Search Results (9 total)

This paper was published online on 20 May 2005 without several of the authors’ corrections incorporated. Equation (13) has been replaced. The captions of Figs. 16–18 have also been replaced. Typographical errors on pages 4, 6, 14, 15, 18, 19, 22, and 24 have all been corrected. The paper has been corrected as of 8 June 2005. The text is correct in the printed version of the journal.

The ratio of the proton elastic electromagnetic form factors, G_Ep/G_Mp, was obtained by measuring P_t and P_ℓ, the transverse and longitudinal recoil proton polarization components, respectively, for the elastic e→p→ep→reaction in the four-momentum transfer squared range of 0.5 to 3.5  GeV². In the single-photon exchange approximation, G_Ep/G_Mp is directly proportional to P_t/P_ℓ. The simultaneous measurement of P_t and P_ℓ in a polarimeter reduces systematic uncertainties. The results for G_Ep/G_Mp show a systematic decrease with increasing Q², indicating for the first time a definite difference in the distribution of charge and magnetization in the proton. The data have been reanalyzed and their systematic uncertainties have become significantly smaller than those reported previously.

High-precision ¹H(e,e^′p)π⁰ measurements at Q²  =  0.126 (GeV/c)² are reported, which allow the determination of quadrupole amplitudes in the γ^*N→Δ transition; they simultaneously test the reliability of electroproduction models. The derived quadrupole-to-dipole ( I  =  3/2) amplitude ratios, R_SM  =  (-6.5±0.2_stat+sys±2.5_mod)% and R_EM  =  (-2.1±0.2_stat+sys±2.0_mod)%, are dominated by model error. Previous R_SM and R_EM results should be reconsidered after the model uncertainties associated with the method of their extraction are taken into account.

The ratio of the proton's elastic electromagnetic form factors, G_Ep/G_Mp, was obtained by measuring P_t and P_ℓ, the transverse and the longitudinal recoil proton polarization, respectively. For elastic e→p→ep→, G_Ep/G_Mp is proportional to P_t/P_ℓ. Simultaneous measurement of P_t and P_ℓ in a polarimeter provides good control of the systematic uncertainty. The results for the ratio G_Ep/G_Mp show a systematic decrease as Q² increases from 0.5 to 3.5 GeV², indicating for the first time a definite difference in the spatial distribution of charge and magnetization currents in the proton.

We present a measurement of the induced proton polarization P_n in π⁰ electroproduction on the proton around the Δ resonance. The measurement was made at a central invariant mass and a squared four-momentum transfer of W=1231 MeV and Q²=0.126 GeV²/c², respectively. We measured a large induced polarization, P_n=-0.397±0.055±0.009. The data suggest that the scalar background is larger than expected from a recent effective Hamiltonian model.

A Reply to the Comment by F. Klein et al.

The first measurements of the induced proton polarization P_n for the ¹²C(e,e^′p→) reaction are reported. The experiment was performed at quasifree kinematics for energy and momentum transfer (ω,q)≈ (294 MeV, 756 MeV/c) and sampled a missing momentum range of 0–250 MeV/c. The induced polarization arises from final-state interactions and for these kinematics is dominated by the real part of the spin-orbit optical potential. The distorted-wave impulse approximation provides good agreement with data for the 1p_3/2 shell. The data for the continuum suggest that both the 1s_1/2 shell and underlying ℓ>1 configurations contribute.

Recoil proton polarization observables were measured for both the p(e→,e^′p→) and d(e→,e^′p→)n reactions at two values of Q² using a newly commissioned proton focal plane polarimeter at the MIT-Bates Linear Accelerator Center. The hydrogen and deuterium spin-dependent observables D_ℓℓ and D_ℓt, the induced polarization P_n, and the form factor ratio G_E^p/G_M^p were measured under identical kinematics. The deuterium and hydrogen results are in good agreement with each other and with the plane-wave impulse approximation (PWIA).