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

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22 Outline 1.Background 2. Improvements over previous study 3. Results 4. Summary

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3 Background Proton, the only stable hadron 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

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44 big surprise!!! Polarization transfer experiment at Jlab: M.K. Jones et al., Phys. Rev. Letts. 84, 1398 (2000). exp. in Hall A, Jlab with E lab = GeV G E falls faster than G M G M /μ p G D is approximately constant

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55 Ensuing efforts to verify the discrepancy - experimental New global analysis of the worlds cross section data (Arrington, 2003) still inconsistent with the polarization measurements High-precision Super-Rosenbluth experiment (Qattan et al., 2005) with 4 - 8% precision,

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

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7 Ensuing efforts to understand the discrepancy - theoretical Re-examination of the radiative corrections O ( α 2 ) Maximon and Tjon: ε dependence comes only from proton vertex and TPE corrections; proton vertex corr. < 0.5% Two photon exchange effects ??

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

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

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

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11 Improvements over KBMTs calculation 1. correct γN Δ vertex function 2. realistic γNΔ coupling constants, the Coulomb quardruploe one g c in particular (g 1, g 2, g 3 ) = (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 g c in particular (g 1, g 2, g 3 ) = (7, 9, 0) ---- KBMT (6.59, 9.08, 7.12) ---- ZY 3. realistic γNΔ form factors

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12 Effects of realistic γNN form factors

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Details of the three improvements 13

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

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15 realistic γNΔ form factors

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17 Realistic coupling constants from experiments (g 1, g 2, g 3 ) = (7, 9, 0) ---- KBMT (6.59, 9.08, 7.12) ---- ZY

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18 Results Effects of correct vertex function Effects of realistic γNΔ form factors

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19 Effects of realistic γNΔ coupling constants with correct vertex function but KBMTs f.f.s Combined effects of all three improvements

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20 Combined effects

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

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24 Summary 1.Three improvements, correct vertex function, realistic form factors, and coupling constants for 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.

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25 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 G E /G M obtained from LT and PT analyses 4. Substantial discrepancy remains between predictions of hadronic model with Born plus TPE and other data like R(e+p/e-p) and P L /P L (Born) More experimental and theoretical efforts are called for !!

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26 The End Thanks you!!

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27 Backup slides

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