NUCLAT, April 21-22, 2008, 1

Kees de JagerJefferson LabNUCLATApril 21 - 22, 2008

Nucleon Electro-Weak Form Factors

•General Introduction•Electromagnetic Form Factors

•Formalism•Neutron•Proton

•Parity Violation•Formalism•Strange Form Factors

•Summary

Thomas Jefferson National Accelerator Facility

Z0

NUCLAT, April 21-22, 2008, 2

(Hadronic) Structure and (EW) Interaction

StructureInteraction

Probe Object

|Form factor|2 =

Electroweak probe

Interaction

Structure

(structured object)

(pointlike object)

Hadronic object

→ Interference!

Factorization!

Lepton scattering

→ Utilize spin dependence of electroweak interaction to achieve high precision

Inelastic Elastic

Born Approximation

NUCLAT, April 21-22, 2008, 3

Kinematics

→ Q2 > 0 space-like region, studied through elastic electron scattering

→ Q2 < 0 time-like region, studied through creation or annihilation

e+ + e- -> N + N or N + N -> e+ + e-

_ _

rq =

rp−

rp'

ω =Ee − ′Ee

−Q2 ≡t≡q2 =ω 2 −rq2

Q2 ≈4EE'sin2θe / 2

J μ =U(P') μF1(Q2 ) +

i μνqv

2M p

κF2 (Q2 )

⎧⎨⎪

⎩⎪

⎫⎬⎪

⎭⎪U(P)

NUCLAT, April 21-22, 2008, 4

Nucleon Electro-Magnetic Form Factors

Fundamental ingredients in “Classical” nuclear theory

• A testing ground for theories constructing nucleons from quarks and gluons

• Provides insight in spatial distribution of charge and magnetization

• Wavelength of probe can be tuned by selecting momentum transfer Q:

< 0.1 GeV2 integral quantities (charge radius,…)

0.1-10 GeV2 internal structure of nucleon

> 20 GeV2 pQCD scaling

Caveat: If Q is several times the nucleon mass (~Compton wavelength), dynamical effects due to relativistic boosts are introduced, making physical interpretation more difficult

NUCLAT, April 21-22, 2008, 5

Formalism

€

dσdΩ

(E,θ) =α 2E 'cos2 (

θ

2)

4E 3 sin4 (θ

2)

[(F12 +κ 2τ F2

2) + 2τ (F1 +κF2)2 tan2 (

θ2

)]

Dirac (non-spin-flip) F1 and Pauli (spin-flip) F2 Form Factors

with E (E’) incoming (outgoing) energy, θ scattering angle, κ anomalous magnetic moment and = Q2/4M2

In the Breit (centre-of mass) frame the Sachs FF can be written as the Fourier transforms of the charge and magnetization radial density distributions

ddΩ

(E,θ) =M [GE

2 +GM2

1++ 2GM

2 tan2 (θ2)]

M =α 2E 'cos2 (

θ

2)

4E 3 sin4 (θ

2)

F1 =GE +GM F2 =GM −GE

κ(1+ ) =

Q2

4M 2

Alternatively, Sachs Form Factors GE and GM can be used

NUCLAT, April 21-22, 2008, 6

The Pre-JLab Era

• Stern (1932) measured the proton magnetic moment µp ~ 2.5 µDirac

indicating that the proton was not a point-like particle• Hofstadter (1950’s) provided the first measurement of the

proton’s radius through elastic electron scattering• Subsequent data (≤ 1993) were based on:

Rosenbluth separation for proton, severely limiting the accuracy for GE

p at Q2 > 1 GeV2

• Early interpretation based on Vector-Meson Dominance

• Good description with phenomenological dipole form factor:

€

GD =Λ2

Λ2 + Q2

⎧ ⎨ ⎩

⎫ ⎬ ⎭

2

with Λ = 0.84GeV

corresponding to (770 MeV) and ω (782 MeV) meson resonances in timelike region and to exponential distribution in coordinate space

NUCLAT, April 21-22, 2008, 7

Global Analysis (1995)

P. Bosted et al. PRC 51, 409 (1995)

Three form factors very similar

GEn zero within errors ->

accurate data on GEn early goal

of JLab

GEp / GD =GM

p /GD = 1+ aiQi

i=1

5

∑⎛⎝⎜

⎞⎠⎟;

GMn /GD = 1+ biQ

i

i=1

4

∑⎛⎝⎜

⎞⎠⎟; GE

n =0

NUCLAT, April 21-22, 2008, 8

Modern Era

Akhiezer et al., Sov. Phys. JETP 6, 588 (1958) andArnold, Carlson and Gross, PR C 23, 363 (1981) showed that:

accuracy of form-factor measurements can be significantly improved by measuring an interference term GEGM through the beam-helicity asymmetry with a polarized target or with recoil polarimetry

Had to wait over 30 years for development of

• Polarized beam with high intensity (~100 µA) and high polarization (>70 %)

(strained or superlattice GaAs, high-power diode/Ti-Sapphire lasers)

• Beam polarimeters with 1-3 % absolute accuracy• Polarized targets with a high polarization or• Ejectile polarimeters with large analyzing powers

NUCLAT, April 21-22, 2008, 9

Double Polarization Experiments to Measure GnE

€

GEn

GMn =

A⊥

A||

τ +τ (1+ τ )tan2 (θ2

)

• Study the (e,e’n) reaction from a polarized ND3 targetlimitations: low current (~80 nA) on target

deuteron polarization (~25 %)

• Study the (e,e’n) reaction from a LD2 target and measure the neutron polarization with a polarimeter limitations: Figure of Merit of polarimeter

• Study the (e,e’n) reaction from a polarized 3He targetlimitations: current on target (12 µA)

target polarization (40 %)nuclear medium corrections

NUCLAT, April 21-22, 2008, 10

GnE Experiment with Neutron Polarimeter

€

ξ ∝ sin χ +δ( )⇒GE

n

GMn = −tanδ

τ 1 +ε( )

2ε

• Use dipole to precess neutron spin• Up-down asymmetry ξ

proportional to neutron sideways polarization

• GE/GM depends on phase shift w.r.t. precession angle

NUCLAT, April 21-22, 2008, 11

Neutron Electric Form Factor GEn

Galster: a parametrization fitted to old (<1971) data set of very limited quality

For Q2 > 1 GeV2 data hint that GE

n has similar Q2-behaviour as GE

p

Most recent results (Mainz, JLab) are in excellent agreement, even though all three different techniques were used

GEn (Q2 ) =

−μnQ2

4Mn2 +bQ2( )

GD Q2( )

Schiavilla and Sick analyzed data on deuteron elastic quadrupole form factor

NUCLAT, April 21-22, 2008, 12

Exclusive QE scattering: 3He(e,e′n)

e′

n

2 veto planes

7 Iron/scintillatorsandwich planes

e

Beam polarization

84%

Target polarization

~50%

Q2 = 1.3, 1.7, 2.5, 3.5 GeV2

NUCLAT, April 21-22, 2008, 13

First physics result from Hall A GEn

1.7 GeV2 datum is well above Galster Nuclear corrections include neutron polarization and FSI estimate (5%) Present error (∼20%) dominated by preliminary “neutron dilution factor”, and is expected to be ∼7% stat. and 8% syst. with further analysis 3.4 GeV2 datum, however, is on top of Galster

NUCLAT, April 21-22, 2008, 14

Measuring GnM

•Measure (en)/(ep) ratioLuminosities cancelDetermine neutron detector efficiency

•On-line through e+p->e’+π+(+n) reaction (CLAS)•Off-line with neutron beam (Mainz)

€

RD =

d3σ (eD ⇒ e 'n(p))

dE ' dΩe 'dΩn

d3σ (eD ⇒ e ' p(n))

dE 'dΩ e'dΩ p

Old method: quasi-elastic scattering from 2Hlarge systematic errors due to subtraction of proton contribution

€

A =− cosθ *vT 'RT ' + 2sinθ *cosϕ *vTL'RTL'( )

vLRL + vTRT

RT’ directly sensitive to (GMn)2

• Measure inclusive quasi-elastic scattering off polarized 3He

NUCLAT, April 21-22, 2008, 15

Measurement of GnM at low Q2

NUCLAT, April 21-22, 2008, 16

Preliminary CLAS results for GMn

A systematic difference of several % between results ( ) in Q2 range 0.4 - 0.8 GeV2. A final analysis and paper from CLAS is coming soon.Reminder that at least two independent experiments are always needed.

NUCLAT, April 21-22, 2008, 17

Early Measurements of GEp

•relied on Rosenbluth separation•measure d/dΩ at constant Q2

•GEp inversely weighted with Q2, increasing the systematic error

above Q2 ~ 1 GeV2

€

Q2 = 4EE 'sin2(θ2

)

At 6 GeV2 R changes by only 8% from =0 to =1 if GE

p=GMp/µp

Hence, measurement of GEp

with 10% accuracy requires 1.6% cross-section measurement

=1

1+ 2(1+ τ ) tan2 (θ / 2)

R Q2 ,ε( ) = ε 1+1

τ⎛⎝⎜

⎞⎠⎟

E

E '

σ E,θ( )

σ Mott

= (GMp )2 Q2( ) +

ε

τ(GE

p )2 Q2( )

NUCLAT, April 21-22, 2008, 18

Spin Transfer Reaction 1H(e,e’p)

€

r

€

r

€

Pn = 0

±hPt = mh2 τ (1+ τ )GEpGM

p tanθe

2

⎛ ⎝

⎞ ⎠/ I0

±hPl = ±h Ee + Ee'( ) GMp

( )2

τ (1+ τ )tan2 θe

2

⎛ ⎝

⎞ ⎠/ M / I0

I0 = GEp Q2( ){ }

2+ τ GM

p Q2( ){ }

21+ 2(1+ τ )tan2 θ e

2

⎛ ⎝

⎞ ⎠

⎡

⎣ ⎢ ⎤

⎦ ⎥

€

GEp

GMp = −

Pt

Pl

Ee + Ee'

2Mtan

θ e

2

⎛ ⎝

⎞ ⎠

No error contributions from• analyzing power• beam polarimetry

NUCLAT, April 21-22, 2008, 19

Recoil Polarization Measurement

Observed angular distribution

Focal-Plane Polarimeter

NUCLAT, April 21-22, 2008, 20

JLab Polarization Transfer Data

•E93-027 PRL 84, 1398 (2000)Used both HRS in Hall A with FPP

•E99-007 PRL 88, 092301 (2002)used Pb-glass calorimeter for electron detection to match proton HRS acceptance

•Reanalysis of E93-027 (Punjabi, nucl-ex/0501018)Using corrected HRS properties

•No dependence of polarization transfer on any of the kinematic variables

NUCLAT, April 21-22, 2008, 21

Super-Rosenbluth (E01-001)

I. Qattan et al. (PRL 94, 142301 (2005))• Detect recoil protons in HRS-L to diminish

sensitivity to:• Particle momentum• Particle angle• Rate

• Use HRS-R as luminosity monitor• Very careful survey

RosenbluthPol TransMC simulations

NUCLAT, April 21-22, 2008, 22

Rosenbluth Compared to Polarization Transfer

• John Arrington performed detailed reanalysis of SLAC data• Hall C Rosenbluth data (PRC 70, 015206 (2004)) in agreement with

SLAC data• No reason to doubt quality of either Rosenbluth or polarization transfer

data• Investigate possible theoretical sources for discrepancy

NUCLAT, April 21-22, 2008, 23

Two-Photon Exchange Proton form factor measurements

• Comparison of precise Rosenbluth and polarization measurements of GEp/GMp show clear discrepancy at high Q2

Two-photon exchange corrections believed to explain the discrepancy

Compatible with e+/e- ?• Yes: previous data limited to

low Q2 or small scattering angle

Still lack direct evidence of effect on cross section• Transverse single spin

asymmetry is the only observable in elastic e-p where TPE observed

P.A.M.Guichon and M.Vanderhaeghen, PRL 91, 142303 (2003)

Chen et al., PRL 93, 122301 (2004)

NUCLAT, April 21-22, 2008, 24

24

Two-Photon Exchange Measurements

Comparisons of e+-p and e--p scattering [VEPP-III, JLab-Hall B, BLAST@DORIS]

ε dependence of polarization transfer and unpolarized σe-p [JLab-Hall C]

• More quantitative measure of the discrepancy• Test against models of TPE at both low and high Q2

TPE effects in Born-forbidden observables [JLab-Hall A, Hall C, Mainz]

• Target single spin asymmetry Ay in e-n scattering

• Induced polarization py in e-p scattering

• Vector analyzing power AN in e-p scattering

World’s dataNovosibirskJLab – Hall B

Evidence (3 level) for TPE in existing data

J. Arrington, PRC 69, 032201(R) (2004)

NUCLAT, April 21-22, 2008, 25

Two-Photon Exchange Calculations Significant progress in theoretical understanding

• Hadronic calculations appear sufficient up to 2-3 GeV2

• GPD-based calculations used at higher Q2

Experimental program will quantify TPE for several e-p observables

Before TPEAfter TPE (Blunden, et al)

• Precise test of calculations

• Tests against different observables

Want calculations well tested for elastic e-p, reliable enough to be used for other reactions

NUCLAT, April 21-22, 2008, 26

Reanalysis of SLAC data on GM

p

E. Brash et al. (PRC 65, 051001 (2002)) have reanalyzed SLAC data with JLab GE

p/GMp results

as constraint, using a similar fit function as BostedReanalysis results in 1.5-3% increase of GM

p data

NUCLAT, April 21-22, 2008, 27

Charge and Magnetization Radii

€

r2 ≡ 4π ρ (r)r4 dr = −6

G(0)dG(Q2)

dQ2∫Experimental values<rE

2>p1/2= 0.895+0.018 fm

<rM2>p

1/2= 0.855+0.035 fm<rE

2>n= -0.119+0.003 fm2

<rM2>n

1/2= 0.87+0.01 fm

€

dGEn (Q2)

dQ2 Q 2= 0=

dF1n (Q2)

dQ2 Q 2= 0−

F2n(0)

4M 2

Foldy term = -0.126 fm2 canceled by relativistic corrections (Isgur)implying neutron charge distribution is determined by GE

n

Even at low Q2-values Coulomb distortion effects have to be taken into accountThe three real radii are identical within the experimental accuracy

NUCLAT, April 21-22, 2008, 28

Low Q2 Systematics

All EMFF show minimum (maximum for GEn) at Q ≈ 0.5 GeV

NUCLAT, April 21-22, 2008, 29

Pion Cloud

•Kelly has performed simultaneous fit to all four EMFF in coordinate space using Laguerre-Gaussian expansion and first-order approximation for Lorentz contraction of local Breit frame

€

˜ G E,M (k) = GE ,M (Q2) 1+ τ( )2 with k 2 =

Q2

1+ τ and τ =

Q

2M

⎛ ⎝

⎞ ⎠

2

• Friedrich and Walcher have performed a similar analysis using a sum of dipole FF for valence quarks but neglecting the Lorentz contraction

• Both observe a structure in the proton and neutron densities at ~0.9 fm which they assign to a pion cloud

• Hammer et al. have extracted the pion cloud assigned to the NN2π component which they find to peak at ~ 0.4 fm

_

NUCLAT, April 21-22, 2008, 30

New Polarization Transfer Results for GEp

• Parasitic to G0

• 40% Polarized Beam• Approx. 12 hours/point

Cross Section from C. Berger et al., Phys. Lett. B35 (1971) 87Deviation in Ratio is due to Electric Form Factor

PRL 99, 202002 (2007)

NUCLAT, April 21-22, 2008, 31

Approved Experiment E04-007

One Day of Beam Time per Point!Will run in May/June 2008

Extensive program of accurate cross-section measurements at low Q2 ongoing at MAMI

NUCLAT, April 21-22, 2008, 32

High-Q2 behaviour

Basic pQCD (Bjørken) scaling predicts F1 1/Q4 ; F2 1/Q6

F2/F1 1/Q2 (Brodsky & Farrar)Data clearly do not follow this trend

Schlumpf (1994), Miller (1996) andRalston (2002) agree that by• freeing the pT=0 pQCD condition• applying a (Melosh) transformation to

a relativistic (light-front) system• an orbital angular momentum

component is introduced in the proton wf (giving up helicity conservation) and one obtains

F2/F1 1/Q• and equivalently a linear drop off of

GE/GM with Q2

Brodsky argues that in pQCD limit non-zero OAM contributes to both F1 and F2

NUCLAT, April 21-22, 2008, 33

High-Q2 BehaviourBelitsky et al. have included logarithmic corrections in pQCD limit

They warn that the observed scaling could very well be precocious

NUCLAT, April 21-22, 2008, 34

Theory

• Relativistic Constituent Quark ModelsVariety of q-q potentials (harmonic oscillator, hypercentral, linear)Non-relativistic treatment of quark dynamics, relativistic EM currents

• Miller: extension of cloudy bag model, light-front kinematicswave function and pion cloud adjusted to static parameters

• Cardarelli & SimulaIsgur-Capstick oge potential, light-front kinematicsconstituent quark FF in agreement with DIS data

• Wagenbrunn & Plessaspoint-form spectator approximationlinear confinement potential, Goldstone-boson exchange

• Giannini et al.gluon-gluon interaction in hypercentral modelboost to Breit frame

• Metsch et al.solve Bethe-Salpeter equation, linear confinement potential

NUCLAT, April 21-22, 2008, 35

Relativistic Constituent Quark

charge magnetization

proton

neutron

NUCLAT, April 21-22, 2008, 36

Time-Like Region

•Can be probed through e+e- -> NN or inverse reaction_

•Data quality insufficient to separate charge and magnetization contributions•No scaling observed with dipole form factor•Iachello only model in reasonable agreement with data•Large new accurate data set from BaBar/ CLEO /DANE

NUCLAT, April 21-22, 2008, 37

In the simple valence-quark model a nucleon consists of u and d quarks.

Valence and Sea Quarks in the Nucleon

However, ss pairs are continuously created and annihilated and are known to contribute to a variety of nucleon properties, such as mass (0-30%), momentum (2-4%) and spin (0-20%).

Just as pions can cause the zero-charge neutron to have a non-zero charge distribution, the strange sea is expected to contribute to the charge and magnetic moment distribution of nucleons.

but how much?

NUCLAT, April 21-22, 2008, 38

Elastic Electroweak Scattering

GEs(Q2), GM

s(Q2)

p

AMEF AAAQGA

πα++

⎥⎦

⎤⎢⎣

⎡−=

24

2

APV for elastic e-p scattering:

Forward angle Backward angle

( ) eA

pMWA

ZM

pMM

ZE

pEE GGAGGAGGA '2sin41 , , εθτε −−===

Helium: unique GE sensitivityDeuterium: enhanced GA sensitivity

Z0

Kaplan & Manohar (1988)McKeown (1989)

NUCLAT, April 21-22, 2008, 39

Flavor Separation of Nucleon Form Factors

•If we can measure

with

:,, ,,, npZp GGG

( )psME

pdME

puME

pME GGGG ,

,,,

,,

,, 3

1

3

2+−=γ

nsps

nupd

ndpu

GG

GG

GG

,,

,,

,,

=

=

=

dropping the p superscripts on the left

then we can write

by assuming charge symmetry

G ~ N eii∑ qiΓμqi N

GE ,Mu =(3−4sin2θW )GE,M

,p −GE,MZ,p

GE,Md =(2 −4sin2θW )GE,M

,p +GE,M,n −GE,M

Z,p

GE,Ms =(1−4sin2θW )GE,M

,p −GE,M,n −GE,M

Z,p

NUCLAT, April 21-22, 2008, 40

Instrumentation for PVES

Large -Acceptance Detectors (G0, A4) Large kinematic range Poor background suppression

Spectrometer (HAPPEx) Good background rejection Scatter from magnetized iron

Cumulative Beam Asymmetry• Helicity-correlated asymmetryx~10 nm, I/I~1 ppm, E/E~100 ppb

Helicity flips• Pockels cell• half-wave plate flips

!!!1010 1413 −≈N

%52

11==

NAAA

610−−+

−+

≈+−

=

PVA

Need• Highest possible luminosity• High rate capability• High beam polarization

Detectors Integrating (HAPPEx) vs. Counting (G0, A4)

NUCLAT, April 21-22, 2008, 41

Overview of Experiments

GMs, (GA) at Q2 = 0.1 GeV2

SAMPLE

HAPPEx GEs + 0.39 GM

s at Q2 = 0.48 GeV2

GEs + 0.08 GM

s at Q2 = 0.1 GeV2

GEs at Q2 = 0.1 GeV2 (4He)

open geometry, integrating

A4

GEs + 0.23 GM

s at Q2 = 0.23 GeV2

GEs + 0.10 GM

s at Q2 = 0.1 GeV2

GMs, GA

e at Q2 = 0.1, 0.23, 0.5 GeV2

open geometry

fast-counting calorimeter for background rejection

Electron Beam

LH2 Target

SuperconductingCoils

Particle Detectors

G0

GEs + GM

s over Q2 = [0.12,1.0] GeV2

GMs, GA

e at Q2 = 0.23, 0.62 GeV2

open geometry

fast-counting with magnetic spectrometer + TOF for background rejection

NUCLAT, April 21-22, 2008, 42

Experimental Technique

Compton1.5-3% syst

Continuous

Møller2-3% syst

Polarimeters

High Resolution SpectrometerS+QQDQ 5 mstr over 4o-8o

Target400 W transverse flow20 cm, LH220 cm, 200 psi 4He

Cherenkovcones

PMT

PMTElastic Rate:1H: 120 MHz4He: 12 MHz

100 x 600 mm

12 m dispersion

sweeps away inelastic events

NUCLAT, April 21-22, 2008, 43

New HAPPEx Results

Asym

metr

y

(pp

m)

Slug

Hydrogen

Araw correction ~11 ppb

Slug

Asym

metr

y

(pp

m)

Helium

normalization error ~ 2.5%

APV = -1.58 0.12 (stat) 0.04 (syst) ppmA(Gs=0) = -1.66 ppm 0.05 ppm

HydrogenSystematic control ~ 10-8

APV = +6.40 0.23 (stat) 0.12 (syst) ppmA(Gs=0) = +6.37 ppm

HeliumNormalization control ~ 2%

Helicity Window Pair Asymmetry

Hydrogen

NUCLAT, April 21-22, 2008, 44

World Data near Q2 ~0.1 GeV2

Caution: the combined fit is approximate. Correlated errors and assumptions not taken into account

GMs = 0.28 ± 0.20

GEs = -0.006 ± 0.016

~3% ± 2.3% of proton magnetic moment

~0.2 ± 0.5% of Electric distribution

HAPPEX only fit suggests something even smaller:

GMs = 0.12 ± 0.24

GEs = -0.002 ± 0.017

PRL 98, 032301 (2007)

NUCLAT, April 21-22, 2008, 45

New data confront theoretical predictions

16. Skyrme Model - N.W. Park and H. Weigel, Nucl. Phys. A 451, 453 (1992).

17. Dispersion Relation - H.W. Hammer, U.G. Meissner, D. Drechsel, Phys. Lett. B 367, 323 (1996).

18. Dispersion Relation - H.-W. Hammer and Ramsey-Musolf, Phys. Rev. C 60, 045204 (1999).

19. Chiral Quark Soliton Model - A. Sliva et al., Phys. Rev. D 65, 014015 (2001).

20. Perturbative Chiral Quark Model - V. Lyubovitskij et al., Phys. Rev. C 66, 055204 (2002).

21. Lattice - R. Lewis et al., Phys. Rev. D 67, 013003 (2003).

22. Lattice + charge symmetry -Leinweber et al, Phys. Rev. Lett. 94, 212001 (2005) & hep-lat/0601025

NUCLAT, April 21-22, 2008, 46

Summary and Outlook

• Suggested large values at Q2~0.1 GeV2

• Ruled out

• Possible large values at Q2=0.6 GeV2

• G0 back angle, data being analyzed• HAPPEX-III - 2009

• Large possible cancellation at Q2~0.2 GeV2

• Very unlikely given constraint at 0.1 GeV2

• G0 back angle at low Q2 (error bar~1.5% of μp) maintains sensitivity to discover GM

S0.6 GeV2

G0 backward

HAPPEX-III

GMs

GEs

NUCLAT, April 21-22, 2008, 47

Outstanding Precision for Strange Form Factors

This has recently been shown to enable a dramaticimprovement in precision in testing the Standard Model

Q2=0.1 GeV2

Ciq denote the V/A electron-quark coupling constants

NUCLAT, April 21-22, 2008, 48

Summary• Very active experimental program on nucleon electro-weak form factors

thanks to development of polarized beam (> 100 µA, > 75 %) with extremely small helicity-correlated properties, polarized targets and polarimeters with large analyzing powers

• Electromagnetic Form Factors• GE

p discrepancy between Rosenbluth and polarization transfer not an experimental problem, but highly likely caused by TPE effects

• GEn precise data up to Q2 = 3.5 GeV2

• GMn precise data up to Q2 = 5 GeV2, closely following

dipole behaviour, but with experimental discrepancies at ~ 1 GeV2

• Further accurate data will continue to become available as benchmark for Lattice QCD calculations

• Parity-violating asymmetries have provided highly accurate data on strange form factors at ~ 0.1 GeV2 with further data in the near future

• Significant advances in measurement of transverse SSAs• Sensitive test of TPE calculations

NUCLAT, April 21-22, 2008, 49

Extensions with JLab 12 GeV Upgrade

. BLUE = CDR or PAC30 approved, GREEN = new ideas under development~10 GeV2

NUCLAT, April 21-22, 2008, 50

Acknowledgements

Many thanks to the colleagues who willingly provided me with figures/slides/discussions:

• John Arrington• Roger Carlini• Doug Higinbotham• Michael Kohl• Paul Souder• Bogdan Wojtsekhowski