LumiCal concept including the tracker

1
LumiCal concept including the tracker

• R. Ingbir, P.Ružicka, V. Vrba

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Outline

• Motivation
• LumiCal concept
• Algorithms
• Performance
• Luminosity measurement
• Summary and outlook

Motivation

• Improvement of position reconstruction of the
  luminosity detector by adding the silicon tracker
  to the front of the calorimeter.
• The same concept was used at the L3 experiment at
  LEP (Si Tracker BGO Calorimeter) for luminosity
  measurement . The luminosity was also measured by
  measurement of Bhabha event rate.
• The tracker had 3 silicon layers ( 2 layers for ?
  measurement, 1 layer for f measurement)
• The total systematic error on the luminosity was
  about 1.5?10-3.

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Environment

• The simulation was done in G4.8 stand alone code
  developed in Prague.
• The 4000 events with single electron with uniform
  polar and azimuth angle distribution were used.
• The data were analyzed by our code which uses
  ROOT libraries.
• The station with 8 CPU was used for the
  simulation and analysis.

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LumiCal concept

• LumiCal is composed of the silicon tracker and
  Si-W calorimeter
• Two silicon layers are situated in front of the
  calorimeter

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Tracker geometry
Z distance from the IP 227 cm
Inner radius 80 mm
Outer radius 195 mm
Number of layers 2
Gap between the layers 50 mm
Silicon thickness 250 µm
Electronics support thickness 500 µm
Cylinder pitch 70 µm (1643 cylinders)
Sector pitch 0.36º (1000 sectors)

• Tracker is made of two fine granularity silicon
  layers
• The silicon layers have ministrip structure for
  the (r,f) measurement
• The small amount of material in front of the
  calorimeter is required

Definition of sector and cylinder
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Calorimeter geometry
Z distance from the IP 237 cm
Inner radius 80 mm
Outer radius 195 mm
Number of layers 30
Tungsten thickness 3.4 mm
Silicon thickness 300 µm
Electronics support thickness 2.5 mm
Number of sectors 48
Number of cylinders 48

• Si/W calorimeter
• The layer has about 1 X0 (WSiElectronics)
• The total depth is 30 X0

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Reconstruction algorithm

• The f, ? are reconstructed in the calorimeter
  using the algorithm with logarithmic weighting

• Then the algorithm looks for the hit cells in
  each tracker layer in cone (?calord?)?
  (fcalordf).

• The information from the tracker is used only if
  the following conditions are fulfilled
• There are one or two hit cells in the cone in
  each tracker layer
• If two cells are hit in the cone they must be in
  same sector and they must be in neighbors
  cylinders (cylinders r and r 1) in each tracker
  layer.

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Reconstruction algorithm

• The (Rlayer, flayer) position in each layer is
  computed as average position of hit pixels in the
  cone

If the pixel is hit ?i1 and if not ?i0

• Finally the ?track and ftrack are computed using
  the formula

, where n is number of layers
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Energy reconstruction(using the calorimeter)
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Polar angle performance of the calorimeter
Bias 5?10-6 rad Resolution 3 - 5 ?10-5 rad
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Azimuth angle performance of the calorimeter
Bias 2?10-5 rad Resolution 2 - 7 ?10-3 rad
Comparison of different weighting methods
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Tracker performance - polar angle
Bias 7?10-7 rad Resolution 5 -12 ?10-6 rad
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Tracker performance - azimuth angle
Bias 2?10-5 rad Resolution 18 - 20 ?10-4 rad
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LumiCal performance
Efficiency 96
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Luminosity measurement

• High statistics studies of luminosity error
• The angle ?gen was generated using BHWIDE Bhabha
  event generator
• 12 millions of events were generated
• Fast detector simulation was done using the
  results presented in previous page.
• Then the luminosity error was computed as

where, Ngen..number of particle generated
in acceptance region Nrec.. number of
particle reconstructed
in acceptance region
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Luminosity measurement
Calorimeter (Bias5?10-6)
TrackerCalorimeter (Bias7 ?10-7)
Using the bias 7.00?10-7 and resolution 5 ?10-6
the ?L/L(4.760.74) ?10-5
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Summary

• Stand alone G4 simulation that included the
  calorimeter with the tracker was studied.
• Significant improvement in the position
  reconstruction.
• Improvement in polar resolution is about factor
  of 7.
• Improvement in the theta bias is about factor of
  6.
• Improvement in the azimuth resolution and bias.
• The new design leads to better performance. The
  improvement of the luminosity measurement is
  about factor 7.
• This was done in very clean environment. The
  study was done only for single electron. Full
  simulation with Bhabha events was not done. The
  electronics noise was not included. There were no
  background from the other detectors. No selection
  cuts were applied.

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Outlook

• Study of this detector design in more real
  environment. Full simulation with Bhabha events
  including the Beamstrahlung and beam energy
  spread have to be done. The simulation have to
  include physics background and background from
  the other detector. Simulation of electronics
  have to be added.
• Further optimization of the detector geometry
  and the reconstruction algorithms for real
  environment.