The Detector and Interaction Region for a Photon Collider at TESLA

1
The Detector and Interaction Region for a Photon
Collider at TESLA

• Aura Rosca
• DESY Zeuthen
• Aachen, Germany, 17-23 July 2003

2
Motivation

• Higgs Physics
• Measure two-photon partial width and search for
  heavy Higgs states in extended Higgs models
• Electroweak Physics
• Excellent W factory allowing precision study of
  anomalous gauge boson interactions
• Physics beyond SM
• Search for new charged particles, such as
  supersymetric particles, leptoquarks, excited
  states of electrons, etc.

3
Principle of a Photon Collider
Crab Crossing Angle 2 deg.
2 mm
2 mm

• Run in mode
• Convert electrons in high energy photons via
  Compton backscattering of laser photons
• High energy photons follow electron direction

4
Layout of the Beams
Electrons Out
Electrons Out
IP
Laser in
Laser Out
Electrons In
Electrons in

• Disruption angle is larger then in because
  of beam-laser interaction
• Outgoing beam no longer fits through final
  quadrupole
• need crossing angle to have separate beam pipe
  for in- and outgoing beam
• Four beam pipes will enter the detector from each
  side.

5
Laser Requirements

• Laser wavelength
• Laser energy
• Pulse duration
• Rayleigh length
• Repetition rate TESLA collision rate
• Average power
• Pulsed laser with correct time structure and
  relaxed power requirements feed a resonant cavity
  with quality factor Q 100

6
Proposed Ring Cavity

• Cavity mounted around detector
• Round trip time repetition rate of the electron
  bunches
• Stabilization of the cavity length within about
  0.5 nm

Detector
focusing mirror
e
e
focusing mirror
12 m
laser
7
Laser-Electron Crossing Angle

• Need crossing angle electron beam-laser

• opening angle laser
• distance to e-beam

Laser crossing angle

• Laser collision angle reduces conversion
• Compensated by higher laser energy

8
Electron-Photon Conversion Probability
9
Luminosity

GeV
/
1
-
unpolarized
s
2
-
cm
32
10

??
s
d
/
dL
helicity --
10
Background
Background can be a factor 10 higher than in
LC

• Disrupted beam
• larger than in case and additionally
  widened by crab crossing
• Beam-beam interactions
• Incoherent pair production (ICP)
• Coherent pair production (CP)
• Neutrons from beam dump
• Background from physics processes, ex.

• Energy distribution on calorimeter face from
  one BX at z3.8 m

14 mrad
2
Units GeV/mm
11
Design of the Mask
HCAL
ECAL

• Redesign of TESLA detector in forward region to
  minimize background in TPC and VTX
• Two masks
• Longer outer mask
• Tungsten parts

TPC
outer mask (tungsten)
tungsten parts
IP
inner mask (tungsten)
100 cm
183 cm
12
Background in VTX

• With Mask
• Incoherent pairs
• 368 hits
• Coherent pairs
• 1 hit in the first layer and 3 hits in three
  last layers, from one event each
• 0.03 hits/mm in L1

• Hits per layer for ICP

1 layer
2 layer
2
no change necessary wrt design
4 layer
3 layer
5 layer
13
Background in TPC

• No mask
• Incoherent pairs
• 12900 photons / bunch
• Coherent pairs
• 400000 photons / bunch
• With Mask
• Incoherent pairs
• 927 photons / bunch
• Coherent pairs
• 2440 photons / bunch
• Reduction by a factor 125

• lt 1 occupancy
• factor 2.4 higher than in
• OK for TPC

14
Beam Steering

• Feedback e-e IP 88 nm x 4.3 nm
• Feedback Compton IP

Work in progress..
15
Beam Steering
1

• Electron beams are stabilized by fast feedback
  system measuring beam deflection at IP
• BPMs need large aperture because disrupted beam
  is larger
• Solution undisrupted Pilot bunches for beam
  steering
• Electron bunches stable over one train
• Photon beams follow electron direction
• Separate electrons and photons on dump

2
Dump
IP
16
Beam Dump

• Photons cannot be deflected electrically or
  magnetically
• Direct line of sight from IP to dump
• High neutron flux at vertex detector
• Narrow photon beam cannot be spread out and will
  always hit same window
• High thermal load on window
• High radiation damage to window

WIP
17
Conclusion

• Tesla offers the possibility to work as a Photon
  Collider
• The expected luminosity might be 20 of the
  luminosity at the LC
• Beam-beam backgrounds are larger but can be
  reduced redesigning the forward region
• Some more items need to be studied for a
  realistic design of a Photon Collider

18
Acknowledgements

• Many thanks to all my colleagues for providing me
  with their results.