We report new precise H(e,e(')p)pi(0) measurements at the Delta(1232) resonance at Q(2)=0.127 (GeV/c)(2) obtained at the MIT-Bates out-of-plane scattering facility which are particularly sensitive to the transverse electric amplitude (E2) of the gamma(*)N-->Delta transition. The new data have been analyzed together with those of earlier measurements to yield precise quadrupole to dipole amplitude ratios: Re(E(3/2)(1+)/M(3/2)(1+))=(-2.3+/-0.3(stat+syst)+/-0.6(model))% and Re(S(3/2)(1+)/M(3/2)(1+))=(-6.1+/-0.2(stat+syst)+/-0.5(model))% for M(3/2)(1+)=(41.4+/-0.3(stat+syst)+/-0.4(model))(10(-3)/m(pi(+))). The derived amplitudes give credence to the conjecture of deformation in hadrons favoring, at low Q2, the dominance of mesonic effects.

The occupation probabilities of shell-model orbits in the lead region are estimated by the addition of random-phase-approximation corrections to nuclear-matter results. The occupation probabilities of single-particle states in shells just below and above the Fermi energy are found to be ~ 0.7 and 0.1, respectively. It is shown that these estimates explain the quenching of the single-particle contribution observed in many elastic and inelastic electronscattering experiments.PACS numbers: 21.60. Cs, 27.80. + W Substantial empirical evidence indicates that the independent-particle model provides only a partially successful description of some of the simplest nuclear excitations in heavy nuclei. Mean-field theories correctly predict the shape of transition densities but they systematically underestimate the strength of these simple excitations. Various mechanisms have been proposed to explain this quenching of single-particle strength, particularly for the high-spin magnetic transitions in lead. Hamamoto, Lichtenstadt, and Bertsch 1 have studied the quenching due to first-order core polarization, but subsequent studies 2 " 4 found this effect to be small. While Krewald and Speth 2 could explain the quenching with particle-vibration coupling, selfconsistent first-and second-order random-phaseapproximation (RPA) calculations 3 could not. The effect of meson exchange currents, left out in these calculations, was estimated by Suzuki and Hyuga 4 and found to be small.In this Letter we examine the empirical evidence for the quenching of the single-particle strength in both elastic and inelastic scattering measurements. We find that the mechanisms explored so far account only for part of the effect. Reasonable agreement with the data is achieved when the effect of short-range correlations is also included.In Table I we list transitions to relatively pure shell-model states. The magnitude of the quenching is directly obtained by comparison of the observed and the single-particle form factors for electron scattering. It is central to our argument that in all these cases the observed form factor can be separated, mainly through its momentum dependence, into a quenched single-particle term and a background term: F(q) = QF sv (q)+F H (q).(1)The form factors of the differences in the charge and current densities of neighboring nuclei, measured in the elastic electron-scattering experiments listed in Table II, also have a quenched singleparticle part. The quenching Q observed in both elastic and inelastic scattering experiments is ~ 0.6. We first review the connection between Q, the quenching factor, and the occupation numbers. Consider a transition from a state |/) to the state \f). Let \i m ) and \f m ) denote the independentparticle-model approximations of |/) and \f). The single-particle states occupied in |/ m ) are denoted by h n , n = 1,A, and the unoccupied states are p n , n = 1, oo. The single-particle part of the transition corresponds to a nucleon being transferred from orbit h to p, so that \f m ) =p^h \i m ), and the...

High-precision 1H(e,e'p)pi(0) measurements at Q2 = 0.126 (GeV/c)2 are reported, which allow the determination of quadrupole amplitudes in the gamma*N-->Delta 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.

Abstract. To determine nonspherical angular-momentum amplitudes in hadrons at long ranges (low Q 2 ), data were taken for the p( e, e p)π 0 reaction in the ∆ region at Q 2 = 0.060 (GeV/c) 2 utilizing the magnetic spectrometers of the A1 Collaboration at MAMI. The results for the dominant transition magnetic dipole amplitude and the quadrupole to dipole ratios at W = 1232 MeV are M Experimental confirmation of the presence of nonspherical hadron amplitudes (i.e. d states in quark models or p-wave π-N states) is fundamental and has been the subject of intense experimental and theoretical interest (for reviews see [1][2][3]). This effort has focused on the measurement of the electric and Coulomb quadrupole amplitudes (E2, C2) in the predominantly M 1 (maga Current address: