ECAL (inter)calibration and monitoring

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’.

2. Toyoko Orimoto, Caltech 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

3. 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

4. ECAL Detector Design 6.4m • Barrel (EB): • 61200 crystals • 36 Supermodules (SM), each 1.7k crystals 2.6m 1 Endcap Super-Crystal 1 Super-Module • Endcap (EE): • 14648 crystals • 4 Dees • SuperCrystals of 5x5 xtals Pb-Si Pre-shower 1 Dee

5. 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

6. Intercalibration & Energy resolution ‘Energy resolution’: how well do we reconstruct signals as a function of energy? For every calorimeter we have: • Noise term: • Electronic (pre-amps,APD) • Pile-up s = a √E + c E + b It dominates at high energy, so it should be kept small Measured: s = 2.8% √E + 125 MeV + 0.3% E • Stochastic Term: • Photostatistics • Sampling (not for ECAL!) • Gain stage • Constant term: • Calibration & intercalibration • Rear leakage • Light yield non-uniformity

7. 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

8. 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 %

9. What if LHC starts tomorrow Hγγ width Zee width EB 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

10. 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…

11. 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 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 : • 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

12. 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.

13. 4 LASER Monitoring System Hardware • 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 APD PN APD VPT

14. 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 • ‘Classic VPT effect’ induced by LHC on/off changes in cathode current; mitigated by LED constant pulsing to limit current excursions: on average 1% Simulation of transparency: η=0.92 @ L = 2 x 1033cm-2s-1) Scenario comparable to (ECAL TDR): η=3 @1031cm-2s-1 rel. Crystal response • 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

15. Stability of the ECAL response: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% • Long term ageing: irrelevant in 2010 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 Rel. VPT gain Rel. VPT gain ~25% Response to blue laser/LED and orange LED sensitive to VPT gain changes Correction to crystal energies simply proportional to monitored change (α=1)

16. Speriamo bene… • Ci sono circa 30 persone che lavorano alla calibrazione e monitoraggio di ECAL • Per ora sembra che riusciremo a farlo…