1

JLab Low Q2 MeasurementsRon Gilman*, Rutgers University

BackgroundExperimentsE05-103 (2006)E08-007 (2008)E08-007 (2011-12)Other IssuesSummary

Welcome to PINAN Form Factor Fest

Session 2

*Supported by NSF PHY 09-69239Outline

2

Background - Form Factor Fest

Form Factors - Theory Overview

Form Factors and Radii of the Proton

BLAST and OLYMPUS Programs

JLab Low Q2 Measurements

Form Factors - Future Measurements

A new Precision Charge Radius Experiment

Time-like Structure Functions with PANDA

Gerald Miller

Thomas Walcher

Michael Kohl

Ron Gilman

Gerald Gifoyle

Dipangkar Dutta

Ronald Kunne

What does one do as the 4th of 7 form factor talks?

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Background - Form Factor Fest

Form Factors - Theory Overview

Form Factors and Radii of the Proton

BLAST and OLYMPUS Programs

JLab Low Q2 Measurements

Form Factors - Future Measurements

A new Precision Charge Radius Experiment

Time-like Structure Functions with PANDA

Gerald Miller

Thomas Walcher

Michael Kohl

Ron Gilman

Gerald Gifoyle

Dipangkar Dutta

Ronald Kunne

What does one do as the 4th of 7 form factor talks?Remember G Miller did much of the interesting recent

theory / interpretation and probably showed it.

Remember almost everything has been shown before and be brief.

Finish early - most speakers run long anyways.

Be glad you are not speaker 5 or 6.

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The Basics: 1

currents

algebra

cross sections

with form factors:

and kinematic factors:

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Interpretation

The FF is the 3d Fourier transform (FT) of the Breit frame spatial distribution in the Long Range Plan, but the Breit frame is not the rest frame, and doing this confuses people who do not know better.

The FF is the 2d FT of the transverse spatial distribution.

The slope of the FF at Q2 = 0 gives what everyone should call the slope of the FF at Q2 = 0, but for reasons of history and or poor education most people call the radius.

Nucleon magnetic FFs crudely follow the dipole formula, GD = (1+Q2/0.71 GeV2)-2, which a) has the expected high Q2 pQCD behavior, and b) is amusingly the 3d FT of an exponential, but c) has no theoretical significance.

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The Basics: 2Measure cross sections

Perform radiative corrections

Do Rosenbluth separations - or - fit world data with form factor parameterization

The EM interaction is too strong!

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The Basics: 3Use polarizations for

form factor ratios

Sensitive to spin transport, insensitive to almost everything else ... but needs large

statistics

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The Basics: 4

Measuring two angles at the same time allows a ratio to be made, reducing

sensitivity to PbPt, which can vary by 20% or more over time.

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Our story starts ...Friedrich & Walcher fit, EPJA 17, pg 607, 2003

2-dipole fit of the form factors leaves residual bumps, interpreted as evidence for meson-cloud effects

Not in agreement with newest data.

Articles appear studying the Zemach radius and corrections to Hydrogen hyperfine splitting

Friar and Sick, PLB 579 (2004)

Brodsky, Carlson, Hiller, and Hwang, PRL 96 (2005)

Friar and Payne, PRC 72 (2005)

Nazaryan, Carlson, and Griffioen, PRL 96 (2006)

Low Q2 nucleon structure study

re-invigorated!

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Four experimentsBLAST - long planned program for low Q2 nucleon and deuteron structure with polarized beam - internal polarized target

Mainz A1 - already discussed by Th. Walcher

E05-103 run 2006

FPP calibrations for low energy deuteron photodisintegration used to determine proton GE/GM

E08-007 run 2008

Dedicated FPP experiment to more systematically cover the 0.3 - 0.7 GeV2 range with higher statistics

E08-007 part II to run Nov 2011 - May 2012 (along with g2p)

Dedicated polarized beam - polarized target measurements to cover the range about 0.02 - 0.4 GeV2 with high precision

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BLAST Low Q2 DataC.B. Crawford et al., Phys. Rev. Lett. 98, 052301 (2007)

BLAST FF ratio consistent with unity, within ≈2% uncertainties

Consistent with earlier fits / analyses / theory calculations

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E05-103 Low Q2 DataG. Ron et al., Phys. Rev. Lett. 99, 202002 (2007)

Our initial FPP results indicate the FF ratio is lower than previously believed, around 0.4 GeV2

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E05-103 Low Q2 DataG. Ron et al., Phys. Rev. Lett. 99, 202002 (2007)

Our initial FPP results indicate the FF ratio is lower than previously believed, around 0.4 GeV2

Note that the fits ... have a range of slopes near the origin, not well constrained with data

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E05-103 Low Q2 DataG. Ron et al., Phys. Rev. Lett. 99, 202002 (2007)

Combining Berger at al. PLB 35, 1971 dσ/dΩ with new FPP data in G. Ron et al PRL 98, we showed fits

tend to get GM about

right, but tend to over

predict GE

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Mainz A1 DataJ. Bernauer et al., Phys. Rev. Lett. 105, 242001 (2010)

Th. Walcher has already discussed.

The figure is from J. Bernauer’s Ph.D. thesis: Rosenbluth separation results compared to spline fit.

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E08-007 DataX. Zhan et al., ran 2008

M. Paolone et al., Phys Rev Lett 105, 072001, 2010 (Q2 = 0.8 GeV2)

Results essentially unchanged since online data.

About 1% total uncertainty on FF ratio.

Decreased ratio compared to earlier measurements prompted 2 years of thorough systematics studies: cuts, spin transport, backgrounds, ...

Major finding: with very high statistics here one sees changes in ratio as cuts are made very tight.

Reanalyzed G Ron data in very good agreement.

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Large Improvement in FF Ratio

RosenbluthPolarizationE08007 E03104

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E08-007 ImpactFit of world data except Mainz A1 data.

GE reduced up to ≈2% from 0.3 - 1 GeV2

GM increased ≈0.5% from 0.1 - 0.8 GeV2

FF ratio smaller by up to ≈2.5% from 0.3 - 0.8 GeV2

Slopes changed at Q2 = 0 changing slope of form factor at Q2 = 0. (``radii’’)

AMT

w/ E08007

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But some tension between Mainz and JLab

Polarization

Note that the FF ratio agrees better than the individual form factors ... so the difference must arise from Mainz vs. world cross sections.

Is there an issue in the FF ratio at the low Q2 limit, or is it an end-point problem / statistics? We will know better once we have the polarized target results.

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Muonic Hydrogen Puzzle

Polarization

Muonic hydrogen disagrees with atomic physics and electron scattering determinations of slope of FF at Q2 = 0.

Slope of GEp at Q2 = 0 (AU)

# Extraction <rE>2 [fm]

1 Sick 0.895±0.018

2 CODATA 0.8768±0.0069

3 Mainz 0.879±0.008

4 This Work 0.870±0.010

5Combined

2-40.8764±0.0047

6Muonic

Hydrogen0.842±0.001

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Hyperfine Splitting and Zemach radius

EHFS = (1+∆QED+∆pR+∆p

hvp+∆pμvp+∆p

WEAK+∆S) EFp = 1420.405 751 766 7(9)

MHz

Structure term ∆S = ∆Z + ∆POL, with ∆Z = -2amerZ(1+dradZ), and ∆POL an

inelastic structure correction dependent on g2p.

The Zemach radius is

FF rp [fm] rZ [fm]ΔZ

[ppm]

AMT 0.885 1.08 -41.43

AS 0.879 1.091 -41.85

Kelly 0.878 1.069 -40.99

F&W 0.808 1.049 -40.22

Dipole 0.851 1.025 -39.29

New 0.868 1.075 -41.22

Parameterizations vary by ≈2 ppm

Uncertainty from Q2 ≈ 0.01 - 1 GeV2

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E08-007 Phase II

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Note on PV Experiments

For a given experimental asymmetry, with an oversimplified assumption of electric or magnetic dominance, A ≈ GpZ/Gpγ, so a reduced GE

p leads to a reduced GpZ and a reduced GE

s.

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E08-007 & g2p Status

Designers have been busy...

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E08-007 StatusComponents are being ordered...

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E08-007 StatusRun plans have been developed...

g2p and elastic FF are intermixed.

g2p settings

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E08-007 StatusSchedules have been published...

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E08-007 StatusAnd shift signup has started ...

We are getting all set to take data!

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What’s next?Can we do even lower Q2 ep elastic scattering experiments?

Obvious 1st guess: high energy proton beam on atomic electrons

Akin to low Q2 pion form factor measurements

With MEIC/EIC, etc., obvious alternative in the longer term: use a ring with bending magnets to provide access to near 0 degree scattering

And a nice new JLab idea - D Dutta’s talk

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High Energy Protons on Atomic Electrons

E906 at FNAL is taking data with 120 GeV protons.

Inverse kinematics, high E protons on atomic electrons, sample small Q2

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High Energy Protons on Atomic Electrons

Cross section is large.

Counts are plentiful.

Precision required is large - looking for 0.5% effect.

Statistics use E906 POT on 10 mg/cm2 12C for number of atomic electrons, Kelly form factors, and full φ acceptance. Ratio based simply on σ ≈ 1 - Q2 r2 / 6.

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Collider Form Factor Measurements

Q2

(GeV2)10-4 5⋅10-4 10-3 5⋅10-3 0.01

θe 0.19 0.427 0.6 1.35 1.9

XS(cm-2)

2.60E-23 1.00E-24 2.50E-25 1.00E-26 2.50E-27

Rate (Hz)

9.1 1.75 0.875 0.175 0.0875

T0.5%

(hr)1.22 6.35 12.7 63.5 127

Estimates from G. Ron

With MEIC/EIC, etc., obvious alternative in the longer term: use a ring with bending magnets to provide access to near 0 degree scattering

Low Q2 requires very forward particle detection

limits due to systematics - e.g. beam polarization direction

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Collider Form Factor Measurements

Top: θpol = 45o.

Bottom: θpol = 45o. Q2 = 0.001 GeV2.

Lower beam energy is better, but collider luminosity drops with decreasing energy.

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A note on the neutron charge distribution

What are we to make of the neutron charge density at the origin being positive in the Breit frame but negative for the transverse density?

It seems intuitively obvious that as r → 0 or ∞ the sign of the charge density should be the same for the 3d and 2d transverse densities

It seems intuitive to think in the rest frame and to identify the Breit frame with the rest frame, however wrong this is.

It probably makes no sense to talks about the rest frame for a relativistic system anyway.

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Kelly Form Factors

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Why is ρ3d>0 when ρ<0 at r,b=0?Natural to assume they should have the same sign.

G Miller has suggested high Q2 data might change FT so ρT > 0 at b = 0.

ρBreit > 0 since GE > 0.

ρT < 0 since F1 < 0.

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Why is ρ3d>0 when ρ<0 at r,b=0?

Positive ρT requires positive F1, which requires GE grows relative to Q2GM. Seems unlikely. Since GM ≈ GD ≈ 1/Q2, GE grows absolutely. Seems unlikely.

Negative ρBreit requires only that GE goes sufficiently negative at high Q2.

One can generate nonsense that fits existing data and does this. Maybe future data will show this happens.

ρBreit > 0 since GE > 0.

ρT < 0 since F1 < 0.

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Summary

Strong recent program in Low Q2 nucleon structure - form factors and spin structure.

Continued interest in slope of form factor at Q2 = 0, hyperfine splitting, parity violation, which are impacts of form factor measurements, as well as this aspect of nucleon structure for itself - e.g., is there a signature of the pion cloud?

Ongoing interest in future experiments to push precise measurements to even lower Q2.

A suggestion that GEn might go negative at high Q2.