qed radiative corrections for precision measurements of nucleon form factors
DESCRIPTION
QED Radiative Corrections for Precision Measurements of Nucleon Form Factors. Andrei Afanasev Jefferson Lab Nucleon 2005 INFN, Frascati, October 12, 2005. Main problem. Uncertainties in QED radiative corrections limit interpretability of precision experiments on electron-hadron scattering. - PowerPoint PPT PresentationTRANSCRIPT
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
QED Radiative Corrections for Precision Measurements of Nucleon Form Factors
Andrei Afanasev
Jefferson Lab
Nucleon 2005
INFN, Frascati, October 12, 2005
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Main problem
. Uncertainties in QED radiative corrections limit interpretability of precision experiments on electron-hadron scattering
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Plan of talk
Radiative corrections for electron scattering
. Model-independent and model-dependent; soft and hard photons
Two-photon exchange effects in the process e+p→e+p
. Models for two-photon exchange
. Cross sections
. Polarization transfer
. Single-spin asymmetries
. Parity-violating asymmetry
Refined bremsstrahlung calculations
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Elastic Nucleon Form Factors
pm
q
peefi
fifi
utFtFuuueM
MM
))()(( 22121
1
•Based on one-photon exchange approximation
)0(
,
:1
)1(2
:)(
2121
2
220
y
ME
M
E
z
x
z
x
EM
P
FFGFFG
techniqueonPolarizatiG
G
A
A
P
P
techniqueRosenbluthGG
•Two techniques to measure
Latter due to: Akhiezer, Rekalo; Arnold, Carlson, Gross
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Do the techniques agree?
. Both early SLAC and Recent JLab experiments on (super)Rosenbluth separations followed Ge/Gm~const
. JLab measurements using polarization transfer technique give different results (Jones’00, Gayou’02)
Radiative corrections, in particular, a short-range part of 2-photon exchange is a likely origin of the discrepancy
SLAC/Rosenbluth
JLab/Polarization
~5% difference in cross-sectionx5 difference in polarization
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Basics of QED radiative corrections
(First) Born approximation
Initial-state radiation Final-state radiation
Cross section ~ dω/ω => integral diverges logarithmically: IR catastrophe
Vertex correction => cancels divergent terms; Schwinger (1949)
)}(2
1
36
17)1)(ln
12
13{(ln
2,)1( 2
2
exp
fm
Q
E
E
eBorn
Multiple soft-photon emission: solved by exponentiation,Yennie-Frautschi-Suura (YFS), 1961
e )1(
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Complete radiative correction in O(αem )Radiative Corrections:• Electron vertex correction (a)• Vacuum polarization (b)• Electron bremsstrahlung (c,d)• Two-photon exchange (e,f)• Proton vertex and VCS (g,h)• Corrections (e-h) depend on the nucleon structure•Mo&Tsai; Meister&Yennie formalism•Further work by Bardin&Shumeiko; Maximon&Tjon; AA, Akushevich, Merenkov;
•Guichon&Vanderhaeghen’03:Can (e-f) account for the Rosenbluth vs. polarization experimental discrepancy? Look for ~3% ...
Main issue: Corrections dependent on nucleon structureModel calculations: •Blunden, Melnitchouk,Tjon, Phys.Rev.Lett.91:142304,2003•Chen, AA, Brodsky, Carlson, Vanderhaeghen, Phys.Rev.Lett.93:122301,2004
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Separating soft 2-photon exchange
. Tsai; Maximon & Tjon (k→0)
. Grammer &Yennie prescription PRD 8, 4332 (1973) (also applied in QCD calculations)
. Shown is the resulting (soft) QED correction to cross section
. Already included in experimental data analysis
. NB: Corresponding effect to polarization transfer and/or asymmetry is zero
ε
δSoftQ2= 6 GeV2
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Bethe-Heitler corrections to polarization transfer and cross sectionsAA, Akushevich, Merenkov Phys.Rev.D64:113009,2001;AA, Akushevich, Ilychev, Merenkov, PL B514, 269 (2001)
Pion threshold: um=mπ2
In kinematics of elastic ep-scattering measurements, cross sections are more sensitive to RC
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Lorentz Structure of 2-γ amplitude
pAppeee
pm
q
ppeee
pepefi
pm
q
peefi
fififi
uusGuAuuA
uusFusFuVuuV
AAVVM
utFtFuuuM
MMM
55
221
2
2211
21
),(,
)),('),('(,
))()((
Generalized form factors are functions of two Mandelstam invariants;Specific dependence is determined by nucleon structure
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
New Expressions for Observables
AMEM
AEMEy
AMM
AEME
z
x
AMEM
GGGG
GGGGP
GGG
GGGG
P
P
GGGG
*222
**
*22
**
*2220
Re)1)(1(2||||
)1(21)Im()1(2)Im(
)1(2)Re(1||
)1(21)Re()1(2)Re(
)Re)1)(1(2|||(|
We can formally define ep-scattering observables in terms of the new form factors:
For the target asymmetries: Ax=Px, Ay=Py, Az=-Pz
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Calculations using Generalized
Parton Distributions
Hard interaction with a quark
Model schematics: • Hard eq-interaction•GPDs describe quark emission/absorption •Soft/hard separation
•Use Grammer-Yennie prescription
AA, Brodsky, Carlson, Chen, Vanderhaeghen, Phys.Rev.Lett.93:122301,2004; hep-ph/0502013
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Short-range effects; on-mass-shell quark(AA, Brodsky, Carlson, Chen,Vanderhaeghen)
Two-photon probe directly interacts with a (massless) quarkEmission/reabsorption of the quark is described by GPDs
su
tii
t
s
t
s
ut
u
s
su
t
s
ui
t
u
s
tf
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suii
t
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t
s
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u
s
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uf
uuAuuV
fAAfVVt
eA
A
V
qeqeqe
qeqeqe
Aqe
Vqeemq
eqeq
2)]2))(log(log(
1))((log
1[
4
)(
)]log(1
))(log(1
[2
2)]2))(log(log(
1))((log
1[
4
)(
)]log(1
))(log(1
[2
)log(])[log(2
,
),(2
2
222
2
22
22
222
2
22
2
,5,,
,,,
22
Note the additional effective (axial-vector)2 interaction; absence of mass terms
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
`Hard’ contributions to generalizedform factors
GPD integrals
Two-photon-exchange form factors from GPDs
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Two-Photon Effect for Rosenbluth Cross Sections
. Data shown are from Andivahis et al, PRD 50, 5491 (1994)
. Included GPD calculation of two-photon-exchange effect
. Qualitative agreement with data:
. Discrepancy likely reconciled
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Updated Ge/Gm plot
AA, Brodsky, Carlson, Chen, Vanderhaeghen, Phys.Rev.Lett.93:122301,2004; hep-ph/0502013
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Full Calculation of Bethe-Heitler Contribution
212
2
1
1
212
2
1
1
152
2
ˆ
2
ˆ
2
ˆ
2
ˆ
)ˆ)(ˆ1()ˆ(2
1
kk
k
kk
k
kk
k
kk
k
kk
k
kk
k
kk
k
kk
k
mkmkTrL er
Radiative leptonic tensor in full formAA et al, PLB 514, 269 (2001)
Additional effect of full soft+hard brem → +1.2% correction to ε-slopeResolves additional ~25% of Rosenbluth/polarization discrepancy!
Additional work by AA et al., using MASCARAD (Phys.Rev.D64:113009,2001) Full calculation including soft and hard bremsstrahlung
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Polarization transfer
. Also corrected by two-photon exchange, but with little impact on Gep/Gmp extracted ratio
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Charge asymmetry
. Cross sections of electron-proton scattering and positron-proton scattering are equal in one-photon exchange approximation
. Different for two- or more photon exchange
To be measured in JLab Experiment 04-116, Spokepersons W. Brooks et al.
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Single-Spin Asymmetries in Elastic Scattering
Parity-conserving
. Observed spin-momentum correlation of the type:
where k1,2 are initial and final electron momenta, s is a polarization vector
of a target OR beam
. For elastic scattering asymmetries are due to absorptive part of 2-photon exchange amplitude
Parity-Violating
21 kks
1ks
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Quark+Nucleon Contributions to An
. Single-spin asymmetry or polarization normal to the scattering plane
. Handbag mechanism prediction for single-spin asymmetry/polarization of elastic ep-scattering on a polarized proton target
HGPDondependenceNo
BGAGA MER
n
~
)Im(2
1)Im(
1)1(2
Only minor role of quark mass
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Radiative Corrections for Weak Processes
Semi-Leptonic processes involving nucleons
. Neutrino-nucleon scattering
Per cent level reached by NuTeV. Radiative corrections for DIS calculated at a partonic level (D. Bardin et al.)
. Neutron beta-decay: Important for Vud measurements; axial-vector coupling gA
Marciano, Sirlin, PRL 56, 22 (1986); Ando et al., Phys.Lett.B595:250-259,2004; Hardy, Towner, PRL94:092502,2005
. Parity-violating DIS: Bardin, Fedorenko, Shumeiko, Sov.J.Nucl.Phys.32:403,1980; J.Phys.G7:1331,1981, up to 10% effect from rad.corrections
. Parity-violating elastic ep (strange quark effects, weak mixing angle)
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Parity Violating elastic e-N scattering
Longitudinally polarized electrons,
unpolarized target
2
e e pp
unpol
AMEF
LR
LR AAAQGA
224
2
sRFGQG
GGRGRQGe
AZA
eA
sME
nME
nA
pME
pAW
ZME
)()(
)1()1)(sin41()(2
,,,22
,
45.3
sin41
282
W
MeAWA
MZMM
EZEE
GGA
GGA
GGA
)sin41(
)(
2
Neutral weak form factors contain explicit contributions from strange sea
GZA(0) = 1.2673 ± 0.0035 (from decay)
= Q2/4M2
= [1+2(1+)tan2(/2)]-1
’= [(+1)(1-2)]1/2
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Born and Box diagrams for elastic ep-scattering
. (d) Computed by Marciano,Sirlin, Phys.Rev.D29:75,1984, Erratum-ibid.D31:213,1985 for atomic PV
. (c) Presumed small, e.g., M. Ramsey-Musolf, Phys.Rev. C60 (1999) 015501
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
New Expressions for PV asymmetry
. PV asymmetry in terms of generalized form factors including multi-photon exchange
2/1,2/)sin41(
,)1)(1(2
,2,2
)()(24
2
2
22
2
eAW
eV
MpZA
eV
BornA
EpZMp
eA
BornMEp
ZEp
eA
BornE
EpMp
BornA
BornM
BornEF
LR
LRBornPV
gg
GGgA
GGgAGGgA
GG
AAAQGA
PV-asymmetry, Born Approximation
ApEpMp
AMAMEF
LR
LRPV
GGG
AAAAAQGA
')1(2||||
''
24 22'
2
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
2γ-exchange Correction to Parity-ViolatingElectron Scattering
. New parity violating terms due to (2gamma)x(Z0) interference should be added:
xγ γ Z0
Electromagnetic Neutral Weak
)Re()1(2'
)Re()1)(1(2''
'2
ApZA
eVA
ApZM
eAM
GGgA
GGgA
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
GPD Calculation of 2γ×Z-interference. Can be used at higher Q2, but points at a
problem of additional systematic corrections for parity-violating electron scattering. The effect evaluated in GPD formalism is the largest for backward angles:
AA & Carlson, hep-ph/0502128, Phys. Rev. Lett. 94, 212301 (2005): measurements of strange-quark content of the nucleon are affected, Δs may shift by ~10%
Important note: (nonsoft) 2γ-exchange amplitude has no 1/Q2 singularity;1γ-exchange is 1/Q2 singular => At low Q2, 2γ-corrections is suppressed as Q2 P. Blunden used this formalism and evaluated correction of 0.16% for Qweak
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
. Quark-level short-range contributions are substantial (2-3%)
. 2γ-correction to parity-violating asymmetry does not cancel. May reach a few per cent for GeV momentum transfers
. Corrections are angular-dependent, not reducible to re-definition of coupling constants
. Revision of γZ-box contribution and extension of model calculations to lower Q2 is necessary
. Experimentally measurable directly by comparing electrons vs positrons on a spin-0 target- it is difficult => in the meantime need to rely on the studies of 2γ-effect for parity-conserving observables
Two-photon exchange for PV electron-proton scattering
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Parity-Conserving Single-Spin Asymmetries in Scattering Processes(early history). N. F. Mott, Proc. R. Soc. (London), A124, 425 (1929), noticed that
polarization and/or asymmetry is due to spin-orbit coupling in the Coulomb scattering of electrons (Extended to high energy ep-scattering by AA et al., 2002).
. Julian Schwinger, Phys. Rev. 69, 681 (1946); ibid., 73, 407 (1948), suggested a method to polarize fast neutrons via spin-orbit interaction in the scattering off nuclei
. Lincoln Wolfeinstein, Phys. Rev. 75, 1664 (1949); A. Simon, T.A.Welton, Phys. Rev. 90, 1036 (1953), formalism of polarization effects in nuclear reactions
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Proton Mott Asymmetry at Higher Energies
. Due to absorptive part of two-photon exchange amplitude; shown is elastic contribution
. Nonzero effect observed by SAMPLE Collaboration (S.Wells et al., PRC63:064001,2001) for 200 MeV electrons
. Calculations of Diaconescu, Ramsey-Musolf (2004): low-energy expansion version of hep-ph/0208260
Transverse beam SSA, units are parts per million Figures from AA et al, hep-ph/0208260
Spin-orbit interaction of electron moving in a Coulomb field N.F. Mott, Proc. Roy. Soc. London, Set. A 135, 429 (1932);BNSA for electron-muon scattering: Barut, Fronsdal, Phys.Rev.120, 1871 (1960);BNSA for electron-proton scattering: Afanasev, Akushevich, Merenkov, hep-ph/0208260
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Phase Space Contributing to the absorptivepart of 2γ-exchange amplitude
. 2-dimensional integration (Q12, Q2
2) for the elastic intermediate state
. 3-dimensional integration (Q12, Q2
2,W2) for inelastic excitations Examples: MAMI A4
E= 855 MeVΘcm= 57 deg;
SAMPLE, E=200 MeV
`Soft’ intermediate electron;Both photons are hard collinear
One photon is Hard collinear
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
MAMI data on Mott Asymmetry
. F. Maas et al., Phys.Rev.Lett.94:082001, 2005
. Pasquini, Vanderhaeghen:
Surprising result: Dominance of inelastic intermediate excitations
Elastic intermediatestate
Inelastic excitationsdominate
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Beam Normal Asymmetry(AA, Merenkov)
0
ˆ
)ˆ1)(ˆ()ˆ(4
1
)ˆ()ˆ1)(ˆ()ˆ(4
1
2Im
),(
1212
512
512
22
210
3
22
2,
qHqHqHqLqLqL
aa
TMpMpTrH
mkmkmkTrL
HL
k
kd
QsD
QA
p
eeee
Pen
Gauge invariance essential in cancellation of infra-red singularity for target asymmetry0/0 2
22
1 QorandQifHL
Feature of the normal beam asymmetry: After me is factored out, the remaining expression is singular when virtuality of the photons reach zero in the loop integral!But why are the expressions regular for the target SSA?!
Also calculations by Vanderhaeghen, Pasquini (2004); Gorschtein, hep-ph/0505022Confirm quasi-real photon exchange enhancement
2
2
2
222
22
1 log,log~0/e
e
e
eem
Qm
m
QmAQorandQifconstmHL
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Peaking Approximation
. Dominance of collinear-photon exchange =>
. Can replace 3-dimensional integral over (Q12,Q2
2,W) with one-dimensional integral along the line (Q1
2≈0; Q22=Q2(s-W2)/(s-M2))
. Save computing time
. Avoid uncertainties associated with (unknown) double-virtual Compton amplitude
. Provides more direct connection to VCS and RCS observables
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Special property of normal beam asymmetry
. Reason for the unexpected behavior: hard collinear quasi-real photons
. Intermediate photon is collinear to the parent electron
. It generates a dynamical pole and logarithmic enhancement of inelastic excitations of the intermediate hadronic state
. For s>>-t and above the resonance region, the asymmetry is given by:
)2)(log(8
)(2
2
22
21
212
2
e
ep
en m
Q
FF
FFQmA
Also suppressed by a standard diffractive factor exp(-BQ2), where B=3.5-4 GeV-2 Compare with no-structure asymmetry at small θ:
AA, Merenkov, Phys.Lett.B599:48,2004, Phys.Rev.D70:073002,2004;
+Erratum (2005)
3s
mA ee
n
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Input parameters
. Used σγp from parameterization by N. Bianchi at al., Phys.Rev.C54 (1996)1688
(resonance region) and Block&Halzen, Phys.Rev. D70 (2004) 091901
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Predictions for normal beam asymmetry
Use fit to experimental data on σγp and exact 3-dimensional integration over phase space of intermediate 2 photons
HAPPEX
Data from HAPPEX More to come from G0
2γ-exchange is in diffractive regime
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
No suppression for beam asymmetry with energyat fixed Q2
x10-6 x10-9
Parts-per-million vs. parts-per billion scales: a consequence ofsoft Pomeron exchange, and hard collinear photon exchange
SLAC E158 kinematics
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Beam SSA in the resonance region
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Normal Beam Asymmetry on Nuclei
. Important systematic correction for parity-violation experiments (HAPPEX on 4He, PREX on Pb)
. For diffractive 2γ-exchange, scales as ~A/Z~2
Five orders of magnitude enhancement for (HAPPEX) due to excitation of inelastic intermediate states in 2γ-exchange (AA, Merenkov, to be published)
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
Summary on SSA in Elastic ep-Scattering
. Collinear photon exchange present in (light particle) beam SSA
. (Electromagnetic) gauge invariance of is essential for cancellation of collinear-photon exchange contribution for a (heavy) target SSA
. Unitarity is crucial in reducing model dependence of calculations at small scattering angles, in particular for beam asymmetry
. Strong-interaction dynamics for BNSA small-angle ep-scattering above the resonance region is soft diffraction
. For the diffractive mechanism An is a) not supressed with beam energy and b) does not grow with Z
. If confirmed experimentally → first observation of diffractive component in elastic electron-hadron scattering
Andrei Afanasev, Nucleon’05, Frascati, Oct.12, 2005Operated by the Southeastern Universities Research Association for the U.S. Dept. of Energy
. Quark-level short-range contributions are substantial (3-4%) ; correspond to J=0 fixed pole (Brodsky-Close-Gunion, PRD 5, 1384 (1972)).
. Structure-dependent radiative corrections calculated using GPDs bring into agreement the results of polarization transfer and Rosenbluth techniques for Gep measurements
. Full treatment of brem corrections removed ~25% of R/P discrepancy in addition to 2γ
. Collinear photon exchange + inelastic excitations dominance for beam asymmetry
. Experimental tests of multi-photon exchange. Comparison between electron and positron elastic scattering (JLab E04-116). Measurement of nonlinearity of Rosenbluth plot (JLab E05-017). Search for deviation of angular dependence of polarization and/or asymmetries
from Born behavior at fixed Q2 (JLab E04-019)
. Elastic single-spin asymmetry or induced polarization (JLab E05-015)
. 2γ additions for parity-violating measurements (HAPPEX, G0)
Through active theoretical support emerged a research program of
Testing precision of the electromagnetic probeDouble-virtual VCS studies with two space-like photons
Two-photon exchange for electron-proton scattering