1. The CMS Electromagnetic Calorimeter R M Brown on behalf of the CMS ECAL Group
Overview
Detector details
Crystals
Photo-detectors
On-detector electronics
Off-detector electronics
Laser monitoring system
Mechanical assembly & installation
Pre-shower
Inter-calibration with electrons & cosmic muons
Test beam results
Summary
STFC
RAL
HEP2007 July Manchester R M Brown - RAL

2. Compact Muon Solenoid Objectives: Higgs discovery
Physics beyond the Standard Model
Current data suggest a light Higgs
Favoured discovery channel H  gg
Intrinsic width very small
 Measured width, hence S/B given by experimental resolution
High resolution electromagnetic calorimetry is a hallmark of CMS
Target ECAL energy resolution: ≤ 0.5% above 100 GeV
 120 GeV SM Higgs discovery (5s) with 10 fb-1 (100 d at 1033 cm-2s-1)
Length ~ 22 m
Diameter ~ 15 m
Weight ~ 14.5 kt
HEP2007 July Manchester R M Brown - RAL

3. ECAL: Challenges & Choices
Fast response (25ns between bunch crossings)
High radiation doses & neutron fluences
(10 year doses: n/cm2, 1kGy (=0)
2x1014 n/cm2, 50kGy ( =2.6))
Strong magnetic field (4 Tesla)
On-detector signal processing
0/ discrimination
Long term reproducibility
Choices:
Lead tungstate crystals
Avalanche photodiodes (Barrel), Vacuum phototriodes (Endcaps)
Electronics in 0.25 mm CMOS
Pb/Si Preshower in Endcap region
Laser light monitoring system
HEP2007 July Manchester R M Brown - RAL

4. ECAL layout Lead tungstate ~ 11 m3, 90 t
(PbWO4)
~ 11 m3, 90 t
Endcap
‘Supercystals’
(5x5 crystals)
Endcap ‘Dee’
(3662 crystals)
Endcaps: 1.48 < || < 3.0
4 Dees
14648 crystals (3 x 3 x 22 cm3)
Barrel crystals
Pb/Si Preshower
Barrel
‘Supermodule’
(1700 crystals)
Barrel: || < 1.48
36 Super Modules
61200 crystals (2 x 2 x 23 cm3)
HEP2007 July Manchester R M Brown - RAL

5. Lead tungstate properties
Radiation resistant to very high doses
Light loss is characterised by ‘induced absorption’
Dynamic equilibrium between formation/annealing of colour centres under irradiation at room temp
 Light loss depends on dose-rate
CMS specification requires: μ420 < 1.5 m-1 (Under uniform (lateral) 60Co irradiation at > 30 Gy/hr)
Under LHC-like conditions for the Barrel (~ 0.15 Gy/hr)  Light yield loss ≾ 6% 1 2 3 4 5 6 7 8 i n t a l f e r d o w v g h ( m )
T(%)
- emission
Fast light emission: ~80% in 25 ns
Peak emission ~425 nm (visible region)
Short radiation length: X0 = 0.89 cm
Small Molière radius: RM = 2.10 cm
Light Yield Loss (%)
Temp dependence ~2.2%/OC
Stabilise to  0.05OC
Low light yield (1.3% NaI)
Photodetectors with gain in magnetic field DT
Power-on/
Power-off
HEP2007 July Manchester R M Brown - RAL

6. Photodetectors Barrel: Avalanche photodiodes (APD)
40mm
deff ~6mm
Barrel: Avalanche photodiodes (APD)
Two 5 x 5 mm2 APDs/crystal
Gain: QE: lpeak= 420 nm
Temperature dependence: %/oC
Gain dependence on bias: 3 %/V
 = 26.5 mm
MESH ANODE
Endcaps: Vacuum phototriodes (VPT)
More radiation resistant than Si diodes
(with UV glass window)
- Active area ~ 280 mm2
Gain ~10 (B=4T) Q.E.~ 20% (420 nm)
HEP2007 July Manchester R M Brown - RAL

7. On-detector electronics
Trigger Tower (TT)
Very Front End card (VFE)
Front End card
(FE)
Trigger Sums
Data MB
VFE x 5 FE
LVR
Σ 3x3
Mean = 127 MeV
Energy Equivalence (MeV)
Σ 5x5
Mean = 213 MeV
Σ1x1
Mean = 41.5 MeV
Noise measured in Test Beam
With dynamic pedestal sampling:
Cluster noise scales as n for clusters summed over n channels
Trigger primitives computed on the detector
Command & control via token ring
Modularity: Trigger Tower (25 channels in Barrel)
5 VFE Boards (5 channels each) / 1 FE Board
1 Fibre sends trig primitives (every bunch Xing)
1 Fibre sends data (on Level1 accept)
x12 x6 x1
MGPA
Logic
12 bit ADC 2 1
12 bits
2 bits HV
APD/VPT
VFE architecture for single channel
HEP2007 July Manchester R M Brown - RAL

8. Laser light monitoring
During LHC cycles the ECAL response will vary, depending on irradiation conditions
Laser signal
Electron signal (test beam)
Changes in crystal transparency will be monitored with a laser allowing the ECAL response to be corrected
Simulation of crystal transparency evolution at LHC (L = 2 x 1033cm-2s-1) - based on test beam irradiation results
440 nm
796 nm
Laser light is injected into each crystal through optical fibres (normalised with PN diodes to 0.1 %) Blue light tracks response, infrared provides a check The laser is pulsed during the LHC ‘abort gap’
An optical switch directs light to one half-supermodule or one quarter-Dee in turn A complete monitoring cycle takes ~ 20 minutes
Corrected response
Raw response
Time (hours)
ADC
2/ndf =73.9/68
Test beam data
HEP2007 July Manchester R M Brown - RAL

9. Construction & Installation: Barrel
Sub-module: 10 crystals
Module: 400/500 crystals
Super-module: 1700 crystals
Bare SM
Instrumented SM
HEP2007 July Manchester R M Brown - RAL

10. Barrel ECAL Installation
HEP2007 July Manchester R M Brown - RAL

11. Construction & Status: Endcaps
Supercrystal: 25 crystals
Dee (½ endcap): 3662 crystals
Front view with first 20 supercrystals mounted
Rear view with laser fibre harnesses mounted
Backplates successfully test mounted on HCAL
Schedule 1 Endcap (2 Dees, 1 PS) ready Feb 08 inserted before CMS closes 1 Endcap early summer 08
33%
HEP2007 July Manchester R M Brown - RAL

12. Rapidity coverage: 1.65 < || < 2.6 (End caps)
Preshower detector
Rapidity coverage: < || < 2.6 (End caps)
Motivation: Improved 0/ discrimination
2 orthogonal planes of Si strip detectors behind
2 X0 and 1 X0 Pb respectively
Strip pitch: 1.9 mm (63 mm long)
Area: 16.5 m2 (4300 detectors, 1.4 x105 channels)
High radiation levels - Dose after 10 yrs:
~ 2 x1014 n/cm2
~ 60 kGy
 Operate at -10o C
HEP2007 July Manchester R M Brown - RAL

13. Inter-calibration  2 = 0.2% s = 1.5% s = 4.5%
electron beam calibration
reproducibility (Aug - Sept)
9 Supermodules (25%) inter-calibrated with e-
 2 = 0.2%
electron beam – cosmic muon
comparison
s = 1.5%
36 Supermodules (100%) inter-calibrated with cosmics
Cosmic muon intercalibration precision versus  ‘index’
2.0%
1.0%
electron beam - construction database comparison
s = 4.5%
HEP2007 July Manchester R M Brown - RAL

14. Correction for impact position
120 GeV
E (GeV)
Central impact (4x4 mm2)
0.5%
120 GeV
‘Uniform’ impact (20x20 mm2) after impact-position correction
E (GeV)
0.5%
Response for S(3x3) varies by ~3% with impact position in central crystal
Correction made using information from crystals alone (works for g) Does not depend on E,,
position ( )
(3 x 3) around Crystal 184
(3 x 3) around Crystal 204 (3 x 3) around Crystal 224
4x4 mm2 central region
HEP2007 July Manchester R M Brown - RAL

15. Energy resolution: random impact
22 mm
Series of runs at 120 GeV centred on many points within S(3x3)
Results averaged to simulate the effect of random impact positions
Resolution goal of 0.5% at high energy achieved
HEP2007 July Manchester R M Brown - RAL

16. Summary A hallmark feature of CMS is the high resolution crystal ECAL
The ECAL Barrel is complete, pre-commissioned and (almost) fully installed
Crystal production has switched to the Endcaps (1/3 of crystals now delivered)
The first Endcap (crystals + Preshower) to be installed at the start of , the second to be ready in early summer 2008)
Extensive test beam studies demonstrate the CMS ECAL will meet its ambitious design goals
STFC
RAL
HEP2007 July Manchester R M Brown - RAL

17. Spares
HEP2007 July Manchester R M Brown - RAL

18. ECAL design objectives
High resolution electromagnetic calorimetry is central to the CMS design
Benchmark process: H   
m / m = 0.5 [E1/ E1  E2/ E2   / tan( / 2 )]
Where: E / E = a /  E  b  c/ E
Aim (TDR): Barrel End cap
Stochastic term: a = % %
(p.e. stat, shower fluct, photo-detector, lateral leakage)
Constant term: b = % %
(non-uniformities, inter-calibration, longitudinal leakage)
Noise: Low L c = 155 MeV MeV
High L MeV MeV
(dq relies on interaction vertex measurement)
Coloured histograms are separate contributing backgrounds for 1fb-1
(electronic,
pile-up)
Optimised analysis
HEP2007 July Manchester R M Brown - RAL

19. Calibration strategy
Lab measurements
Test Beam LY
Lab LY
Test Beam LY – Lab LY
 = 4.0%
Initial pre-calibration by ‘dead reckoning’ based on lab measurements (~4%)
In-situ calibration
Precision with 11M events
Limit on precision 
Inter-calibration precision %
-symmetry
w/jet trigger (ET > 120 GeV)
Fast in-situ calibration based on principle that mean energy deposited by jet triggers is independent of  at fixed  (after correction for Tracker material) (~2-3% in few hours)
Reference pre-calibration of 9 SM with 50/120 GeV electrons in test beam (<2%)
Pre-calibration
-ring inter-calibration and Z  e + e cross-calibration (~1% in 1 day)
GeV
Z  e + e
Barrel
Finally: calibration to < 0.5% with W   + e in ~2 months
HEP2007 July Manchester R M Brown - RAL

20. Off-Detector electronics
Clock & Control System (CCS)
Trigger Concentrator Card (TCC)
Data Concentrator Card (DCC)
HEP2007 July Manchester R M Brown - RAL