The BESIII Luminosity Monitor

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The BESIII Luminosity Monitor

• High Energy Physics Group
• Dept. of Modern Physics,USTC
• P.O.Box 4 Hefei, 230027

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Side view of the near IP region
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Channels to Measure Luminosity

• The Bhabha Channel
• ee- ? ee- (? ) at small angle with
  
• respect to the IP and beam
• Lowest Order Diff. Cross Section
• d? /dcos? ? ?2(3cos2 ?)2/8Eb2(1-cos?)2

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Event rate estimation for 3 ? regions(Assume L
1033cm-2s-1, Ebeam1.55GeV, ? 2 ? covered)

• Extreme Forward Region (5o to 16o )
• Event Rate 12743 Hz
• End Cap Region (21o to 34o )
• Event Rate 412 Hz
• Barrel Region (34o to 146o)
• Event Rate 423 Hz

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LUM Type I Extremely Forward Luminosity Monitor

• The Defining and Complimentary Counter
• Dimension of ? Scintillation fiber
• or Silicon
  Strips
• Dimension of f Plastic scintillator
• The Calorimeter
• BGO / PWO Crystal

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Requirement on space resolution

• The precision requirement on the inner edge of
  the tracker part should be 160 ?m for
• a tracker put at Z 41.6cm To make the
  Bhabha event accepted within a 1 change
• (This also sets installation precision of
  the micro-beta magnet If it is around 1 mm,
  error of luminosity measured gt 6)

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Arrangement of the EFLM
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LUM Arrangement(Tracker not plotted)
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Front View of Defining/Complimentary Counter
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Cross Section of Fiber Bunch
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Separation Power of the Calorimeter
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Effects of the support Al structure of the MDC

• Effective thickness of the Al plate and tube
• 25 to 50 mm for different angles
• Al plate 20 mm , Al tube surround the beam pipe
  2mm
• R.M.S of the track smearing for the case
• of 45mm thick Al case 0.905 mm
• Corresponding to a 7 of error in event count

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Track distribution 20 cm away from the Al
surfaceEffective Al thickness 35 mm
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Track deflection by the AlEffective thickness
45 mm
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Secondary charge track numbers due to
AlEffective Al thickness 45 mm
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Error estimate for track smearingand
installation precision

• Track smearing due to Al
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• Assuming a 1mm error in the installation
  precision of the micro-beta magnet
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• Total effect
• gt 8

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LUM Type IIZero Degree Luminosity Monitor

• Luminosity Monitor Based on e-(e)single
  Bremsstrahlung(SB)
• The photons ? are emitted along the e-(e)
  direction within a cone of total aperture of
  (me/Eb) with cylindrical symmetry, where Eb and
  me is energy of beam and mass of electron
  respectively.

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Position of the ZDLM
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Photon energy

• Maximum energy
• ? is the total energy in the center of
  momentum system. For BES3 of BEPC2, the cone of
  total aperture of photon radiated is about 0.33
  mrad.and kmax is 1550MeV
• if Ebeam 1.55GeV

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Formula for Luminosity calculation

• If a photon detector is located coaxially
  with the incident beam line and is subtended to
  IP with a solid angle of ?D, the counting rate of
  NsB(kt) is measured the luminosity can be
  obtained by

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Photon energy spectrum with different Kt
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Angular distribution for different Kt
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The acceptance and the rate estimationSuppose a
calorimeter is located behind the splitter
magnets at the position of 10 meter away from the
IP. An aperture of ?20 mm lead collimator coaxial
with the incident beam line is assembled in the
cross sections of the calorimeter with various
photon energy cuts kt
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kt dependence of FAC(kt). Its shown that the
total aperture of 2 mrad for the calorimeter is
able to accept more than 87 of the SB-photons
for ?lt 1mrad. 66 for ?lt0.5mrad.
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Background

• Beam gas Bremsstrahlung (GB) background. The
  calorimeter faces the direction of the incident
  ebeam, so that the beam gas Bremsstrahlung in
  the IP region (30meter straight part) is the
  main background of SB photonGB-background. GB
  has a very similar energy spectrum and angular
  distribution with the SB photon

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Energy spectrum of GB photonsAssuming 10-7 mmHg
vacuum in the 30 m long chamber
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Background caused by beam lost

• The lost beam (BL) hits the vacuum chamber, the
  spread secondary photons and electrons would be
  another background source of SB counting. A veto
  counter, which is sensitive to charged particles
  in the front of the calorimeter, could
  effectively suppress the secondary charged
  particles and make the beam lost background
  negligible.

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Calorimeter system

• The SB photon rates are so high, Its
  difficult to count photons one by one, doing
  energy analysis is apparently impossible. We
  could not be able to set kt cut for readout
  electronics. So absolute luminosity measurement
  based on SB process is hardly to do. High SB
  photon flux is an advantage for relative
  luminosity monitoring, the integrated currents
  output from the photon calorimeter will be a
  relative measurement for the real time luminosity.

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• Detector GSO crystal 5515cm3 coupled
  with photodiode.
• The high flux of SB photons (from 10 to
  1550 MeV) will deposit their energies in the
  crystal and the absorb dose will be up to 0.23
  Mrad/day. So that the radiation hardness of GSO
  should be good.

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The photo-diode Hamamstsu S3584-09 will be
coupled through the air light guide and concave
mirror to the GSO like the Belle design
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The sensitivity to the parameters of IP,
transverse positions (x,y) and crossing angles

• Fixing the e beam 11mrad relative to z axis
  and the e- beam 11mrad relative to z axis, the
  axis of the calorimeter, which faces the IP and
  subtends a half angle of ?,is coincided with the
  axis of incident e- beam, steering the e- beams
  axis deviated from the original axis with an
  amount of ??

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Factor of photons accepted changes due to
crossing angle error (1mrad acceptance)
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Factor of photons accepted changes due to
crossing angle error (0.5mrad acceptance)
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The relative acceptance changes with the ?x
(1mrad acceptance)
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The relative acceptance changes with the ?x
(0.5mrad acceptance)
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Conclusion

• The EFLM can be used as a relative luminosity
  online monitor for BESIII while
• the precise value of luminosity can be
  completed by end cap and barrel detectors.
• SB photons measurement by the ZDLM can be used
  as a sensitive real time and relative luminosity
  monitor for BEPC2