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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
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