Low Q^2 measurements of the proton form factor ratio $mu_p G_E / G_M$

Physical Review C - PHYS REV C 03/2011; 84(05). DOI: 10.1103/PhysRevC.84.055204
Source: arXiv

ABSTRACT We present an updated extraction of the proton electromagnetic form factor
ratio, mu_p G_E/G_M, at low Q^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_p
G_E/G_M compared to the original analysis.

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    ABSTRACT: A high precision measurement of the proton elastic form factor ratio muGE/GM in the range of Q^2=0.3 - 0.7 (GeV/c)^2 has been made using recoil polarimetry in Jefferson Lab Hall A. In this low Q^2 range, previous data (BLAST: C. B. Crawford et al. 2007, Phys. Rev. Lett. 98 052301, LEDEX: G. Ron et al. 2007, Phys. Rev. Lett. 99 202002) along with many fits and calculations indicate substantial deviations of the ratio from unity, and continue to suggest that structures might be present in the individual form factors, and in the ratio. In this new measurement, we used the high resolution sepctrometer to detect recoil protons incoincidence with the elastic scattered electrons tagged by BigBite calorimeter. With 80% polarized electron beam for 24 days, we are able to achieve ˜0.5% statistical uncertainty. This high precision result will confirm or refute all existing suggestions of few percent structures in the form factors ratio. Beyond the intrinsic interest in nucleon structure, the improved form factor measurements also have implications for DVCS, determinations of the proton Zemach radius and for parity violation experiments.

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