1 JLab Low Q 2 Measurements Ron Gilman*, Rutgers University Background Experiments E (2006) E (2008) E ( ) Other Issues Summary Welcome to PINAN Form Factor Fest Session 2 *Supported by NSF PHY Outline

2 Background - Form Factor Fest Form Factors - Theory Overview Form Factors and Radii of the Proton BLAST and OLYMPUS Programs JLab Low Q 2 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 4 th of 7 form factor talks?

3 Background - Form Factor Fest Form Factors - Theory Overview Form Factors and Radii of the Proton BLAST and OLYMPUS Programs JLab Low Q 2 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 4 th 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 Q 2 = 0 gives what everyone should call the slope of the FF at Q 2 = 0, but for reasons of history and or poor education most people call the radius. Nucleon magnetic FFs crudely follow the dipole formula, G D = (1+Q 2 /0.71 GeV 2 ) -2, which a) has the expected high Q 2 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 P b P t, which can vary by 20% or more over time.

9 Our story starts... Friedrich & Walcher fit, EPJA 17, pg 607, 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 Q 2 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 E run 2006 FPP calibrations for low energy deuteron photodisintegration used to determine proton G E /G M E run 2008 Dedicated FPP experiment to more systematically cover the GeV 2 range with higher statistics E part II to run Nov May 2012 (along with g 2p ) Dedicated polarized beam - polarized target measurements to cover the range about GeV 2 with high precision

11 BLAST Low Q2 Data C.B. Crawford et al., Phys. Rev. Lett. 98, (2007) BLAST FF ratio consistent with unity, within ≈2% uncertainties Consistent with earlier fits / analyses / theory calculations

12 E Low Q2 Data G. Ron et al., Phys. Rev. Lett. 99, (2007) Our initial FPP results indicate the FF ratio is lower than previously believed, around 0.4 GeV 2

13 E Low Q2 Data G. Ron et al., Phys. Rev. Lett. 99, (2007) Our initial FPP results indicate the FF ratio is lower than previously believed, around 0.4 GeV 2 Note that the fits... have a range of slopes near the origin, not well constrained with data

14 E Low Q2 Data G. Ron et al., Phys. Rev. Lett. 99, (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 G M about right, but tend to over predict G E

15 Mainz A1 Data J. Bernauer et al., Phys. Rev. Lett. 105, (2010) Th. Walcher has already discussed. The figure is from J. Bernauer’s Ph.D. thesis: Rosenbluth separation results compared to spline fit.

16 E Data X. Zhan et al., ran 2008 M. Paolone et al., Phys Rev Lett 105, , 2010 (Q 2 = 0.8 GeV 2 ) 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 E08007 E03104

18 E Impact Fit of world data except Mainz A1 data. G E reduced up to ≈2% from GeV 2 G M increased ≈0.5% from GeV 2 FF ratio smaller by up to ≈2.5% from GeV 2 Slopes changed at Q 2 = 0 changing slope of form factor at Q 2 = 0. (``radii’’) AMT w/ E08007

19 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 Q 2 limit, or is it an end-point problem / statistics? We will know better once we have the polarized target results.

20 Muonic Hydrogen Puzzle Polarization Muonic hydrogen disagrees with atomic physics and electron scattering determinations of slope of FF at Q 2 = 0. Slope of G E p at Q 2 = 0 (AU) #Extraction 2 [fm] 1Sick0.895± CODATA0.8768± Mainz0.879± This Work0.870± Combined ± Muonic Hydrogen 0.842±0.001

21 Hyperfine Splitting and Zemach radius E HFS = (1+∆ QED +∆ p R +∆ p hvp +∆ p μvp +∆ p WEAK +∆ S ) E F p = (9) MHz Structure term ∆ S = ∆ Z + ∆ POL, with ∆ Z = -2am e r Z (1+d rad Z ), and ∆ POL an inelastic structure correction dependent on g 2 p. The Zemach radius is FFr p [fm]r Z [fm]ΔZ [ppm] AMT AS Kelly F&W Dipole New Parameterizations vary by ≈2 ppm Uncertainty from Q 2 ≈ GeV 2

22 E Phase II

23 Note on PV Experiments For a given experimental asymmetry, with an oversimplified assumption of electric or magnetic dominance, A ≈ G pZ /G pγ, so a reduced G E p leads to a reduced G pZ and a reduced G E s.

24 E & g 2 p Status Designers have been busy...

25 E Status Components are being ordered...

26 E Status Run plans have been developed... g 2 p and elastic FF are intermixed. g 2 p settings

27 E Status Schedules have been published...

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

29 What’s next? Can we do even lower Q 2 ep elastic scattering experiments? Obvious 1 st guess: high energy proton beam on atomic electrons Akin to low Q 2 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 Q 2

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/cm 2 12 C for number of atomic electrons, Kelly form factors, and full φ acceptance. Ratio based simply on σ ≈ 1 - Q 2 r 2 / 6.

32 Collider Form Factor Measurements Q 2 (GeV 2 ) ⋅ ⋅ θe XS (cm -2 ) 2.60E E E E E-27 Rate (Hz) T 0.5% (hr) 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 Q 2 requires very forward particle detection limits due to systematics - e.g. beam polarization direction

33 Collider Form Factor Measurements Top: θ pol = 45 o. Bottom: θ pol = 45 o. Q 2 = GeV 2. 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 ρ 3d >0 when ρ<0 at r,b=0? Natural to assume they should have the same sign. G Miller has suggested high Q 2 data might change FT so ρ T > 0 at b = 0. ρ Breit > 0 since G E > 0. ρ T < 0 since F 1 < 0.

37 Why is ρ 3d >0 when ρ<0 at r,b=0? Positive ρ T requires positive F 1, which requires G E grows relative to Q 2 G M. Seems unlikely. Since G M ≈ G D ≈ 1/Q 2, G E grows absolutely. Seems unlikely. Negative ρ Breit requires only that G E goes sufficiently negative at high Q 2. One can generate nonsense that fits existing data and does this. Maybe future data will show this happens. ρ Breit > 0 since G E > 0. ρ T < 0 since F 1 < 0.

38 Summary Strong recent program in Low Q 2 nucleon structure - form factors and spin structure. Continued interest in slope of form factor at Q 2 = 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 Q 2. A suggestion that G E n might go negative at high Q 2.