1. The Interaction Region
M. Sullivan
5th SuperB Workshop
Paris
May 9-11, 2007

2. Outline
Design Issues
IR Design
Toward an improved design
Summary

3. Detector Considerations
Reasonable angular acceptance
±300 mrad
Small radius beam pipe
10 mm radius
Thin beam pipe
SR backgrounds
Rates comparable to PEP-II
Few hits per crossing on Be beam pipe
Little or no hits on nearby beam pipes

4. Detector Considerations (2)
BGB backgrounds
Keep nearby upstream bending to a minimum
Suggest upstream bending further away from the detector (>10 m) to minimize the BGB integral
Low vacuum pressure upstream of the detector

5. Detector Considerations (3)
Luminosity backgrounds
Beam lifetimes
Radiative bhabhas
Beam-beam
Local HOM power
Small diameter beam pipes trap higher frequencies
Always get modes when two pipes merge to one

6. Accelerator parameters
LER HER
Energy (GeV)
Current (A)
No. bunches
Bunch spacing (m)
Beat x* (mm)
Beta y* (mm)
Emittance x (nm-rad)
Emittance y (pm-rad) 4 4
Full crossing angle (mrad) 34
These parameters constrain or define the IR design

7. Summary of Present Design
Crossing angle of ±17 mrad
Beam pipe diameter of 20 mm at the end of QD0 for both beams (same size as IP pipe)
This leaves enough room (~10 mm) to place a permanent magnet quadrupole and get the required strength (Using Br = 14 kG)
We have placed small bending magnets between QD0 and QF1 on the incoming beam lines to redirect the QF1 SR
The septum QF1 magnets for the outgoing beams are tilted in order to let the strong SR fans escape
The outgoing beams B0 magnets are a C shape design in order to allow the strong SR fans to escape

8. IR design parameters Length Starts at Strength Comments
L* m Drift
QD m m kG/m Both HER and LER
QD0H m m kG/m HER only
B00L m m kG Incoming LER only
B00H m m kG Incoming HER only
QF1L m ±1.45 m kG/m LER only
QF1H m ±1.45 m kG/m HER only
B0L m ±2.05 m kG LER only (sign?)
B0H m ±2.05 m kG HER only (sign?)
QD0 offset 6.00 mm Incoming HER
QD0 offset mm Incoming LER

9. SR Power Numbers
The design (G3) has a total SR power comparable to PEP-II
SR power in QD0 (kW) for beam currents of 1.44A HER and 2.5A LER
No QD0 offsets Ver. F Ver. G PEP-II
3A on 1.8A
Incoming HER
Incoming LER
Outgoing HER
Outgoing LER
Total

11. LER SR fans

12. HER SR fans

13. ±1 meter

14. SR fans

15. Some SR background details
We are using a gaussian beam distribution with a second wider and lower gaussian simulating the “beam tails”
The beam distribution parameters are the same as the ones used for PEP-II
We allow particles out to 10 in x and 35 in y to generate SR
Unlike in PEP-II the SR backgrounds in the SuperB are dominated by the particle distribution at large beam sigma, so we are more sensitive to the exact particle distribution out there

16. Radiative Bhabhas
The outgoing beams are still significantly bent as they go through QD0
Therefore the off-energy beam particles from radiative bhabhas will get swept out
Knowing this, we will have to build in shielding for the detector

17. HER radiative bhabhas

18. LER radiative bhabhas

19. How to improve the design
The best improvement would be to reduce the radiative bhabha background
Note that there is only a small gain in beam separation from the strong outgoing bending because one has to allow the outgoing SR to escape (see slide 14)
The only gain comes from the BSC moving away from the septum

20. Attempts to improve the design
Three possibilities so far looked at
Reduce the strength of the shared element
Difficult to control beta functions (Can’t let the beta functions get too big)
Try a high strength but very short and close to the IP shared element (minimal off-axis trajectories)
Need a VERY high strength field to control beta functions
High field still bends a beam even with a small off-axis traj.
Eliminate the shared element
Wants a maximum crossing angle (±24 mrads?)
Can start one focusing magnet for one of the beams first and then follow with the focusing magnet for the other beam as soon as possible
Still need to control beta functions
Just got started on this option: no conclusion yet

21. More designs Other possibilities thought about
A longer, weaker shared element
End up with more bending at the outboard end
Wants a minimal crossing angle
Difficult to control beta functions
Asymmetric IR (more like ILC?)
Well controlled incoming beta functions
Outgoing beta functions allowed to get bigger
OK for ILC—not so good for storage rings

22. Summary
We have an IR design that has acceptable SR backgrounds with a crossing angle of ±17 mrad and an energy asymmetry of 7x4
The BGB and coulomb scattered beam particles as a background need to be calculated and controlled (been done?)
Radiative bhabha backgrounds are still high due to the strong bending of the outgoing beams
The total SR power generated by the IR is high for the same reason. This can cause emittance growth. Especially vertical emittance growth since this is in a coupled region.
A through exploration of parameter space is needed to find the best IR design