SBS Magnet, Optics, and Spin Transport

1. SBS Magnet, Optics, and Spin Transport John J. LeRose Technical Review of the Super BigBite Project January 22, 2010

2. SBS: a “large” acceptance, small angle, moderate resolution device

3. 48D48 Basic Geometry

4. The magnet is “available”. To guarantee that it’s ours we need to formally transfer ownership.

5. Needed Modifications to the Magnet • For small angles at short distance • Cut opening in Yoke • Modify coils • For Polarized Target & background control • Add field clamp to reduce field at target • For beam transport to the dump • Field clamp (again) • Add magnetically shielded beam pipe • Add solenoid

6. Layout of system with part of yoke removed Field Clamp Shielded Beam pipe Modified Coils

7. With magnetically shielded pipe with 1kA/cm current density solenoid B at target < 2 Gauss Calculations by StepanMikhailov Using “Mermaid” (units are kG, cm) Nice clean magnetic field

8. y y’ x’ θ x 1 mm @ 30m Effect on beamline by transverse field is effectively eliminated

9. Various Views of the Modified Magnet

10. Optics It’s really very simple!

11. This is what it looks like to me!

12. Arbitrary Trajectory Reference Trajectory y x r0 Magnetic Midplane TRANSPORT formalism • References: • K.L. Brown, D.C. Carey, C. Iselin and F. Rothacker, Designing Charged Particle Beam Transport Systems, CERN 80-04 (1980) • K.L. Brown, SLAC Report-75 (http://www.slac.stanford.edu/cgi-wrap/getdoc/slac-r-075.pdf) • …...

13. x x x x z TRANSPORT formalism cont’d All trajectories are characterizedby their difference from a reference trajectory* *”The Central Trajectory” z y z l = length difference between trajectory and the reference trajectory Relative change in momentum

14. TRANSPORT formalism cont’d General Solution of the equation of motion: Each component can be expressed as a Taylor series around the Central Ray:

15. TRANSPORT formalism cont’d The first order transfer matrix: For static magnetic systems with midplane symmetry: If you know how well you can measure x, θ, y, and φ, you know how well you can determine the target parameters.

16. Projected Errorsbased on projected detector performance and general setup

17. Higher Order Effects? • Strategy: • Use SNAKE to create a database of trajectories and then fit the reconstruction tensor. (higher order terms) • Use the reconstruction tensor in a Monte-Carlo fashion to evaluate the errors. δ0-δmeas θ0-θmeas δ0-δmeas θ0-θmeas y0-ymeas y0-ymeas φ0-φmeas φ0-φmeas

18. Higher order terms, while necessary to accurately reconstruct the target variables, don’t contribute to the uncertainties in the measurements. i.e. They’re small corrections!

19. Momentum Dependence of ΔΩ 8 GeV/c 1 GeV/c

20. Spin Transport Non-dispersive precession Dispersive precession Target Target to Reaction Plane Reaction Plane

21. Spin Transport Because of Pl - Pt mixing, the non-dispersive bend angle contributes by a factor of ~100 to the FF ratio systematic error. However, it is very small: ±1.1 mrad (FWHM) and can be reconstructed with high precision (~0.1mrad). Systematic error is 10% of projected statistical error

22. Calibration Scheme Will need to: • Calibrate Momentum (P0 and δ) • Calibrate Angle reconstruction (θ0 & φ0) • Calibrate Vertex reconstruction (y0)

23. Calibration scheme cont’d • Do a series of elastic scattering runs (H2(e,e’p)) • δ scans (P0 and δ) • with and without sieve slit (θ0 & φ0) • Requires a proton arm in coincidence • Segmented extended target (y0) • i.e. a series of thin targets along the beamline • Has been very successfully done with BigBite • Compare to Magnet off • straight throughs

24. Conclusion • Magnet exists and is available • Magnet will work nicely • with proposed modifications • Optics are very simple