Nucleon Form Factors from Elastic Scattering of Polarized Leptons (e,μ,τ) from Polarized Nucleons
Recently nucleon form factors have regained much interest, both theoretical and experimental. Future electron scattering facilities (BATES, CEBAF) will devote considerable efforts to the study of the neutron charge form factor G E n (q 2 ). In all these experiments, however, the lepton beam is an electron beam. At the “meson factories” (SIN, LAMPF, TRIUMF) the pion beams produce a longitudinally polarized muon (μ) beam (through spin rotation techniques a transverse polarization of the muon beam can be achieved) which can favourably be used to remove persistent uncertainties in our knowledge of the nucleon form factors: i) the size of the proton (measured by the slope of the charge form factor G E p (q 2) near zero) is 0.86±0.01 fm, known only with a rather large uncertainty [1,2]; the corresponding neutron charge form factor G E n (q 2 ) is badly known away from the rather precise thermal neutron on atomic electrons scattering data (at very low 4-momentum transfer squared (-q 2 ) ); ii) the magnetic form factors G M p,n (q 2) are badly known for very low (-q 2 ) (due to a kinematical suppression ~q 4 in the cross section, for a massless lepton beam) where the charge form factor G E p (q 2 ) dominates (at high (-q2), G M p (q 2 ) dominates by far the cross section).
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