6 × 28.6 mm2 and a length of 22 cm, corresponding to 24.7X 0. In front of the endcaps there is a preshower detector (ES) to discriminate between prompt photons and pairs of photons that result from a neutral pion decay.

Fig. 6.4Layout of the electromagnetic calorimeter within the CMS detector. As shown here, the subdetector is divided into the barrel and endcap

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