The Interaction Region

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) 4.0 7.0 Current (A) 3.95 2.17 No. bunches 3466 Bunch spacing (m) 0.63 Beat x* (mm) 20 20 Beta y* (mm) 0.2 0.2 Emittance x (nm-rad) 1.6 1.6 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* 0.30 m 0.0 Drift QD0 0.46 m 0.30 m -820.6 kG/m Both HER and LER QD0H 0.29 m 0.76 m -820.6 kG/m HER only B00L 0.40 m -1.05 m -2.2 kG Incoming LER only B00H 0.40 m 1.05 m 1.5 kG Incoming HER only QF1L 0.40 m ±1.45 m 293.2 kG/m LER only QF1H 0.40 m ±1.45 m 589.1 kG/m HER only B0L 2.0 m ±2.05 m 0.3 kG LER only (sign?) B0H 2.0 m ±2.05 m 0.526 kG HER only (sign?) QD0 offset 6.00 mm Incoming HER QD0 offset 7.50 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. F1 Ver. G3 PEP-II 3A on 1.8A Incoming HER 41 9 4 49 Incoming LER 28 1 1 16 Outgoing HER 41 152 93 49 Outgoing LER 28 67 55 16 Total 138 230 153 130

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