Calibration of the ZEUS calorimeter for electronstapper/talks/ringberg_electrons.pdf · Calibration...
Transcript of Calibration of the ZEUS calorimeter for electronstapper/talks/ringberg_electrons.pdf · Calibration...
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Calibration of the ZEUScalorimeter for electrons
Alex TapperImperial College, London
for the ZEUS CollaborationWorkshop on Energy Calibration of the ATLAS Calorimeters, 21-24 July, 2002, Ringberg Castle
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Outline• HERA physics• The ZEUS detector
– Dead material– Non-uniformity
• Combining information– Presampler detectors– Tracking detectors– Hadron Electron
Separators
• RCAL Calibration• BCAL Calibration• Results• Summary
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HERA physics
22
q2P
q
2
22
P)(qW
y
P)(qx
2
+=
=
+=
!=
"
!
resolving power of probe
fraction of proton momentum
inelasticity parameter
mass of hadronic system
• Neutral Current DIS exchanged boson γ or Z0
• Charged Current DIS exchanged boson W±
• Photoproduction exchanged boson γ with Q2~0 GeV2
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HERA physics requirements
• Neutral Current– Energy and angle measurement for electrons
• Charged Current– Measurement of inclusive hadronic final state– Measurement of missing momentum
• Jets & tests of QCD– Measurement of jet energy and angle– Measurement of jet shape
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HERA kinematics
• Measure energy and angle of scatteredlepton and hadronic final state
• Over constrained system - only twodegrees of freedom– Transverse momentum balance PT
e=PTh
– Longitudinal momentum (E-pz)e + (E-pz)h = 2Ee(beam)– Double angle method (next slide…)
• Use all possible methods to studysystematic uncertainties
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Double Angle Method• Predict E’e and Eh from scattering angles γ and θ
• Insensitive to overall energy scale of the CAL• Sensitive to ISR• Relies on good understanding of the hadronic final
state and precise position reconstruction• Angles measured more accurately than energies
at ZEUS
ã)sin(èsinèsinã
sinã
e
'
DA E2E+!+
"=
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The ZEUS detector
p 920 GeVe ±
27.5 GeV
BCAL
RCALFCAL
CTDSOLENOID
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The ZEUS calorimeter - test beam
• Uranium-Scintillator• Compensating (e/h=1)• Calibration from UNO• ~6000 cells• Precise timing
information• Solenoid between
tracking and CAL
E(GeV)
18%E
ó(E)=
E(GeV)
35%E
ó(E)=
Electrons
Hadrons
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The ZEUS calorimeter - geometry
• EMC cells– 5x20 cm2 (10x20 cm2 in
RCAL)
– 1 interaction length
• HAC cells– 20x20 cm2
– 3 interaction lengths (2in BCAL)
• Readout 2 PMTs percell
• Imbalance givesposition
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The ZEUS calorimeter - calibration
• Uniform structure throughout the entirecalorimeter
• Natural uranium activity provides absoluteenergy calibration
• Each cell is calibrated back to its testbeam result
• Calibration runs taken between physicsruns
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Dead material• In barrel region
solenoid ~ 1X0
• Endplates of CTD• Support structures
for forward trackingdetectors
• Cryogenics in rear ofdetector
• Measuring electronenergy precisely is achallenge!
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Non-uniformity
• Geometry of CAL leads to non-uniform electron energy response
• Energy lost between CAL cells• Energy leakage between CAL sections• Energy leakage around beam pipe
holes• Variation typically between 5-10%
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Combining detector information
• Presampler detectors– Information on dead material
• Tracking detectors– Absolute energy calibration at low energy and
alignment
• Hadron Electron Separators– Distinguish between EM and HAD showers– Position information
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RCAL Electron Calibration
Significant deadmaterial betweenIP and RCALRCAL has largestEMC cells(10x20cm2)Cannot rely ontracking
High statistics
• Use NN e-finder• Kinematic Peak
events– E’e~27.5 GeV
• Double AngleMethod– 15 < E’e < 25 GeV
• QED Compton– 5 < E’e < 20 GeV
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SRTD & Rear presampler
• 20x20 cm2 tiles• Mounted on face of RCAL• Use MIPS signal to
calibrate for dead material
• Scintillator strips• Precise position
measurement• Use MIPS signal for
energy calibration
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Non-uniformity• Choose events with low
MIPS in SRTD and PRES• Compare position of KP on
cell-by-cell basis tocorrect each cell touniform response
• Consider ECAL/EDA as afunction of position
• Derive corrections for non-uniformity between CALcells independently fordata and MC
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Dead Material• PRES and SRTD give MIPS
signal proportional toenergy loss
• Calibrate using KP and DAevent samples
• No dependence on electronenergy
• Dependence on radius i.e.angle of incidence– Also seen in test-beam
• Derive suitablecorrections
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Hadron Electron Separators• 20 m2 of diodes• 300 µm Si pad detectors• Located at EM shower max
~4X0 in RCAL and FCAL• Highly segmented (3x3
cm2) gives improvedposition measurement
• Separation of e± and γfrom hadrons, in particularinside jets
• Input to NN e-finders
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BCAL Electron Calibration
Solenoid betweenIP and BCAL
Use CTD track forelectron angle –good resolution inDA method
Compare CTDtrack momentumwith CAL energy
• Elastic J/ψ events• QED Compton
events• Double Angle
Method
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Central Tracking Detector• Drift chamber• 15° < θ < 164°
• ~5K sense wires• Resolution:
• Use to calibrate CALat lower energies
TT
T
p0.0014
Tp
)ó(p0.00650.0058p !!=
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Barrel presampler• Most electrons shower in
solenoid before hitting CAL• BPRES signal proportional
to losses in dead material• Produce dead material
“map”• Correct electron energy• Can also use for hadrons
and γ/π0 separation….
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Barrel presampler
• Fit BPRES MIPS signal to ECAL
• Correction is energy independent
E’e 20 GeV
E’e 25 GeV
E’e 30 GeV
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Non-uniformity• Clearly see the
structure of theBCAL cells inuncorrected data andMC
• Consider ECAL/EDA as afunction of position
• Use track position toderive corrections interms of z and ϕ
• Become limited bystatistics in +zdirection
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Physics channels – Elastic J/ψ
• J/ψ -> e+e-
• e+e- in BCAL• Compare track
momentum and CALenergy
• Range 1-3 GeV
Absolute energycalibrationOnly low energyLow stats
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Physics channels - Elastic QEDCompton
• Clean signal• e in BCAL• Compare track
momentum and CALenergy
• Range 2 < E’e < 15 GeV Absolute calibration Bridges gap between
high and low energyLow stats
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Physics channels - NC DIS• Use DA method Good position
resolution from track Spans all energy
rangesLimited by statisticsfor higher energiesBias in DAreconstruction limitsaccuracy at lowerenergies
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Results
• Uncertainty in RCAL ±2% at 8 GeV falling to±1% for energies of15 GeV and higher
• Uncertainty ±1% inBCAL
• Insufficient statisticsin FCAL– Use result of test-
beam and assignuncertainty of ±3%
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Summary• Combined information
from sub-detectorsto improve the CALelectron energymeasurement
• Used physics channelswith overlappingenergy ranges
• Systematicuncertainty of ±1%for most physicsanalyses