Low Q^2 measurements of the proton form factor ratio $mu_p G_E / G_M$
We present an updated extraction of the proton electromagnetic form factor ratio, {mu}{sub p}G{sub E}/G{sub M}, at low Q{sup 2}. The form factors are sensitive to the spatial distribution of the proton, and precise measurements can be used to constrain models of the proton. An improved selection of the elastic events and reduced background contributions yielded a small systematic reduction in the ratio {mu}{sub p}G{sub E}/G{sub M} compared to the original analysis.
Two-photon exchange and elastic scattering of electrons/positrons on the proton. (Proposal for an experiment at VEPP-3)
It has been suggested that two-photon exchange corrections could explain the discrepancy between Rosenbluth extractions of the proton form factors and recent polarization transfer measurements. We propose a precise comparison of electron-proton and positron-proton scattering at Q^2=1.6 GeV^2 to determine if two-photon exchange at moderate Q^2 values is in fact able to explain the discrepancy.
New Measurement of GE / GM for the Proton
The structure of the proton is clearly a matter of universal interest in nuclear and particle physics. Charge and current distributions are obtained through measurements of the electric and magnetic form factors, GE and GM , and as such it is extremely important to determine these quantities as accurately as possible. In elastic e-p scattering magnetic scattering dominates at all but the lowest four-momentum transfer, Q, and therefore e-p elastic scattering di erential cross section measurements leave GM quite well established. For the very same reason GE is much harder to determine but a recent, and probably ongoing, experiment in Hall A uses a novel and elegant polarization transfer method to measure GE/GM . This group has reported results up to Q = 3.47 GeV with an accuracy of ranging from about 0.03