1. The Detector and Interaction Region for a Photon Collider at TESLA
Aura Rosca
DESY Zeuthen
Aachen, Germany, July 2003

2. Aura Rosca DESY-Zeuthen
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.
17 July 2003
Aura Rosca DESY-Zeuthen

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
17 July 2003
Aura Rosca DESY-Zeuthen

4. Aura Rosca DESY-Zeuthen
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.
17 July 2003
Aura Rosca DESY-Zeuthen

5. Aura Rosca DESY-Zeuthen
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
17 July 2003
Aura Rosca DESY-Zeuthen

6. Aura Rosca DESY-Zeuthen
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
17 July 2003
Aura Rosca DESY-Zeuthen

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
17 July 2003
Aura Rosca DESY-Zeuthen

8. Electron-Photon Conversion Probability
17 July 2003
Aura Rosca DESY-Zeuthen

9. Aura Rosca DESY-Zeuthen
Luminosity ]
GeV / 1 -
unpolarized s 2 - cm 32 10 [ s γγ d / dL
helicity --
17 July 2003
Aura Rosca DESY-Zeuthen

10. Aura Rosca DESY-Zeuthen
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 z=3.8 m
14 mrad 2
Units: GeV/mm
17 July 2003
Aura Rosca DESY-Zeuthen

11. Aura Rosca DESY-Zeuthen
Design of the Mask
ECAL
HCAL
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
17 July 2003
Aura Rosca DESY-Zeuthen

12. Aura Rosca DESY-Zeuthen
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
3 layer
4 layer
no change necessary wrt design
5 layer
17 July 2003
Aura Rosca DESY-Zeuthen

13. Aura Rosca DESY-Zeuthen
Background in TPC
No mask:
Incoherent pairs
~ photons / bunch
Coherent pairs
~ photons / bunch
With Mask
~ 927 photons / bunch
~ 2440 photons / bunch
Reduction by a factor ~ 125
< 1% occupancy
factor 2.4 higher than in
OK for TPC
17 July 2003
Aura Rosca DESY-Zeuthen

14. Aura Rosca DESY-Zeuthen
Beam Steering
Feedback e-e IP: nm x 4.3 nm
Feedback Compton IP:
Work in progress..
17 July 2003
Aura Rosca DESY-Zeuthen

15. Aura Rosca DESY-Zeuthen
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
17 July 2003
Aura Rosca DESY-Zeuthen

16. Aura Rosca DESY-Zeuthen
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 July 2003
Aura Rosca DESY-Zeuthen

17. Aura Rosca DESY-Zeuthen
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
17 July 2003
Aura Rosca DESY-Zeuthen

18. Aura Rosca DESY-Zeuthen
Acknowledgements
Many thanks to all my colleagues for providing me with their results.
17 July 2003
Aura Rosca DESY-Zeuthen