Nucleon Form Factors

1
Nucleon Form Factors

• Robert Bradford
• Jupiter/MINERnA Group Meeting
• May 16, 2005

2
Outline

• Little about nucleon form factors
• BBBA2005 form factors
• Recent developments
• Gep from BBBA2005
• Influence of Gmp on Gep fit
• Current shape of BBBA form factors
• Conclusions

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What are the elastic nucleon form factors?

• Arise from spatial size of nucleon in elastic
  cross section formula in 1-photon exchange
  approximation

where ?e electron scattering angle Ee
electron energy Ee scattered electron energy
and
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Gep Controversy

• Two methods for measuring
• Rosenbluth Separation
• Good for measuring Gep for Q2 lt 1.0 GeV2
• Recoil polarization
• 2 methods dont agree

Ratio should have been close to 1 if consistent
with older Rosenbluth data
Source M.K. Jones et al, PRL 84, 1398 (2000)
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BBBA2005 Fits

• Uses fit function borrowed from J. Kelly
• Emphasizes recoil polarization data for GMP
• Uses 2 additional conditions from duality to
  constrain poorly-measured neutron form factors

4 parameters for Gep, Gmn, and Gmn 7 for Gen
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Gep Issue
Our fit is a good match to J. Kellys Gep - here
they are plotted side-by-side
Then Arie noticed this graphic from my NuInt05
talk.... 2 figures are consistent (Gd is small so
differences are emphasized)
Q2 (GeV2)
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So, I got to work...

• To extract Gep from recoil polarization data, we
  scale ratio by parameterization of Gmp
• Arie suggested our Gep differs from Kelly because
  we scale the data differently.
• Didnt think this would change anything, but
  tried it anyways....
• Compared results using 2 different
  parameterizations for Gmp Brash and BBBA05

8
Déjàvu

• Essentially got the same results...
• Gmp well measured to high Q2, so
  parameterizations all well constrained...
• Not a lot of effect on fit

Q2 (GeV2)
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What else could be going on...

• Low-Q2 data appears to be driving our fit.
• Lots of it
• More precise
• Most is Rosenbluth data
• Tried to run fit with only polarization data, but
  high-Q2 behavior was wrong
• May try to fudge error bars so that polarization
  data is emphasized.

Gep/Gd
Q2 (GeV2)
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Other Efforts

• Arie had me grab the Kelly parameterizations of
  Gep and Gmp and then re-fit the neutron form
  factor data.
• He wasnt happy with the results....
• Finally grabbed plots using Gep from Kelly and
  Gmp, Gen, and Gmn from BBBA05.
• Cant really do that form factor fits are all
  coupled
• Will show at conference, but well keep working..

11
Future Efforts

• Arie has in mind what he wants the curves to do,
  I just have to keep trying.
• Final fits should be similar to BBBA05, but need
  to emphasize high-Q2 polarization data in fits.
• Artificially decrease error bars on polarization
  data
• Remove some low-Q2 Rosenbluth data