A Precision Measurement of G E p /G M p with BLAST

1. A Precision Measurement of GEp/GMp with BLAST Chris Crawford MIT Laboratory for Nuclear Science May 18, 2005

2. Introduction Formalism World Data Experiment overview Experimental Setup LDS polarized target BLAST detector Calibrations Analysis Cuts & Yields Asymmetry Extraction of mGE/GM Systematic errors Conclusion Results: mGE/GM Separation of GE, GM Outline

3. Introduction • GE,GM fundamental quantities describing charge/magnetization in the nucleon • Test of QCD based calculations and models • Provide basis for understanding more complex systems in terms of quarks and gluons • Probe the pion cloud • QED Lamb shift

4. Form Factors of the Nucleon • Form Factor definition • Nucleon current • Breit frame

5. Elastic Cross Section b = target spin angle w/r to the beam line

6. World Data World Unpolarized Data

7. Polarization Transfer • Recoil proton polarization • Focal Plane Polarimeter • recoil proton scatters off secondary 12C target • Pt, Pl measured fromφ distribution • Pb, and analyzing powercancel out in ratio

8. GE/GM — World Data

9. Theory and Models • Direct QCD calculations • pQCD scaling at high Q2 • Lattice QCD • Meson Degrees of Freedom • Dispersion analysis, Höhler et al. 1976 • Soliton Model, Holzwarth 1996 • VMD + Chiral Perturbation Theory, Kubis et al. 2000 • Vector Meson Dominance (VMD), Lomon 2002 • QCD based constituent quark models (CQM) • LF quark-diquark spectator, Ma 2002 • LFCQM + CBM, Miller 2002 †Nucleon Electromagnetic Form Factors, Haiyan Gao, Int. J. of Mod. Phys. E, 12, No. 1, 1-40(Review) (2003)

10. Models Consistent with Polarized Data

11. Form Factor Ratio @ BATES • Exploits unique features of BLAST • internal target: low dilution, fast spin reversal • large acceptance: simultaneously measure all Q2 points • symmetric detector: ratio measurement • Different systematics • also insensitive to Pb and Pt • no spin transport • Q2 = 0.1 – 0.9 (GeV/c) 2 • input for P.V. experiments • structure of pion cloud

12. Asymmetry Super-ratio Method • Beam-Target Double Spin Asymmetry • Super-ratio • b = 45

13. Storage Ring E = 850 MeV Imax=225 mA Pb = 0.65 Internal ABS Target 60 cm storage cell t = 4.91013 cm-2 Pt = 0.80 Polarized Beam and Target • isotopically pure internal target • high polarization, fast spin reversal • L = 3.1  1031 cm-2s-1 • H2: 98 pb-1 D2: 126 pb-1+2005 run

14. 1 1 1 1 MFT (2->3) 3 3 2 2 nozzle 4 6-pole 6-pole Atomic Beam Source • Standard technology • Dissociator & nozzle • 2 sextupole systems • 3 RF transitions Spin State Selection:

15. Laser Driven Source (LDS) • Optical pumping& Spin Exchange • Spincell design • Target and Polarimeter • Results

16. Spin-Exchange Optical pumping

17. LDS Experimental Setup

18. Pictures of the LDS

19. Atomic Dissociation

20. Atomic Polarization

21. Comparison of Polarized Targets

22. BLAST Detector Package Detector Requirements • Definition of q • e  2, e  .°, z  1 cm • e/p/n/ separation • PID: t  1, Čerenkov • Optimize statistics • Large Acceptance • Asymmetry Super-ratios • Symmetric Detector • Polarized targets • 1 m diameter in target region • Zero field at target • B-gradients  50 mG/cm

23. TOF Scintillators • timing resolution: σ=350 ps • velocity resolution: σ= 1% coplanarity cuts ADC spectrum

24. Cosmics TOF Calibration L 15 L 12 channels L 9 L 6 L 3 L 0 channels R 0 R 3 R 6

25. TOF Scintillator Cuts TOF paddle, proton TOF paddle, electron

26. 1 cm thick aerogel tiles Refractive index 1.02-1.03 White reflective paint 80-90 % efficiency 5" PMTs, sensitive to 0.5 Gauss Initial problems with B field Required additional shielding 50% efficiency without shielding Čerenkov Detectors

27. Drift Chambers • 2 sectors × 3 chambers • 954 sense wires • 200μm wire resolution • signal to noise ratio 20:1

28. NSED (Online Display)

29. Scintillators timing, calibration Wire chamber hits, stubs, segments link, track fit PID, DST Reconstruction

30. Newton-Rhapson Track Fitter

31. Hyperbolic timedist function D TDC

32. Linear T2D Calibration 72 33 ~ 1mm resolution c2 28 MeV 12 MeV D p (GeV/c)

33. Wire Chamber Efficiency

34. peqefe ze ppqpfp zp WC Offsets/Resolution/Cuts

35. Tracking Efficiency

36. Comparison of Yields with MC

37. Experimental Spin Asymmetry

38. Single-asymmetry Method measure P first, use to calculate R model-dependent Super-ratio Method 2 equations in P, R in each Q2 bin j independent measure of polarization in each bin! 2n parameters Pj, Rj Global Fit Method fit for P, R1, R2, … from all Aijtogether model independent better statistics n+1 parameters can also fit for b i = left,right sector j = Q2 bin (1..n) b = spin angle

39. Extractions of m GE/GM

40. Single AsymmetryExtraction

41. DQ2 (1.8%) comparison of qe and qp difference between left and right sectorsmost problematic appeal to TOF timing ! Db (0.8%) fieldmap: 47.1°± 1° Hohler: 47.5°± 0.8° Fit Method: 42° ± 3° (1st 7 bins) 48° ± 4° T20 analysis: 46.5°± 3° Systematic Errors

42. GE/GM Results

43. Extraction of GE and GM

44. BLAST + World Data GE and GM Results

45. Q2 Corrections from TOF

46. Conclusion • 1st measurement of mGE/GM using double spin asymmetry • 2 – 3.5× improvement in precision of mGE/GM at Q2 = 0.1– 0.5 GeV2 • sensitive to the pion cloud • is dip in GE around Q2=0.3 GeV2 real? • systematic errors are being reduced