Chapter V: The Compact Muon Solenoid experiment
5.2 Electromagnetic calorimeter
Within the superconducting solenoid volume and just outside of the tracker, lies the electromagnetic calorimeter (ECAL) schematically pictured in Fig.5.4.
The ECAL is a hermetic homogenous calorimeter composed of 61,200 lead- tungstate (PbWO4) scintillating crystals mounted in the barrel covering|η|<
1.479, and 7,324 crystals mounted in each of the two endcap disks cover-
Crystals in a
supermodule Preshower
Supercrystals
Modules
Preshower
End-cap crystals Dee
Figure 5.4: Layout of the CMS ECAL, showing the barrel supermodules, the two endcaps, and the preshower detectors. The ECAL barrel coverage is up to|η| = 1.48; the endcaps extend the coverage to|η| = 3.0; the preshower detector fiducial area is approximately 1.65<|η|<2.6 [116].
ing 1.479 < |η| < 3.0. The crystals in the barrel are arranged in a quasi- projective geometry, meaning that they point back to the center of the de- tector. The high-density (8.28 g/cm3), short radiation length (X0 =0.89 cm), and small Moli´ere radius (RM= 2.2 cm) of PbWO4allow the construction of a compact calorimeter with fine granularity.
The crystal length of 23 (22) cm, corresponding to 25.8 (24.7) radiation lengths in the barrel (endcaps), is sufficient to contain more than 98% of the energy of electrons and photons up to 1 TeV. The crystal material also amounts to about one nuclear interaction length, causing about two thirds of the hadrons to start showering in the ECAL.
The barrel crystal front face has an area of 2.2×2.2 cm2, equivalent to 0.0174×0.0174 in the (η,φ) plane, while in the endcaps, the crystals are ar- ranged instead in a rectangular (x,y) grid, with a front-face area of 2.9×2.9 cm2. The crystal transverse size in the barrel matches the small Moli`ere ra- dius of PbWO4 (2.2 cm). This fine transverse granularity makes it possible to fully resolve hadron and photon energy deposits as close as 5 cm.
The PbWO4crystals emit predominantly blue scintillation light with a broad
maximum at wavelengths 420–430 nm. The quantum efficiency and surface coverage of the photodetectors are such that a particle depositing 1 MeV of energy in a crystal produces an average signal of about 4.5 photoelectrons.
The ECAL barrel energy resolution for electrons is measured in an electron test beam to be [117,116],
σE
E = √ S
E(GeV) ⊕ N
E(GeV) ⊕C (5.1)
= √2.8%
E(GeV) ⊕E(12%GeV) ⊕0.3% (5.2) where the three contributions are the stochastic, noise, and constant terms.
The actual energy resolution in CMS for electrons and photons are mea- sured using known resonances, such as Z → e+e− and H → γγ, in data and simulation. Fig.5.5 (left) shows an example of the Z → e+e− invari- ant mass distibutons, in which each electron is well measured and has a single-cluster supercluster in the barrel. The distributions in data and in simulation are fitted with a Breit–Wigner function convolved with a Crystal Ball function [118],
P(me+e−|mZ,ΓZ,α,n,mCB,σCB) =BW(me+e−|mZ,ΓZ)⊗fCB(me+e−|α,n,mCB,σCB), (5.3)
wheremZandΓZare fixed to the nominal values of 91.188 GeV and 2.485 GeV [119].
The effective resolutionσeff, defined as the half-width of the narrowest inter- val containing 68.3% of the distribution, in data for the Z→e+e−invariant mass in this category is 1.13±0.01 GeV (or about 1%). Considering only the Gaussian core of the distribution, the resolution isσCB =1.00±0.01 GeV.
Since there is excellent agreement between data and simulation for recon- structed photons [120], the energy resolution of photons in simulated events provides an accurate estimate of their resolution in data. Fig. 5.5 (right) shows the distribution of reconstructed energy divided by the true energy, Emeas/Etrue, of photons in simulated H →γγevents that pass the selection requirements given in Ref. [121], with 0.2 <|η|<0.3 and R9≥0.94, where theR9is the energy sum of the 3×3 crystals centered on the most energetic crystal in the supercluster divided by the energy of the supercluster1 The σeff in simulated H→γγ events forEmeas/Etruein this category is 1%.
1The showers of photons that convert before reaching the calorimeter have wider trans- verse profiles and lower values ofR9than those of unconverted photons.
Events / 1 GeV
0 2000 4000 6000 8000 10000 12000 14000 16000
e-
e+
→ Z
BGBG
0.01 GeV
± = 91.04 mpeak
0.01 GeV
± = 1.20 σeff 0.01 GeV
± = 91.09 mpeak
0.01 GeV
± = 1.13 σeff
Simulation Data
a) (8 TeV) 19.7 fb
CMS
(GeV) mee
75 80 85 90 95 100 105
Data/simulation 0.7 0.80.91.11 1.21.3 1.4 1.5 1.6
/Etrue
Emeas
0.8 0.85 0.9 0.95 1 1.05 1.1
Events/0.0025
0 500 1000 1500 2000 2500 3000 3500 4000 4500
| < 0.3 η
≤ | 0.2
0.94
9≥ R
= 0.010 σeff
= 0.009 σHM
8 TeV
CMS
Simulation
| < 0.3 η
≤ | 0.2
0.94
9≥ R
= 0.010 σeff
= 0.009 σHM
(MC) γ γ
→ H
Figure 5.5: (Left) Dielectron invariant mass distribution from Z → e+e− events in data (solid squares) compared to simulation (open circles) fit- ted with a convolution of a Breit–Wigner function and a Crystal Ball func- tion [118], for the best-resolved event category with two well-measured single-cluster electrons in the barrel. The masses at which the fitting func- tions have their maximum values, termedmpeak, and the effective standard deviations σeff are given in the plots. The data-to-simulation scale factors are shown below the main panels [116]. (Right) The distribution of mea- sured over true energy, Emeas/Etrue, for photons in simulated H → γγ events, in a narrow η range in the barrel, 0.2 < |η| < 0.3 for photons with R9≥0.94 [120].
A finer-grained detector, known as the preshower, is installed in front of each endcap disks. It consists of two layers, each comprising a lead radiator followed by a plane of silicon strip sensors, with a pitch of 1.9 mm. The goal of the preshower is to enhance photon identification capabilities.
Since the ECAL crystals are approximately one Moli´ere radius in the lat- eral dimension, high energy electromagnetic showers spread laterally over several crystals. Clustering algorithms are used to sum together energy de- posits in adjacent crystals belonging to the same electromagnetic shower.
The clustering algorithm proceeds first with the formation of “basic clus- ters”, corresponding to local maxima of energy deposits. The basic clusters are then merged together to form a “supercluster,” which is extended inφ (because charged particle tracks bend inφ, but not inη), to recover the radi- ated energy. Because of the differences between the geometric arrangement of the crystals in the barrel and endcap regions, a different clustering algo-
rithm is used in each region. The clustering algorithm used in the barrel, called the “hybrid” algorithm, is described in Ref. [122]. In the endcap and preshower, the algorithm merges together fixed-size 5×5 crystal basic clus- ters and associates each with corresponding preshower energy deposits.