ECAL
(inter)calibration and monitoring
ECAL è un rivelatore bellissimo ma non esattamente facile per farlo
funzionare. La difficoltà aumenta tanto più il calorimetro è
preciso, ogni cosa diventa importante per raggiungere la
precisione voluta.
ECAL ha tanti fisici quanti DT, RPC e CSC sommati (ma siamo la
meta’ del TRK…), circa 30 persone lavorano in ‘calibration &
monitoring’.
Nov. 13th , 2009
1
Nicolo Cartiglia, INFN, Turin, Italy
1
ECAL Intercalibration
Problem: the same photon (or electron) gives a different answer (in ADC counts)
depending upon the crystals it hits.
• each crystal has a specific light yield
• each photodetector has its specific gain (important in the endcaps)
=> poor resolution
Solution: find 75848 coefficients which make every crystal answer in the same way
2100 ADC
2000 ADC
Nov. 13th , 2009
Photons with CMS
2
Nicolo Cartiglia, INFN, Turin, Italy
Toyoko Orimoto,
2
ECAL monitoring
The calorimeter response varies due to many factors:
Temperature:
• Crystal light yield changes -2.1%/C
• Barrel photodetectors (APD) -2.4%/C
Magnetic field:
• Endcaps photodetectors (VPT)
Rate:
• Endcaps photodetectors (VPT)
Radiation:
• Crystals
Solution: a very powerful monitoring system which has 4 lasers, 2 sets of LED
flashers and an almost crystal-by-crystal temperature monitoring system
Nov. 13th , 2009
3
Nicolo Cartiglia, INFN, Turin, Italy
3
ECAL Detector Design
6.4m
Barrel (EB):
• 61200 crystals
• 36 Supermodules (SM),
each 1.7k crystals
Endcap (EE):
• 14648 crystals
• 4 Dees
• SuperCrystals of 5x5
xtals
Nov. 13th , 2009
2.6m
1 Endcap
Super-Crystal
1 Super-Module
1 Dee
Pb-Si Pre-shower
4
Nicolo Cartiglia, INFN, Turin, Italy
4
Crystal production
Crystals are grown in ingots (in Russia and China) and then cut into the right shape.
Each crystal is different, with a different value of transparency and light yield
Nov. 13th , 2009
Nicolo Cartiglia, INFN, Turin, Italy
5
Intercalibration & Energy resolution
‘Energy resolution’: how well do we reconstruct signals as a
function of energy?
Noise term:
For every calorimeter we have:
•Electronic (pre-amps,APD)
•Pile-up
s = a √E
+ b
+cE
Measured: s = 2.8% √E + 125 MeV + 0.3% E
It dominates at high
energy, so it should
be kept small
Stochastic Term:
Constant term:
•Photostatistics
•Sampling (not for ECAL!)
•Gain stage
•Calibration & intercalibration
•Rear leakage
•Light yield non-uniformity
Nov. 13th , 2009
Nicolo Cartiglia, INFN, Turin, Italy
6
From ADC to GeV
Calibration aims at the best estimate of the energy of e and ’s
Energy deposited over multiple crystals:
Ee/ = Fe/ G i ci Ai [ +EES ]
• Amplitude in ADC counts Ai
• Intercalibration: uniform single channel response to a reference ci
• Global scale calibration G
• Particle-specific corrections (containment, clustering for e/’s) Fe/
• Preshower included in the sum in endcaps
There’s inter-play across the different terms and a strategy to dis-entangle
Nov. 13th , 2009
Nicolo Cartiglia, INFN, Turin, Italy
7
Present status of ci
Intercalibration has been achieved in several ways, with different precision:
BARREL:
- Using data collected in the laboratories (all): Crystal response, APD gain, electronics
constants: 4.5-6%
- Cosmic ray (all): expose each SM to cosmic rays: 1-2 %
- TestBeam (11 SM): electrons at a given energy in each crystal ~ 0.3 %
ENDCAP:
Using data collected in the laboratories (all): Crystal response, VPTgain, electronics
constants. Production: 9%, Pre-production:15%
- Beam splash (all): expose each Dee to muons: 15 %
- TestBeam (450): electrons at a given energy in each crystal < 1 %
Nov. 13th , 2009
Nicolo Cartiglia, INFN, Turin, Italy
8
What if LHC starts tomorrow
Zee width
EB
Hγγ width
EB
EE
• Performance acceptable for most physics in EB, nearly in EE
→ Target:
• Target precision: 0.5% set by H benchmark channel
• Approach a.s.a.p. in view of  resonances
Nov. 13th , 2009
Nicolo Cartiglia, INFN, Turin, Italy
9
Next step: in situ intercalibration
Once we will be taking data we will exploit several channels to bring intercalibration
coefficient to a much higher precisions:
 symmetry: based on the phi invariance, actually severely more complicated that it
looked on the beginning (Stefano, Margherita). Statistically limited after a few hours
of data taking.
Goal: 1-2% in barrel, a few in the endcaps
po mass: huge rate, 1 week at 2*1030.
Goal: 0.5 % in barrel, a few in the endcap
Z mass: needs good luminosity…
Nov. 13th , 2009
Nicolo Cartiglia, INFN, Turin, Italy
10
In situ strategy
• Derive intercalibrations ci from phi-inv. and p0/η
• Fix absolute scale G and corrections (η, ET and cluster shape dependent) Fe/ with
electrons from Ze+e• ES calibration (mip) and EE-ES inter-calibration
• Long-term also other channels: isolated electrons Weν
• There’s sufficient redundancy of calibration sources to disentangle interplay between
G/Fe/ and ci :
Ee/ = Fe/ G i ci Ai
→ Validation and combination of calibration sets
• Release new sets for reconstruction as long as precision improves. Further sets for
monitoring.
Nov. 13th , 2009
Nicolo Cartiglia, INFN, Turin, Italy
11
Monitoring
Il nostro calorimetro cambia la risposta per varie ragioni:
• Temperatura: sia i cristalli che gli APD diminuiscono la risposta (luce o
guadagno) se la temperatura aumenta (la combinazione dei due è -3.8%/C)
• Irradiazione: i cristalli si ingialliscono a causa del danno da radiazione, tuttavia
un pochino recuperano…
• Fluenza: i VPT cambiano la risposta quando sono sottoposti ad un flusso
continuo di particelle, quindi durante il ‘fill’ perdono brillantezza, ma poi la
recuperano nell’interfill
• Flusso totale: i VPT perdono brillantezza tanto più carica viene depositata sul
loro catodo
Soluzione: un sistema di laser/led che continuamente spara segnali ‘calibrati’ nei
cristalli per monitorare la loro risposta.
Nov. 13th , 2009
Nicolo Cartiglia, INFN, Turin, Italy
12
LASER Monitoring System Hardware
APD
PN
APD




Laser light sources
Light distribution system (fibers, optical switches, diffusing spheres, etc.)
Very stable PN-diodes used as reference system (MEM)
Precision pulsing system for electronics calibration (separate hardware for
MEMs)
 LED pulsing system for the EE, injecting into level 1 fan-out
4
Nov. 13th , 2009
VPT
Nicolo Cartiglia, INFN, Turin, Italy
13
Stability of the ECAL response:
Crystal transparency
ECAL response will vary, depending on dose rate
with a sequence of crystals transparency
drops and recoveries
rel. Crystal response
Simulation of transparency:
2010 run: transparency change expected in
innermost crystals of EE assuming
‘Classic
31
luminosity will reach L = 10
cm-2VPT
s-1 effect’ induced by LHC on/off
η=0.92 @ L = 2 x 1033cm-2s-1)
Scenario comparable to (ECAL TDR):
η=3 @ 1031cm-2s-1
changes in cathode current; mitigated by LED
constant pulsing to limit current excursions: on
average 1%
• Transparency variation measured via response R/R0 to blue laser pulses injected in each
channel in the LHC abort gap
• Correction to crystal energies proportional to: (R/R0 )α
• with α=1.5 BCTP crystals, α=1 SIC crystals
Nov. 13th , 2009
Nicolo Cartiglia, INFN, Turin, Italy
14
Stability of the ECAL response:
VPT gain
Peak (VPT/PIN)
1.00
normalised
Rel. VPT gain
1.01
Long term ageing: irrelevant in 2010
Rel. VPT gain
‘Classic VPT effect’ induced by LHC on/off
changes in cathode current; mitigated by LED
constant pulsing to limit current excursions: on
average 1%
0.99
0.74
Black: load=10kHz, <IC>~0.25nA; 46 days
h=2.1 and L=2.5*1033cm2s-1
Grey : load=20kHz, <IC>~1.0nA; 134 days
h=2.1 and L=1034cm2s-1
0.72
0.70
0.68
0.66
0.64
0.62
~25%
0.60
0.58
0.98
0.56
0.54
0.97
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
Incremental charge at Cathode (mC)
0.96
0
1
2
3
4
5
6
7
Elapsed Time (days)
Response to blue laser/LED and orange LED sensitive to VPT gain changes
Correction to crystal energies simply proportional to monitored change (α=1)
Nov. 13th , 2009
Nicolo Cartiglia, INFN, Turin, Italy
15
8.0
Speriamo bene…
•
•
Ci sono circa 30 persone che lavorano alla calibrazione e
monitoraggio di ECAL
Per ora sembra che riusciremo a farlo…
Nov. 13th , 2009
Nicolo Cartiglia, INFN, Turin, Italy
16