Nov. 13 th, 2009 Nicolo Cartiglia, INFN, Turin, Italy 1 1 ECAL (inter)calibration and monitoring ECAL è un rivelatore bellissimo ma non esattamente facile

Nov. 13th , 2009 Nicolo Cartiglia, INFN, Turin, Italy 1

1

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’.


Photons with CMS Detector

Toyoko Orimoto, Caltech

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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

2000 ADC

2100 ADC


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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


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ECAL Detector Design

2.6m

6.4m

1 Super-Module1 Super-Module

1 Dee

1 Endcap Super-Crystal

Pb-Si Pre-showerPb-Si Pre-shower

Barrel (EB): • 61200 crystals • 36 Supermodules (SM),

each 1.7k crystals

Endcap (EE): • 14648 crystals• 4 Dees• SuperCrystals of 5x5

xtals


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


Intercalibration & Energy resolution

‘Energy resolution’: how well do we reconstruct signals as a function of energy?

For every calorimeter we have:

Stochastic Term:•Photostatistics•Sampling (not for ECAL!)•Gain stage

= a √E + c E

Constant term:•Calibration & intercalibration •Rear leakage•Light yield non-uniformity

+ b

Noise term:•Electronic (pre-amps,APD)•Pile-up

It dominates at high energy, so it should

be kept small

Measured: 2.8% √E + 125 MeV + 0.3% E


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


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 %


What if LHC starts tomorrow

EB EE

Zee width Hγγ width

EB

• 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


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

o mass: huge rate, 1 week at 2*1030. Goal: 0.5 % in barrel, a few in the endcap

Z mass: needs good luminosity…


In situ strategy

• Derive intercalibrations ci from phi-inv. and 0/η

• Fix absolute scale G and corrections (η, ET and cluster shape dependent) Fe/ with electrons from Ze+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 :

→ Validation and combination of calibration sets

• Release new sets for reconstruction as long as precision improves. Further sets for monitoring.

Ee/ = Fe/ G i ci Ai


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.


LASER Monitoring System Hardware

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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

APDPN

APD

VPT


Stability of the ECAL response: Crystal transparency

ECAL response will vary, depending on dose rate with a sequence of crystals transparency drops and recoveries

2010 run: transparency change expected in innermost crystals of EE assuming luminosity will reach L = 1031 cm-2s-1

Simulation of transparency:

η=0.92 @ L = 2 x 1033cm-2s-1)

Scenario comparable to (ECAL TDR):

η=3 @ 1031cm-2s-1

rel.

Cry

stal

res

pons

e • 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

‘Classic VPT effect’ induced by LHC on/off changes in cathode current; mitigated by LED constant pulsing to limit current excursions: on

average 1%


Stability of the ECAL response:VPT gain

Incremental charge at Cathode (mC)

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

Pe

ak (

VP

T/P

IN)

0.54

0.56

0.58

0.60

0.62

0.64

0.66

0.68

0.70

0.72

0.74

Black: load=10kHz, <IC>~0.25nA; 46 days=2.1 and L=2.5*1033cm2s-1

Grey : load=20kHz, <IC>~1.0nA; 134 days=2.1 and L=1034cm2s-1R

el. V

PT

gai

n

~25%

0 1 2 3 4 5 6 7

norm

alis

ed

0.96

0.97

0.98

0.99

1.00

1.01

Elapsed Time (days)

Rel

. VP

T g

ain

Response to blue laser/LED and orange LED sensitive to VPT gain changesCorrection to crystal energies simply proportional to monitored change (α=1)

Long term ageing: irrelevant in 2010 ‘Classic VPT effect’ induced by LHC on/off changes in cathode current; mitigated by LED constant pulsing to limit current excursions: on average 1%


Speriamo bene…

• Ci sono circa 30 persone che lavorano alla calibrazione e monitoraggio di ECAL

• Per ora sembra che riusciremo a farlo…

Documents

Nov. 13 th, 2009 Nicolo Cartiglia, INFN, Turin, Italy 1 1 ECAL (inter)calibration and monitoring ECAL è un rivelatore bellissimo ma non esattamente facile