Roma, 22/11/01CMS Software & Computing Workshop - E. Longo 1 Calibrazioni del calorimetro:...

Roma, 22/11/01 CMS Software & Computing Workshop - E. Longo

1

Calibrazioni del calorimetro:esperienze (L3) e prospettive

Egidio Longo

Roma, 22/11/01 CMS Software & Computing Workshop - E.

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a ~ 2.5%/√Eb < 200 MeV c ~ 0.5%

and an angular resolution ~ 50 mrad/E

CMS ECAL benchmark

m = 2 E1 E2 (1 - cos)

σm

m=

12

σ1

E1

⎛

⎝ ⎜ ⎞

⎠ ⎟

2

+σ 2

E2

⎛

⎝ ⎜ ⎞

⎠ ⎟

2

+σθ

tgθ /2

⎛ ⎝ ⎜ ⎞

⎠

2

+⎡

⎣ ⎢

⎤

⎦ ⎥

1/ 2

σ E( )E

=aE

⊕bE

⊕ c

H (mH 100 GeV) ~ 2 – 100 MeV H /mH ≤ 10-3

target

low mass Higgs discovery:


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resolution: stocastic term a

• photostatistics contribution, including– LY– light collection efficiency– geometrical efficiency of the photodetector– photocatode quantum efficiency

Npe/GeV = 4000 for 0.5 cm2 APD 1.6%

• electron current multiplication in APD, contributinga square root of excess noise factor, F = 2

1.61.4 = 2.25%

• Lateral containment (55 matrix) 1.5%

Total stochastic term a = 2.7 %


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resolution: noise term b40 ns shaping time, summed over 5x5 channels• Serial noise (p.d. capacitance) 1/t

– 150 MeV• Parallel noise (dark current) t, mostly radiation

induced– negligible at the start of the experiment– 30 MeV after one year at low luminosity– 100 MeV after one year at high luminosity

• Physics pile-up (simulated, with big uncertainties)– low luminosity 30 MeV– high luminosity 100 MeV

Total contribution– low luminosity 155 MeV– high luminosity 210 MeV


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resolution: constant term c

• leakage (front, rear, blind material)CMS full shower simulation < 0.2 %

• system instabilities designed to be at the permill level– temperature stabilization < 0.1 ˚C ( LY = -1.9 % per ˚C)– APD bias stable at 20 mV (dM/dV = 3%/V)

• light collection uniformity,Specifications to stay < 0.3% reached by

single face depolishing

• Key issue to have c 0.5 % intercalibration by monitor and physics signals at 0.5 % including the radiation damage effect


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resolution at test beam on 1999 prototype30 preproduction crystals and APDs

fit as a function of E:

(few crystal intercalibration)

σΕ

=2.74%

E⊕0.40%⊕

142MeVE


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• Is it realistic to require a target0.5 % intercalibration

for such a large scale calorimeter?

• Look at L3 experience

intercalibration


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L3 BGO calorimeter

• 7680 (barrel) + 3840 (EC) BGO Crystals

• PIN diode read-out (no gain)

• 12 bit ADC for 6 gains (1 to 512)

• noise level around 1 MeV

• sparse scan (zero suppression) read-out


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L3 calibration tools

• Individual Test beam calibration (2-50 GeV e-)

• Xenon monitor of the response once per day

• Bhabha events at Z pole (45 GeV e)

• at LEP200, RFQ calibration (17 MeV )

Standard in situ procedure:• Individual intercalibration with Xe lamps• Absolute calibration with bhabha electrons

for groups of crystals sharing the same Xe lamp


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test beam calibrations

• Individual calibration for 7680 crystals– two half barrels in two years (1987,1988)– first half barrel recalibrated in 1988– 4 energies (2, 10, 20 and 50 GeV) – temperature control (nominal ± 0.5 ºC)– temperature monitor (1300 sensors) at 0.2

ºC– fully automated procedure:

sequence, positioning, monitor and quality check


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intercalibration in 1990

after Xe corrections 1.25 %- 0.9 % 0.87 %

measured resolution MC predictedintercalibration spread

(average response decreased by 5 %, due to ageing of some optical component)

1990 Bhabha spectrum


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10 years of follow-up

1990

1991

1992

1993

1994

Barrel

Time (days)

0 200 400 600 800 1000 1200 1400 1600 1800

Electron energy/Beam energy

0.88

0.9

0.92

0.94

0.96

0.98

1

1.02

RB26 (Hb 1)

RB24 (Hb 2)

• system able to track this response decrease (few %/year)

• porting of previous year calibration: 1.3%

• spread after Xe+Bhabha corrections: 0.8%


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follow-up at LEP200• at LEP200 (too few Bhabha) add RFQ

gunsend protons on Li target, producing17 MeV photons, million events in few days

• year after year, algorithms improved – several months of work to get final

calibrations every year

• latest result: 1.06% resolution measured on 45-100 GeV Bhabha electrons intercalibration contribution 0.5 %


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conclusions for L3• L3 BGO calibration combined test-beam

precalibration, monitor and physics events• Porting from test-beam to the experiment was

performed at < 1.5%• After all corrections, first year intercalibration

was known at 0.9%• After ten years of improvements,

intercalibration known at 0.5%


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CMS-ECAL calibration scheme

• individual crystal precalibration at beam test at several energies

• intercalibration porting to the experiment via laser monitor system

• follow-up of radiation damage by laser monitor system

• calibration by physics events every few months


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calibration ground rules


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ECAL installation in

V31

SM assembly

electronics

calibration


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precalibrationWe probably will not be able to precalibrate all

ECAL supermodulesWe may be forced to choose between • the “production serial precalibration” of the

maximum number of modules, or• the precalibration of a smaller number of

modules with deeper understanding of a few modules to be recalibrated several times


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intercalibration• Issue is crystal intercalibration

(the energy scale can be fixed relatively simplyreconstructing the Z invariant mass with just a few Z ee)

– Raw intercalibration from lab measurements < 6%

– Target test beam pre-calibration < 2%

– Target final intercalibration using electrons 0.4

• Is precalibration crucial?– Not clear how well HLT system will

work with 6 % intercalibration

– Much easier to understand what is going on with 2 % intercalibration

– Precalibration ultimate system test before LHC data taking

(F. Cavallari)


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in situ calibrationsBaseline intercalibration uses W electrons measured in tracker• Calibration electron selection and its efficiency

What efficiency to select ‘good’ (not much brem) electrons that can be well reconstructed in 5x5 crystals?

• Calibration algorithmHow many electrons per crystal needed; what is best algorithm to unscramble

the calibration constants?

• To do: simulate the whole procedure (milestone 3/02):Introduce inter-calibration errors and show how well they can be recovered

Use of Z ee invariant mass• 5 times less events

• “internal” ECAL calibration (minimal tracker information needed)

• can be combined with monitor for absolute calibration

Large samples of W’s and Z’s to study these issues

( P. Meriadiani talk)


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alternative intercalibrations

What to do if missing pre-calibration and poor electron capability (tracker) ?

• look at the energy flow in minimum bias– dependence, simmetry (?)– trigger bias

• study on sample of500k fully simulatedmb events available

• cut out noise


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energy flow calibration (1)Total energy after truncation cuts (200 MeV). • Each entry in histogram represents total

energy in a single crystal. • 360 crystals at the same Since MC crystals are perfectly calibrated,

the width represents the error on intercalibration achievable with 500k

events

2% requires 11 M min-bias events

6.6 M crossings

183 hours (at 10Hz)

(< 2 months for 30% duty cycle)

2 hours (at 1kHz —to do in HLT farm)


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energy flow calibration (2)• limitations:

– intercalibration per raws only (combine with Z absolute calibration ?)

– tracker inhomogeneitiesneeds for realistic tracker services simulation

(and a combined tracker-ECAL test beam?)

– sentitivity to channel-to-channel noise variations

• better algorithms? – slope of ET rather than truncated mean


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conclusioni• intercalibrazioni @ 0.5% difficili ma

possibili• baseline calibrazioni in situ con W

milestone per 03/02• Roma partecipa attivamente agli studi

di calibrazioni con W e Z• precalibrazione completa impossibile;

studi su possibili alternative iniziati da poco

• simulazione realistica del tracker essenziale per le prossime produzioni

• test beam con sezioni di tracker e calorimetro auspicabili

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Roma, 22/11/01CMS Software & Computing Workshop - E. Longo 1 Calibrazioni del calorimetro:...

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