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International Conference on the Structure of Baryons (Baryons2013),

Contribution of Two-Photon Exchange with Excitation to ep Scattering Revisited Shin Nan Yang National Taiwan University. In collaboration with Haiqing Zhou, Southeast University, Nanjing.

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International Conference on the Structure of Baryons (Baryons2013),

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  1. Contribution of Two-Photon Exchange with Excitation to ep Scattering RevisitedShin Nan YangNational Taiwan University In collaboration with Haiqing Zhou, Southeast University, Nanjing International Conference on the Structure of Baryons (Baryons2013), Glasgow, Scotland, June 24 – 28, 2013 1

  2. Outline Background 2. Improvements over previous study 3. Results 4.Summary 2

  3. Background • Proton, the onlystablehadron and the lightest baryon, is most amenable to experimental and theoretical studies. • Experimental measurements of proton EM form factors started in 1950s (Hofstadter, 1961 Nobel prize) • Unpolarized data before 2000, analyzed via Rosenbluth formular (LT method), can be fitted by as quoted in the textbooks and often called as SCALING LAW

  4. big surprise!!! • Polarization transfer experiment at Jlab: • M.K. Jones et al., • Phys. Rev. Letts. 84, 1398 (2000). • exp. in Hall A, Jlab with • Elab = 0.934 - 4.090 GeV • GE falls faster than GM • GM/μpGD is approximately constant 4

  5. Ensuing efforts to verify the discrepancy - experimental • New global analysis of the world’s cross section data (Arrington, 2003) → still inconsistent with the polarization measurements • High-precision Super-Rosenbluth experiment (Qattan et al., 2005) → with4 - 8% precision, 5

  6. Pun05 Gay02 proton e.m. form factor : status green : Rosenbluth data (SLAC, JLab) JLab/HallA recoil pol. data 6

  7. Ensuing efforts to understand the discrepancy - theoretical • Re-examination of the radiative corrections O (α2) • Maximon and Tjon: — ε dependencecomes only from proton vertex and TPE corrections; proton vertex corr. < 0.5% Two photon exchange effects ??

  8. Two-photon exchange calculation : hadronic Blunden, Melnitchouk, & Tjon, 2003 N Blunden, Melnitchouk, Tjon, PRL 91 (2003) 142304; with only nucleon in the intermediate states.

  9. Two-photon exchange :partonic calculation GPDs Chen, et al., PRL, 93, (2004) 122301 TPE can account for at least 50% of the discrepancy in the value of μpGE/GM extracted from LT and PT methods !!

  10. Δ Contribution to TPE:hadronic Kondratyuk, Blunden, Melnitchouk, Tjon, (KBMT) PRL, 95 (2005) 172503. Δ(1232) contribution to TPE is not negligible !

  11. Improvements over KBMT’s calculation • 1. correct γN → Δ vertex function • 2. realistic γNΔ coupling constants, the • Coulomb quardruploe one gc in particular • (g1, g2, g3) = (7, 9, 0) ---- KBMT • (6.59, 9.08, 7.12) ---- ZY • 3. realistic γNΔ form factors 1. correct γN → Δ vertex function 2. realistic γNΔ coupling constants, the Coulomb quardruploe one gc in particular (g1, g2, g3) = (7, 9, 0) ---- KBMT (6.59, 9.08, 7.12) ---- ZY 3. realistic γNΔ form factors

  12. Effects of realistic γNN form factors

  13. Details ofthe three improvements

  14. correct γN → Δ vertex function KBMT used (+) sign here

  15. realistic γNΔ form factors

  16. Realistic coupling constants from experiments (g1, g2, g3) = (7, 9, 0) ---- KBMT (6.59, 9.08, 7.12) ---- ZY

  17. Results Effects of correct vertex function Effects of realistic γNΔ form factors

  18. Effects of realistic γNΔ coupling constants with correct vertex function but KBMT’s f.f.’s Combined effects of all three improvements

  19. Combined effects

  20. Comparison with preliminary rad-uncorr. data from CLAS Comparison with preliminary rad-corr. data from Novosibirsk

  21. Summary • Three improvements, correct vertex function, realistic form factors, and coupling constantsfor the γNΔ vertex,have been implemented 2. Each improvement, implemented separately, all produced substantial effect, especially the f.f.’s as in the case of TPE/N. However, the combined effects are modest, but non-negligible, in many cases.

  22. 3. TPE effects from N and Δ within hadronic model can provide a fair account for the unpolarized cross sections and the discrepancy found for the ratio GE/GM obtained from LT and PT analyses 4. Substantial discrepancyremains between predictions of hadronic model with Born plus TPE and otherdata like R(e+p/e-p) and PL/PL (Born) More experimental and theoretical efforts are called for !!

  23. The End Thanks you!!

  24. Backup slides

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