Chapter IV: The Compact Muon Solenoid
4.3 The electromagnetic calorimeter (ECAL)
The electromagnetic calorimeter of CMS (ECAL) [24] is a homogeneous and her- metic calorimeter made of 61,200 lead tungstate (PbWO4) scintillating crystals mounted in the central barrel part (EB), closed at each endcap (EE) by7,324crys- tals. A preshower detector (PS) is placed in front of the endcap crystals. Avalanche photodiodes (APDs) [25, 26] are used as photodetectors for the EB and vacuum phototriodes (VPTs) [27] for the EE.
The lead tungstate (PbWO4) crystals are chosen due to they have high density (8.28 g/cm3), short radiation length (X0 = 0.89 cm), and small Molière radius (R" = 2.19 cm), which has allowed the design of a fast, compact and radiation resistant calorimeter with fine granularity [24]. In Table 4.1, the properties of PbWO4 are compared with those of other crystals used in electromagnetic calorimeters.
Figure 4.8: Tracking efficiency (a,c) and fake rate (b,d) for theC¯C sample as a function of track [, for the current detector (a,b) and the upgrade pixel detector (c,d). Results are shown for zero pileup (blue squares), an average pileup of 25 (red dots), an average pileup of 50 (black diamonds), and an average pileup of 100 (brown triangles) with ROC data loss simulation expected at the given luminosities as detailed in the text [22].
Figure 4.9: Transverse section through the ECAL, showing geometrical configuration [28].
Table 4.1: Comparison of properties of various crystals [24].
NaI(Tl) BGO CSI BaF2 CeF3 PbWO4
Density [g/cm3] 3.67 7.13 4.51 4.88 6.16 8.28
Radiation length [cm] 2.59 1.12 1.85 2.06 1.68 0.89
Interaction length [cm] 41.4 21.8 37.0 29.9 26.2 22.4
Molière radius [cm] 4.80 2.33 3.50 3.39 2.63 2.19
Light decay time [ns] 230 60 16 0.9 8 5 (39%)
300 630 25 15 (60%)
100 (1%)
Refractive index 1.85 2.15 1.80 1.49 1.62 2.30
Maximum of emission [nm] 410 480 315 210 300 440
310 340
Temperature coefficient [%/ C] 0 -1.6 -0.6 -2/0 0.14 -2
Relative light output 100 18 20 20/4 8 1.3
scintillation light from PbWO4[30]. Fig.4.10shows history of ECAL response in six different[bins with laser data during 2011-2018. The response change observed in the ECAL channels is up to 13% in the barrel (|[| < 1.4) and it reaches up to 62% at[ ⇠ 2.5, the limit of the tracker acceptance. The response change is up to 96% in the region closest to the beam pipe (|[| > 2.7).
Figure 4.10: Relative response to laser light (440 nm in 2011 and 447 nm from 2012 onwards) injected in the ECAL crystals, measured by the ECAL laser monitoring system, averaged over all crystals in bins of pseudorapidity ([), for the 2011, 2012, 2015, 2016, 2017 and 2018 data taking periods, with magnetic field at 3.8 T. The response change observed in the ECAL channels is up to 13% in the barrel and it reaches up to 62% at[ 2.5, the limit of the tracker acceptance. The response change is up to 96% in the region closest to the beam pipe. The recovery of the crystal response during the periods without collisions is visible. The bottom plot shows the instantaneous LHC luminosity delivered during this time period [31].
The energy resolution of ECAL is measured by fitting a Voigtian (Breit-Wigner convolved with Gaussian) function to the reconstructed energy distributions [24,28, 30,32]. It has been parameterized as a function of energy, with the function:
⇣f⇢
⇢
⌘2
=
✓ ( p⇢
◆2
+
✓#
⇢
◆2
+⇠2, (4.3)
where S is the stochastic term, N the noise and C the constant term [28]. There are three main sources that contribute to the stochastic term (S):
i) fluctuations on the lateral containment that contributes⇠1.5%,
ii) fluctuations on the energy deposited in the preshower absorber that contributes
⇠5%,
iii) a photostatistics contribution of 2.3%.
There are also three contributions to the noise term (N):
i) preamplifier noise (⇠153 MeV, a quadrature sum of 30 MeV per channel in EB and 150 MeV per channel in EE),
ii) digitization noise (150 MeV at EB and 750 MeV at EE ),
iii) pileup noise (significant at the highest pseudorapidity[regions at high lumi- nosity).
The most relevant contributions for the constant term (C) are:
i) non-uniformity of the longitudinal light collection (0.3%), ii) crystal-to-crystal intercalibration errors (0.4%),
iii) leakage of energy from the back of the crystal (< 0.2%),
iv) uncorrected and imperfectly corrected geometrical effects (< 0.2%).
Table 12.1 in Ref. [24] has a more detailed breakdown of those terms for both the EB ([=0) and the EE ([=2). Fig.4.11shows Run 2 ECAL energy resolution as a function of pseudorapidity[measured by Z!ee decays. The relative ECAL energy resolution (f⇢/⇢) is around 2% at the EB ([ =0) and 4% at the EE ([ =2). The CMS ECAL detector has maintained a stable energy resolution throughout Run 2 at LHC.
0 0.5 1 1.5 2 2.5 η| Supercluster |
0 0 0.5 1 1.5 2 2.5
η| Supercluster | 0
Figure 4.11: Relative electron (ECAL) energy resolution unfolded in bins of pseudorapidity [for the ECAL Barrel and Endcap. Electrons from Z!ee decays are used. The resolution is shown separately for all electrons (inclusive, left), and for low bremsstrahlung electrons (right). The plot compares the resolution achieved after a refined calibration of the data collected at 13 TeV during Run 2 in 2016, 2017, and 2018. The relative resolutionf⇢/⇢ is extracted from an unbinned likelihood fit to Z!ee events, using a Voigtian (Breit- Wigner convolved with Gaussian) as the signal model. A stable ECAL energy resolution is observed over the course of Run 2 despite the increased LHC luminosity and the ageing of the detector [32].
The fast signal from the %1,$4 scintillation also enables time measurements in proton-proton collisions with high-energy electrons and photons [33]. The time resolution for seed crystals of the clusters of the two electrons from Z!ee decays is calculated as following:
f(C1 C2)2= ( #
⇢eff)2+2⇠2, (4.4)
where⇢effis the effective energy. The ombined result of the global timing resolution, which is on the order of 200 ps for energies above 40 GeV in EB, for the 2016, Legacy 2017 and 2018 data is shown in Fig.4.12.
Figure 4.12: The resolution of time difference between the times of the seed crystals of the clusters of the two electrons from Z!ee decays, as a function of the effective energy in the ECAL Barrel for 2016, Legacy 2017 and 2018 data combined together. A global timing resolution of the order of 200 ps for energies above 40 GeV is measured [34].