jlab low q 2 measurements ron gilman*, rutgers university
DESCRIPTION
Outline. *Supported by NSF PHY 09-69239. Background Experiments E05-103 (2006) E08-007 (2008) E08-007 (2011-12) Other Issues Summary. JLab Low Q 2 Measurements Ron Gilman*, Rutgers University. Welcome to PINAN Form Factor Fest Session 2. Background - Form Factor Fest. Gerald Miller - PowerPoint PPT PresentationTRANSCRIPT
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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
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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?
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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.