Instrumentation of the very forward region of the TESLA detector –
summary of the Workshop on Forward Calorimetry and
Luminosity Measurement, Zeuthen, 13-14 November 2002
L.Suszycki†‡
Faculty of Physics and Nuclear Techniques, University of Mining and Metallurgy,Cracow, Poland
ECFA DESY Linear Collider Workshop
Prague, 15th-18th November 2002
†For the LCAL group: K. Afanaciev, V. Drugakov, G. Klämke, E. Kousnetzova, W. Lohmann, A. Stahl, M. Ternick
‡For the LAT group: H. Abramowicz, A. Eskreys, S. Kananov, D. Kisielewska, A. Kowal, A. Levy, L. Suszycki, W. Wierba
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Very forward region of TESLA
Two calorimeters:
• LCAL – Luminosity CALorimeter covering angles 5 – 27.5 mrad, actually it is“Beam Calorimeter”
• LAT – Low Angle Tagger covering angles 27.5 – 83.1 mrad, actually it is“Luminosity Calorimeter”
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Tasks for the very forward region detectors
• Precision luminosity measurement
• Beam diagnostics
• Detection and measurement of electrons and photons at small angles
• Extension of the energy flow measurement down to small angles
• Shielding of the tracking detectors against backscattered beamstrahlung
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Beamstrahlung• Energy deposition via e+e- pairs ~20 TeV/bunch cr.
One year of runs makes a dose ~10 Mgy/year in LCAL a need of radiation hard sensors
• Radial distribution up to ~4 - 5cm LAT should be save
• Azimuthal anizotropy caused by magnetic field
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LCAL design
• Shape
• Two technology options: crystal PbWO4 – Moliere
radius ~2 cmLSO crystals considered
diamond-tungsten sandwich - Moliere radius ~1 cmSegment size ~half of RM
• Readout via optical fibres
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LCAL segmentation
R-z projection x-y projection
30 layers = 30 radiation lengths 12 rings
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LCAL calorimeter• Half barrel of
the diamond-tungsten sandwich calorimeter
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Background in LCALExample of a 250 GeV electron event
1. Generated 2. Background added 3. Reconstructed
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Detection of particles in LCAL
Simple algorithm to find electron or gamma:
• search for cells with signal > 3 sigma of background
• require longitudinal chain of such cells
Efficiency and energy resolution
depend on number of ADC bits 10 bits sufficient
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Energy measurement in LCALEnergy resolution as functions of : R Energy
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Angular resolution of LCALEnergy dependence R dependence
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Fake events in LCAL• High energetic particles in background• Beamstrahlung fluctuations
Electron energy spectrum
generated (tail >20 GeV only is shown) ...and reconstructed
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Beam diagnostics in LCALMeasurement of
x and z
ExerciseInput:
x = 600 nm, z = 250 mResult:x = 597 nm, z = 241 m
More about: see A.Stahl talk at this workshop
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LAT geometry
• 14 `cylinders` in R24 `sectors` in 40 `rings` in z13440 cells assumed for MC studies
• Conical setup
• R: 4 to 12 cmz: 140 to 200 cm
• Silicon-tungsten sandwich 40 X0 deep
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Bhabha scattering• Elastic e+e- e+e-
radiative e+e- e+e-
• Example of elastic Bhabha event (scale ratio 8:1!)
• Born approximation
d/d ~ -3
tot(27.5, 83.1mrad)5nb
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Luminosity measurementR = L · obs , where obs = theor acceptance
For L = 3.4 · 1034 cm-2s-1 rate R 170 Hz `one minute` luminosity possible on-line
• Systematic errors:1. from detector acceptance
Since tot(min, max) ~ min-2 - max
-2 min-2
then L/L = 2min/ min = 2rmin / rmin
With rmin 5 cm, L/L = 10-4 needs rmin = 2.5 m
• Challenging for mechanics and thermal stability!Use of interferometry to monitor position and shape?
2. from theoryAt LEP energy theor. error achieved 5. 10-4 .At TESLA may be harder
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Luminosity measurement (cont`d)Luminosity spectrum due to• ISR• Beamstrahlung• Beam energy spreadcan be measured through acolinearity of Bhabha events -
- resolution better than 0.1 mrad necessary
Background and corrections• Beamstrahlung• Synchrotron radiation• Beam-gas bremsstrahlung• Thermal photons from the rest gas• Beam size effectSo far no estimations done...
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LAT shower example
Only photons (blue) and electrons (red) over 5 MeV are displayed
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LAT calorimetryShowers well contained within ~ 30 radiation lengths:
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LAT – showers...
Two examples of hits generated by 250 GeV electrons:Upper plots show all hits, lower plots show the shower cores
Strong scattering may affect energy measurement
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LAT energy response and resolution
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LAT angular uniformity Energy response Energy resolution
Much poorer performance in the first bin (first cylinder) the effect increasing with energy
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LAT angular resolution
rec calculated using a simple energy weighting
• Accuracy not satisfactory
• Background not incorporated
• More sophisticated algorithm must be used for Bhabha measurement
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LAT fiducial volume
Remark: LAT size is 27.5 to 83.1 mrad
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LAT fiducial volume (cont`d).
Energy deposit as a function of polar angle All events cut Edep > 2.5 GeV
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LAT performance improvedEnergy cut Edep > 2.5GeV applied
Energy response and resolution Angular resolution
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Summary and outlookLCALLCAL• Advanced studies of the several technologies• Detection of hard electrons and gammas with high efficiency feasible• Fake events due to beamstrahlung fluctuations may be a problem• Beam diagnostics looks promising• R&D startedLAT• First MC studies done• Problems with energy resolution and angular resolution• Shape and segmentation is still an open question
flat LAT “ l=5m” option is very recommended:1. Makes mechanical design more realistic2. Improves resolution
• Single module for tests will be prepared soon
The LCAL+LAT proposal will be recommended by the PRC, so let`s look forward in this adventure!