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Figure 3.6: Transverse section illustrating the geometry of the CMS ECAL [16].Preshower DetectorOne of the major reducible backgrounds in the H → γγ decay comes from the misidentificationof photon pairs from the decay of high-energy neutral pions as single highenergyphotons. Especially in the endcap region, the spatial resolution of the crystalcalorimeter is insufficient to make this distinction. Thus, a special preshower detector(ES) is inserted before the endcap calorimeters. It consists of two layers, eachcomprised by a lead radiator to induce showering and a silicon strip detector with1.9 mm strip pitch that measure the energy deposited and the lateral shower profile.The thickness of the preshower absorbers (about 3 X 0 ) was chosen as a compromisebetween the requirement to reliably induce showering for photons of the relevant energyrange versus the negative impact of the preshower detector’s less precise energymeasurement on the total resolution of the crystal calorimeter [18].ECAL PerformanceThe relative energy resolution of the ECAL is usually described through the widthGaussian distribution with contributions from three sources:( σE) 2=( SE) 2 ( ) N√E 2+ + C 2 (3.2)S is the stochastic term, N the noise and C the constant term. Figure 3.7 showssome values obtained from beam tests.25