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Measurement of the Proton Elastic Form Factor Ratio at Low Q 2

Measurement of the Proton Elastic Form Factor Ratio at Low Q 2. Xiaohui Zhan (MIT) On Behalf of E08-007 Collaboration June 14th 2008 Hall A Collaboration Meeting. Outline. Background Experiment Preliminary Analysis. Proton Form Factors. Electron elastic scattering from proton:.

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Measurement of the Proton Elastic Form Factor Ratio at Low Q 2

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  1. Measurement of the Proton Elastic Form Factor Ratio at Low Q2 Xiaohui Zhan (MIT) On Behalf of E08-007 Collaboration June 14th 2008 Hall A Collaboration Meeting

  2. Outline • Background • Experiment • Preliminary Analysis

  3. Proton Form Factors Electron elastic scattering from proton: Sachs Form Factors: Electric:GE(Q2) Magnetic:GM(Q2) Q2=0 Dipole Leading order pQED (single photon exchange)

  4. Rosenbluth Separation (Unpolarized) Method: Vary at fixed Q2, fit linearly Slope G2E Intercept G2M Difficulties: • not sensitive to GE at large Q2 and to GM at small Q2 • Limited by accruaby of cross section measurement at • different settings.

  5. Recoil Polarization 2 Direct measurement of form factor ratios by measuring the ratio of the transferred polarization Pt and Pl . • Advantages: • only one measurement is needed for each Q2. • much better precision than a cross section measurement.

  6. World Data At high Q2, Rosenbluth and Polarization methods do NOT agree. We focus on low Q2 2γ exchange?

  7. E08-007 • Approved by PAC33 • Part I: a high precision survay of Proton FF ratio by recoil • polarization method • Scheduled from May 15th ~ June 9th 2008 • Succesfully completed!

  8. e’ Experiment Setup LHRS Pp : 0.5 ~ 0.9 GeV/c Ee: 1.192GeV Polarization: ~ 80% VDC + FPP S1 + S2 Pion rejector 1 Pion rejector 2 BigBite e’ Scintillator: 13 Preshower: 27x2 Shower: 27x7 LH2 target (6cm, 15cm) E’e: ~ 1 GeV

  9. BigBite • Used for electron tagging (coincidence trigger). • Turned on preshower and shower every time we changed kinematics. • Selected shower blocks were on for production to reduce background

  10. Focal Plane Polarimeter

  11. Q2[GeV2] 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70 0.95 0.70 0.55 0.70 0.46 0.52 0.53 0.44 Preliminary Analysis • Statistical error can be smaller • after adding the weight of different • analyzing power for different • Systematic analysis : • 3 different delta settings for every Kine. • Different spin precession method: • COSY, Pentchev, Dipole • Background substraction: • Aluminum end-cap • accidental E08-007 statistical error and world data & fits

  12. Impact • Many competing effective field theories: • VMD, Lattice, RCQM, Di-Quark, CBM/LFCBM, pQCD … • Combine ratio and cross section data (Mainz). • Proton charge density • Zemach radius/hyperfine splitting • Improved isovector/isoscalar form factors • Possible impacts on other experiments: DVCS, HAPPEX & G0, …

  13. Thank you!

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