11 contribution of two-photon exchange with excitation to ep scattering revisited shin nan yang...
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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)
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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%
Two photon exchange effects ??
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Blunden, Melnitchouk, & Tjon, 2003
Two-photon exchange calculationTwo-photon exchange calculation : : hadronichadronic
N
Blunden, Melnitchouk, Tjon, PRL 91 (2003) 142304; with only nucleon in the intermediate states.
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Two-photon exchange :Two-photon exchange : partonic calculationpartonic 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 !!
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Kondratyuk, Blunden, Melnitchouk, Tjon, (KBMT) PRL, 95 (2005) 172503.
Δ Contribution to Δ Contribution to TPE:TPE: hadronichadronic
2 2 2 2( ) ( ) (1 )R M E Nd G Q G Q
Δ(1232) contribution to TPE is not negligible !
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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
†
0 0
†
0 0
( , ) ( , ) ----- KBMT
( , ) ( , ) ----- ZY,
N N
N N
p q p
p
q
qq p
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
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Effects of realistic γNN form factors
Details ofthe three
improvements
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14
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(1) 2
(2) 2
(3) 2
2
5
1
3
( , ) .
/
(
( , ) .
)
( )
( )
N
N
g
p q const
g p q pF q
F
q p q p q
p q g p q
M q p g p q q p p q
p q const
q
q
g
F
g
(1) 2
(2) 2
(3) 23
1
2
2
( )
( )
- /
(
)
g q p p q p q p q
p q g p q
M q p g p q q p p
F
g
q
q
F q
q
g
g F
1
21 : electric quadrupole tra
nsiti
: magnetic dipo
le transit
i
o
n
on
g
gg
3 - g : Coulomb quadrupole transition
KBMT used (+) sign
here
†
0 0
†
0 0
----- KBM( , ) ( , )
( , ) ( , )
T
----- ZY,
N N
N N
p q p
p p qq
q
correct γN → Δ vertex function
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realistic γNΔ form factors
22(1) 2 (2) 2 (3) 2 1
12 21
2 22(1) 2 (2) 2 31
1 32 2 2 21 3
(3)
KBMT :
ZY:
( ) ( ) ( ) , = 0.84 GeV.
( ) ( ) , = 0.84 GeV, = 2 G
eV,
(
F q F q F qq
F q F qq q
F q
2 22 2 22 31 2 4
2 2 2 2 2 2 2 21 3 2 4
2 4
) (1 ) ,
= 2 GeV, = 0.2 GeV, 0.3.
a aq q q q
a
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Realistic coupling constants from experiments
(g1, g2, g3) = (7, 9, 0) ---- KBMT (6.59, 9.08, 7.12) ---- ZY
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Results
Effects of correct vertex function
Effects of realistic γNΔ form factors
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Effects of realistic γNΔ coupling constants with
correct vertex function but KBMT’s f.f.’s
Combined effects of all three improvements
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2 2 2 2( ) ( ) (1 )R M E Nd G Q G Q
Combined effects
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Comparison with preliminaryrad-uncorr. data from CLAS
Comparison with preliminaryrad-corr. data from Novosibirsk
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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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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 discrepancy remains between
predictions of hadronic model with Born plus
TPE and other data like R(e+p/e-p) and
PL/PL (Born)
More experimental and theoretical efforts are called for !!
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The End
Thanks you!!
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Backup slides
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