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