Instrumentation of the very forward region of the TESLA detector

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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 isBeam Calorimeter
• LAT Low Angle Tagger covering angles 27.5
  83.1 mrad, actually it isLuminosity 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 ee- 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 LCAL

• Example of a 250 GeV electron event1. 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 gt 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 LCAL

• Energy resolution as functions of
  R
• Energy

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Angular resolution of LCAL

• Energy dependence
  R dependence

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Fake events in LCAL

• High energetic particles in background
• Beamstrahlung fluctuationsElectron energy
  spectrum
• generated (tail gt20 GeV only is shown)
  ...and reconstructed

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Beam diagnostics in LCAL

• Measurement of ?x and ?z
• ExerciseInput ?x 600 nm, ?z 250
  ?mResult?x 597 nm, ?z 241 ?mMore about
  see A.Stahl talk at this workshop

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LAT geometry

• 14 cylinders in R24 sectors in ?40 rings
  in z?13440 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 ee- ? ee- radiative ee- ? ee- ?
• Example of elastic Bhabha event ? (scale
  ratio 81!)
• Born approximation
• d?/d? ?-3
• ?tot(27.5, 83.1mrad)?5nb

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Luminosity measurement

• R 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 errors1. from detector acceptance
• Since ?tot(?min, ?max) ?min-2 - ?max-2 ? ?min-2
  
• then ?L/L 2??min/ ?min 2?rmin / 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 (contd)

• Luminosity spectrum due to
• ISR
• Beamstrahlung
• Beam energy spread
• can 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 effect
• So 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 electronsUpper plots show all hits,
lower plots show the shower coresStrong
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 (contd).Energy deposit as
a function of polar angle All events
cut Edep gt 2.5 GeV
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LAT performance improvedEnergy cut Edep gt 2.5GeV
applied Energy response and resolution
Angular resolution
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Summary and outlook

• LCAL
• 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
• RD started
• LAT
• First MC studies done
• Problems with energy resolution and angular
  resolution
• Shape and segmentation is still an open question
  ?flat LAT l5m option is very recommended1.
  Makes mechanical design more realistic2.
  Improves resolution
• Single module for tests will be prepared soon
• The LCALLAT proposal will be recommended by the
  PRC, so lets look forward in this adventure!