Update on Large Angle Beamstrahlung detector for SuperKEKB

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Update on Large Angle Beamstrahlung detector for
SuperKEKB

• J. Flanagan, K. Kanazawa, KEK
• H. Farhat, R. Gillard, G. Bonvicini, Wayne State
  University
• Goal to build a 1 monitor of beam-beam
  interaction parameters

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Admin. status

• 2 graduate students added
• Hosei Yosan 50,000 spent for first prototype
  hardware
• WSU in-kind contribution of 43,000 (graduate
  students salary)
• Nichibei 5,000 (not spent yet)
• NSF 2 proposals pending

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What is beamstrahlung

• The radiation of the particles of one beam due to
  the bending force of the EM field of the other
  beam
• Many similarities with SR but
• Also some substantial differences due to very
  short magnet (L?z/2v2),very strong magnet (10T
  at KEKB). Short magnets produce a much broader
  angular distribution
• Discrimination against machine backgrounds done
  MOSTLY by angular collimation. At SuperKEKB,
  small leftover backgrounds to be further
  subtracted through spectral analysis
• Beamstrahlung POLARIZATION at specific azimuthal
  points provides unique information about the
  beam-beam geometry.

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Some examples of Large Angle BMST pattern
recognition
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¼ CESR Set-up principal scheme

• Transverse view
• Optic channel
• Mirrors
• PBS
• Chromatic mirrors
• PMT numeration

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Set-up general view

• East side of CLEO
• Mirrors and optic port 6m apart from I.P.
• Optic channel with wide band mirrors

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On the top of set-up

• Input optics channel
• Radiation profile scanner
• Optics path extension volume

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Main CESR results page

• Signal(x) strongly correlated to II-2
• Signal strongly polarized according to ratios of
  vertical sigmas
• Total rates consistent with expectations at 10.3
  mrad

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DESIGN OF THE SuperKEKB DETECTOR

• Numerous changes compared to CESR device provide
  far better signal, signal stability, control of
  systematics, detector uniformity
• Current test bench aims at characterization of
  detector spectral response to 0.3, and test
  bench measurement of angular acceptance

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Most important change much stronger beams at
SuperKEKB. Comparison at ?5mrad, ?300-600nm,
0.5mrad2 acceptance)
Qty CESR-c S.KEKB Ratio
Sx(Hz) 6E4 3E11(L),1E11(H) (Prel.) 2-6E6
Sy(Hz) 6E4 6E10(L),2E10(H) (Prel.) 0.3-1E6
Bx(Hz)(?N/?) 2E6 (est.) 2E5(L),1E5(H) 0.05-0.1
By(Hz)(?N/?) 2E6 (est.) 2E6(L),1E6(H) 0.5-1
B(from beam) Very small Very small
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Beam pipe insert

• View port location at 90 degrees minimizes
  backgrounds, polarization measurement errors, and
  provides redundancy against beam orbit errors
• To be located anywhere between 5 and 10 mrad from
  the beam direction at the IP
• Suggested mirror and window sizes 1.5X2mm2 and
  1.7X1.7 mm2 (we could go lower at 10 mrad)

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Beam transport and optics box

• Light is transported to optics boxes by means of
  simple (and replaceable) black-anodized pipes
  (2.5 cm ID) and mirrors
• Device consists of achromatic telescope with
  pinhole optics, pol. Splitter, and two gratings
  illuminating 4 PMT with filters (total system
  32PMTs)
• Many adjustment screws throughout system

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Current activities
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Current activities (all measurements to 0.1
except absolute calibration of PMTs)

• Characterization of PMTs (nearly done)
• Spectral characterization of all optical
  components (mirrors, windows?, splitter,
  gratings, PMTs)
• Uniformity of all components
• Build plywood optics box, check optics,
  achromaticity and focus
• Build and test optics box

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Some results
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Extra slides
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CESR mirrors technical design
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Check for alignment _at_ 4.2GeV
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Directionality

• Scanning is routinely done to reconfirm the
  centroid of the luminous spot.

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If the angle can be considered large and constant

• Assuming (atan(z/?)atan((L-z)/ ?) as the field
  profile, one gets (u????s,ccos,sin(?))

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Large angle beamstrahlung power

• Total energy for perfect collision by beam 1 is
  P00.11?2re3mc2N1N22/(?x2?z)
• Wider angular distribution (compared to
  quadrupole SR) provides main background
  separation
• CESR regime exponent is about 4.5
• ILC regime exponent is very small
• KEKB exponent is small

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2nd major change much better event record

• CESR record contained BMST data, bunch-by-bunch
  currents, luminosity monitors, independent
  measurements of vertical heights, energy, as well
  as other unused quantities. Beam length and beam
  horizontal size were computed by measuring size
  of luminous region using CLEO hadronic events.
• Need at least Beam Position Monitors near the IP
  to monitor beam shifts both in quads and in
  detector-beam axis angle

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Properties of large angle radiation

• It corresponds to the near backward direction in
  electron rest frame (5 degrees at CESR, 2-4
  degrees at KEKB/SuperKEKB, 7 degrees at DAPHNE)
• Lorentz transformation of EM field produces a
  8-fold pattern, unpolarized as whole, but locally
  up to 100 polarized according to cos2(2?),
  sin2(2?) with respect to direction of bending
  force (Bassetti et al., 1983)

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Beam-beam interaction (BBI) d.o.f. (gaussian
approximation)