The Electromagnetic Calorimeter

of the

Babar Detector

Caltech

26 March 2002

Martin Kocian (SLAC)

for the EMC Group and the Babar Collaboration

. The Babar Detector

. The Babar Calorimeter

. Calibration

. Performance

Martin Kocian Calor02 26 March 2002

The B-factory at SLAC

• Asymmetric B factory:• e−-beam at 9 GeV, e+-beam at 3.1 GeV• e+e− −→ Υ(4s) −→ BB̄• Peak luminosity presently at 4.5 · 1033 cm−2s−1• Data recorded until now: 75 fb−1

Martin Kocian Calor02 26 March 2002

The BaBar Experiment

CalorimeterElectromagnetic

Drift Chamber

DIRC

Silicon Vertex Detector

InstrumentedFlux Return

• Measurement of CP violation in B decays• The calorimeter’s main task is the reconstruction of:• Electrons

- “Golden channel” for sin(2β):B0 → J/ΨKs, J/ψ → e+e−- Flavor tagging

• π0s - determination of sin(2α) : B0 → π0π0• Photons

Martin Kocian Calor02 26 March 2002

Calorimetry goals

• High efficiency for photons is needed for B recon-struction:

π0 reco efficiency

B reco efficiency

• High lightyield is needed• Description of the resolution:

σEE

= a4√

E(GeV )⊕ b

• The constant term contains:• Calibration error• Crystal nonuniformity• Leakage

Martin Kocian Calor02 26 March 2002

Calorimeter Overview

11271375920

1555 2295

2359

1801

558

1979

22.7˚

26.8˚

15.8˚

Interaction Point 1-20018572A03

38.2˚

ExternalSupport

• 6580 CsI(Tl) crystals• Barrel: 48 rings in θ, 120 crystals per ring• Forward Endcap: 8 rings in θ, 80/100/120 crystals

per ring

• Angular coverage: 126◦ in θ, 360◦ in φ• Crystals are non-projective in θ by 14 mrad (45 mrad

in the barrel/endcap transition region)

• Gap of ca. 2 mm between barrel and endcap, fullycovered by non-projectivity

Martin Kocian Calor02 26 March 2002

Calorimeter Support Structure

IOB

Aluminum Support Cylinder

Electronic Mini–Crates

Aluminum RF Shield4 m

0.9 m0.48 m

3-20018572A06

Cooling Channels

Cu Heat Sink

Bulkhead

Fan OutBoard

ADBs

Detail of Mini–Crate

Aluminum Strongback

MountingFeatures

Cableway

Carbon Fiber Tubes

Detail of Module

• Barrel• 280 carbon fiber modules• 7 modules along θ, 40 along φ• 7 crystals in θ, 3 in φ per module

• Endcap• 20 carbon fiber modules• 41 crystals per module (8 in θ, 4/5/6 in φ)

=⇒ Readout electronics: see I. Eschrich’s talkMartin Kocian Calor02 26 March 2002

Crystal Housing

CsI(Tl) Crystal

Diode Carrier Plate

Silicon Photo-diodes

Preamplifier BoardFiber Optical Cable

to Light Pulser

AluminumFrame

TYVEK(Reflector)

Mylar(Electrical Insulation)

Aluminum Foil

(R.F. Shield)

CFC Compartments(Mechanical

Support)

OutputCable

11-20008572A02

• Trapezoids: Front 4.7 x 4.7 cm2, back 6.1 x 6.0 cm2• Length between 16.0 X0 (bwd) and 17.5 X0 (fwd)• crystal is held in place by 300 µm of CFC• 2 x 160 µm Tyvek wrapping for reflection and tuning• Aluminum foil/Mylar wrapping• Lightyield was typically 7300 photoelectrons/MeV

during QC

• 2 photodiodes glued with epoxy onto the crystal viaa polystyrene coupling plate - area 2 x 2 cm2

• 2 preamps in readout box above crystalMartin Kocian Calor02 26 March 2002

Calibration Overview

• There are two kinds of calibrations:• Inter crystal calibration:• The crystals have different lightyields• The crystals have different uniformities

−5%

+5%

−2%

+2%

back front

PD

PMT

low energy shower

high energy shower

!!! Lightyield and uniformity change with time throughradiation damage

=⇒ Radiation damage studies: see T. Hryn’ova’s talk• Shower corrections:• Edeposited 6= Etrue• The deposited energy depends on the leakage that

is a function of geometry and material in the de-tector

Martin Kocian Calor02 26 March 2002

EMC Calibration Overview

Properties of the different calibrations

Rad Bhabha 0.3 − 9 GeV1 − 2 days

15 min 0 − 13 GeVElectronics

Duration Energy scale Single Xtal Absolute

3−9 GeV12 h

3 min 0 − 13 GeVLight Pulser

Pi 0

Bhabha

4 h

Source 1/2 h 0.00613 GeV

0.03 − 3 GeV

=⇒ The lightpulser calibration and the electronics cali-bration will be covered in detail in I. Eschrich’s talk

Martin Kocian Calor02 26 March 2002

Source Calibration

• Calibration uses 6.13 MeV photons from16N →16 O∗ →16 Oγ

• 16N has a lifetime of 7 seconds• A fluid, activated by neutrons from a generator, cir-

culates through a system of tubes in front of thecrystals

• All crystals are calibrated with this method• Resolution of the constants: 0.33 %• Source spectrum with two escape peaks:

Martin Kocian Calor02 26 March 2002

Bhabha Calibration I

e- e+

E

E = 9 GeV

E = 3.1 GeVθ

+

-

+

E-

θ

calorimeter

• Single crystal calibration to Eexpecteddeposited (from MC)• Find constants ci so that

χ2 = ∑events(∑

i ciεi−Eexpecteddeposited)2σ2

becomes minimal

Martin Kocian Calor02 26 March 2002

Bhabha Calibration II

• Minimization of χ2 yields a system of linear equa-tions with a 6580 x 6580 matrix−→ numerical solution

• Fit with log-normal distribution

Eσ =E

Eσ =E

Eσ =E

Eσ =E

1.8 % 1.95 %

2.1 % 2.4 %

BaBar

BaBar

BaBar

BaBar

Martin Kocian Calor02 26 March 2002

π0 Calibration

• Deposited energy → Particle’s original energy• Material in front of the calorimeter:

(rad)θ Polar Angle0 0.5 1 1.5 2 2.5 3

) 0

Mat

eria

l (X

10-2

10-1

1

EMC

DRC

DCH

SVT

0.7 X

0.3 X0

0

• Use known π0-mass to correct the photon energies:mπ0 =

√E1E2(1− cos θ), θ:angle between photons

• π0 spectra for different decay types:

GeV GeVGeV GeV

Martin Kocian Calor02 26 March 2002

π0 Calibration

• Make mass plots in bins of log10(E) and cos(θ)200 − 250 MeV150 − 200 MeV125 − 150 MeV100 − 125 MeV

• Interpolate between bins with polynomials

ln(

m(p

i0)

/ m(g

amm

a ga

mm

a))

ln(

m(p

i0)

/ m(g

amm

a ga

mm

a))

cos (theta)ln(E (GeV))

• Calibration function Monte Carlo (Run 1 cuts)• Calibration function data Run 1• Calibration function data Run 2• Data points after correction Run 2

Martin Kocian Calor02 26 March 2002

π0 and η resolution

m/GeV0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24

Ent

ries

0

2000

4000

6000

8000

10000

BaBar

mass = 134.9 MeV

sigma = 6.5 MeV

(GeV)γγm0.3 0.4 0.5 0.6 0.7 0.8

Ent

ries

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

> 1000 MeVγγ Mass Eη

BaBar

-mass = 547.0 MeVη

-width = 15.5 MeVη

Martin Kocian Calor02 26 March 2002

Energy resolution

/ GeVγE10-2

10-1

1

Del

ta E

/ E

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

γγ → 0πγγ → η

Bhabhasγ ψ J/→ cχ

radioact. SourceMonteCarlo+BGMonteCarlo

Martin Kocian Calor02 26 March 2002

Angular resolution

/ GeVγE0 0.5 1 1.5 2 2.5 3

Del

ta T

heta

(ra

d)

0

0.002

0.004

0.006

0.008

0.01

0.012

0.0142σ +

1/2/E1σ) = θ(σ

0.04) mrad± = (4.16 1σ

0.00) mrad± = (0.00 2σ

γγ → 0πγγ → η

MonteCarlo+BG

MonteCarlo

Martin Kocian Calor02 26 March 2002

Electron Identification

• The calorimeter is used for electron identificationby cutting on

• ECalorimeter/pTrack• shower shape

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

0.10.20.30.40.50.60.70.80.9

1

0

0.002

0.004

0.006

0.008

0.01

o < 80labθ < o20

- e-π

BABAR

[GeV/c]labp

Effi

cien

cy

• Electron efficiency• π-mesons misidentified as electrons

Martin Kocian Calor02 26 March 2002

Conclusion

• The Babar Electromagnetic Calorimeter has beenworking reliably for the past 2.5 years

• Performance is close to expectations• Frequent calibration is necessary to compensate for

changes in lightyield

• The calorimeter is a crucial component for doingphysics with Babar

Signal for B0 → J/ψKs with Ks → π0π0

Martin Kocian Calor02 26 March 2002