ECAL calibration reviewOct 15, 2009- 1 -

Calibration of the Electromagnetic Calorimeter of the CMS detector

G. FranzoniUniversity of Minnesota

T. Tabarelli de FatisUniversità & INFN Milano Bicocca

• Calibration definition and targets• Calibrations at start-up• In situ strategy for 2010• Calibration and stability monitoring

ECAL calibration reviewOct 15, 2009- 2 -

Preamble

• General concepts to provide context• Pointing to areas that will be elaborated in following talks• Addressing areas where actions are needed

ECAL calibration reviewOct 15, 2009- 3 -

ECAL layout• High resolution PWO crystal ECAL• Barrel: || < 1.48

• 36 Super Modules• 61200 crystals (2 x 2 x 23 cm3) – 26X0• Avalanche photo diodes

• Endcaps: 1.48 < || < 3.0• 4 Dee’s • 14648 crystals (3 x 3 x 22 cm3) – 25 X0• Vacuum photo triodes

• Preshower 3X0 (Pb/Si) 1.65 < |η| < 2.6

• Monitoring in LHC abort gap:• Laser light injected in all channels• LED light in endcaps

ECAL calibration reviewOct 15, 2009- 4 -

Definitions

• Calibration aims at the best estimate of the energy of e/’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

ECAL calibration reviewOct 15, 2009- 5 -

Status at startup

• Precalibrations ci:• Barrel:

• 0.3% on 9 SM (electron beams)• 1.5-2.5% on 27 SM (cosmic rays)

• ECAL Endcaps: • 6.5% (crystal LY VPT gain) combined w/ local uniformity of splash events• Still a chance to improve with ES@splash09

• Preshower• 2% (cosmic rays)

• Global energy scale G:• Tied to test beam (also ES)

• Corrections: Fe/ • Algorithmic corrections based on MC; η, energy and cluster shape dependent→ Need to be tested/tuned in situ since dependent on material budget

ECAL calibration reviewOct 15, 2009- 6 -

What if LHC start 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

ECAL calibration reviewOct 15, 2009- 7 -

Fast in situ intercalibration methods

• Dedicated HLT filters for for fast intercalibrations:• -invariance of energy flow within an const-η ring• 0/η->γγ mass constraint

calibrations with AlCaRaw (RecHits) to increase yield for calibration• Both methods provide intercalibration sets in a few days of data taking

• No need to go into express stream• AlcaRaw production and CAF workflows tested at CSA08 and CRAFT09→ Performances demonstration still outstanding in endcaps:

• Worse S/N for 0/η; need ~1 week of data, precision to be assessed • Phi-invariance: never reproduced results of CSA06 (1-3%)

HLTFilters AlCaRecoProducers

Calibration Algorithms

P5 Tier0 CAF

RecHitsRawData

RecHits

ECAL calibration reviewOct 15, 2009- 8 -

In situ strategy

• Derive intercalibrations ci from phi-inv. and 0/η (Marat’s talk)• Fix absolute scale G and corrections (η, ET and cluster shape

dependent) Fe/ with electrons from Ze+e- (Riccardo’s talk)• ES calibration (mip) and EE-ES inter-calibration (Ming’s talk)• 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 (tools and procedures in Riccardo’s talk)

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

Ee/ = Fe/ G i ci Ai

ECAL calibration reviewOct 15, 2009- 9 -

In situ strategy

• None of the in situ methods fixes inter-ring scale• η inter-ring scale and correction functions for ci can be fixed using precalibrations

• Inter-ring scale known to better than 0.3%

0 calibration

Ee/ = Fe/ G i ci Ai

ECAL calibration reviewOct 15, 2009- 10 -

Stability of the ECAL response: transparency

• ECAL response will vary, depending on dose rate:• 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

re

spon

se

• Transparency variation measured via response R/R0 to blue laser pulses injected in each channel in the LHC abort gap (Adi’s and David’s talks)

• Correction to crystal energies proportional to: (R/R0 )α

• with α=1.5 BCTP crystals, α=1 SIC crystals

ECAL calibration reviewOct 15, 2009- 11 -

Stability of the ECAL response:VPT gain

• VPT gain varies (Sasha’s talk):• ‘Classic VPT effect’ induced by LHC on/off changes in cathode current; mitigated by LED constant pulsing to

limit current excursions: on average 1%→ Optimal pulsing strategy yet to be defined• Long term ageing: irrelevant in 2010

Incremental charge at Cathode (mC)

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

Pea

k (V

PT/

PIN

)

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-1

Rel

. VP

T ga

in~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 ga

in

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

ECAL calibration reviewOct 15, 2009- 12 -

Calibration and stability

• Due to the different values of α, in general one needs to correct separately for transparency and VPT gain:

Ee/ = Fe/ G i TiVici Ai

Correction for transparency change Ti Correction for VPT gain change Vi

→ End-to-end test applying monitoring correction in RECO: yet to be completed→ Procedures of validation of monitoring corrections (within start of prompt reco) → Capability of monitoring with orange LED yet to be proven. 2010 data need to establish if LED can

provide monitoring of VPT gain alone. • Strategy for 2010:

• Activate monitoring procedure based on blue laser only • Being VPT ageing negligible and classic VPT effect ~1%, acceptable using blue laser

monitoring to correct for VPT and transparency with the same value of α=1.5

ECAL calibration reviewOct 15, 2009- 13 -

Conclusions

• Definitions and procedures in place • ECAL calibration at startup: acceptable for

most physics analysis• Areas needing attention:

• Performance in EE• Stability in EE