1 Contribution of Two-Photon Exchange Contribution of Two-Photon Exchange with with Excitation to ep Excitation to ep Scattering Revisited Scattering Revisited Shin Nan Yang Shin Nan Yang National Taiwan University 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 (1232)
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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.
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Contribution of Two-Photon Exchange with Contribution of Two-Photon Exchange with Excitation to ep Scattering Revisited Excitation to ep Scattering Revisited
Shin Nan YangShin Nan YangNational Taiwan UniversityNational Taiwan University
International Conference on the Structure of Baryons (Baryons2013),
Glasgow, Scotland, June 24 – 28, 2013
In collaboration with Haiqing Zhou, Southeast University, Nanjing
(1232)
22
Outline
1.Background
2. Improvements over
previous study
3. Results 4. Summary
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
2 22 2
2 202 2 2
0
( ) ( )( ) ( ), 0,
( / ) ( / )
1( ) , 0.71 (GeV/c) ,
(1 / )
p np nM ME D E
p N n N
D
G Q G QG Q G Q G
G Q QQ Q
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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
e p e p BBBBBBBBBBBBB B
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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) → with 4 - 8% precision,
2 22.64 4.10 GeV/ 1 ,p E MG QG
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proton proton e.m. form factor : statuse.m. form factor : status proton proton e.m. form factor : statuse.m. form factor : status
green : Rosenbluth data (SLAC, JLab)
Pun05
Gay02
JLab/HallA
recoil pol. data
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Ensuing efforts to understand the discrepancy -
theoretical• Re-examination of the radiative corrections O (α2)
• Maximon and Tjon: — ε dependence comes only from proton vertex and TPE corrections; proton vertex corr. < 0.5%