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JLab Low Q 2 Measurements Ron Gilman*, Rutgers University

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

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JLab Low Q 2 Measurements Ron Gilman*, Rutgers University

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  1. Outline *Supported by NSF PHY 09-69239 • Background • Experiments • E05-103 (2006) • E08-007 (2008) • E08-007 (2011-12) • Other Issues • Summary JLab Low Q2 MeasurementsRon Gilman*, Rutgers University Welcome to PINAN Form Factor Fest Session 2

  2. Background - Form Factor Fest Gerald Miller Thomas Walcher Michael Kohl Ron Gilman Gerald Gifoyle Dipangkar Dutta Ronald Kunne • 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 What does one do as the 4th of 7 form factor talks?

  3. Background - Form Factor Fest Gerald Miller Thomas Walcher Michael Kohl Ron Gilman Gerald Gifoyle Dipangkar Dutta Ronald Kunne • 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 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.

  4. The Basics: 1 currents algebra cross sections with form factors: and kinematic factors:

  5. 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.

  6. The Basics: 2 • Measure cross sections • Perform radiative corrections • Do Rosenbluth separations - or - fit world data with form factor parameterization The EM interaction is too strong!

  7. The Basics: 3 Use polarizations for form factor ratios Sensitive to spin transport, insensitive to almost everything else ... but needs large statistics

  8. 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.

  9. 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!

  10. Four experiments • BLAST - 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

  11. BLAST Low Q2 Data C.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

  12. E05-103 Low Q2 Data G. 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

  13. E05-103 Low Q2 Data G. Ron et al., Phys. Rev. Lett. 99, 202002 (2007) Note that the fits ... have a range of slopes near the origin, not well constrained with data • Our initial FPP results indicate the FF ratio is lower than previously believed, around 0.4 GeV2

  14. E05-103 Low Q2 Data G. 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

  15. Mainz A1 Data J. 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.

  16. E08-007 Data X. 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.

  17. Large Improvement in FF Ratio Rosenbluth Polarization E08007E03104

  18. E08-007 Impact AMT Fit of world data except Mainz A1 data. w/ E08007 • 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’’)

  19. But some tension between Mainz and JLab 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. Polarization

  20. Muonic Hydrogen Puzzle Muonic hydrogen disagrees with atomic physics and electron scattering determinations of slope of FF at Q2 = 0. Polarization Slope of GEp at Q2 = 0 (AU)

  21. Hyperfine Splitting and Zemach radius • EHFS = (1+∆QED+∆pR+∆phvp+∆pμvp+∆pWEAK+∆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 Uncertainty from Q2 ≈ 0.01 - 1 GeV2 Parameterizations vary by ≈2 ppm

  22. E08-007 Phase II

  23. Note on PV Experiments • For a given experimental asymmetry, with an oversimplified assumption of electric or magnetic dominance, A ≈ GpZ/Gpγ, so a reduced GEp leads to a reduced GpZ and a reduced GEs.

  24. E08-007 & g2p Status Designers have been busy...

  25. E08-007 Status Components are being ordered...

  26. g2p settings E08-007 Status Run plans have been developed... g2p and elastic FF are intermixed.

  27. E08-007 Status Schedules have been published...

  28. E08-007 Status And shift signup has started ... We are getting all set to take data!

  29. 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

  30. 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

  31. 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/cm212C for number of atomic electrons, Kelly form factors, and full φ acceptance. Ratio based simply on σ ≈ 1 - Q2 r2 / 6.

  32. Collider Form Factor Measurements • 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

  33. 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.

  34. 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.

  35. Kelly Form Factors

  36. 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.

  37. Why is ρ3d>0 when ρ<0 at r,b=0? ρT < 0 since F1 < 0. ρBreit > 0 since GE > 0. • Positive ρT requirespositive 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.

  38. 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.

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