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[en] We calculate the effects on the elastic electron-proton scattering cross section of the two-photon exchange contribution with an intermediate Δ resonance. The Δ two-photon exchange contribution is found to be smaller in magnitude than the previously evaluated nucleon contribution, with an opposite sign at backward scattering angles. The sum of the nucleon and Δ two-photon exchange corrections has the angular dependence compatible with both the polarization transfer and the Rosenbluth methods of measuring the nucleon electromagnetic form factors

[en] The quenching effects of nuclear and Δ excitations on the beta decay of ⁴⁰K are examined. It is found that although configuration mixing still provides the largest retardation, the inclusion of isobar-hole excitation is necessary to bring calculatoin within calling distance of the experimental decay rate

[en] In light of a recent experiment indicating an anomalously large enhancement of the M3 moment in ³⁹K we examine in detail the possible mechanisms usually at play in the renormalization of magnetic moments. These include meson exchange currents, core polarization, and Δ excitations. We conclude that none of the nuclear structure mechanisms presently known are able to account for any enhancement of the M3 moment and suggest the need for a reevaluation of the experimental analysis

[en] An increasing amount of experimental data indicates that spin-flip quadrupole transitions exhibit quenching effects similar to those reported earlier in (p,n) reactions involving l = 0 and l = 1 transitions. We present here two model calculations suggesting that the E2 spin-flip transitions are more affected than their M1 and M3 counterparts by the tensor and spin-orbit components of the nuclear force and should exhibit the largest quenching. We also review the experimental evidence corroborating our observations

[en] I report on recent work on two-boson exchange contributions (γγ and γZ) and their implications for parity-violating electron scattering measurements. These calculations are particularly relevant for high precision measurements of the strange electric and magnetic form factors, and the weak charge of the proton.

[en] We calculate contributions of hadron resonances to two-photon exchange effects in electron-proton scattering. In addition to the nucleon and P33 resonance, the following heavier resonances are included as intermediate states in the two-photon exchange diagrams: D13,D33,P11,S11, and S31. We show that the corrections due to the heavier resonances are smaller than the dominant nucleon and P33 contributions. We also find that there is a partial cancellation between the contributions from the spin 1/2 and spin 3/2 resonances, which results in a further suppression of their aggregate two-photon exchange effect

[en] Recent investigations of stretched M 6 transitions in the s-d shell have uncovered the unexpected systematic trend that isoscalar M 6 transitions are two to three times more retarded than the isovector ones. We present the result of a microscopic calculation analyzing the different mechanisms usually invoked in the formation of the quenching and suggest that an additional argument based on the difference in structure between isoscalar and isovector transition densities may be playing a predominant role in explaining the data. (orig.)

[en] We investigate how the mechanisms of nuclear excitations and mesonic currents traditionally invoked to explain the quenching of low multipole magnetic transitions apply to higher spin operators. We find that there is a significant quenching of the low multipolarity spin-isospin operators by Δ-hole polarization, but that this effect decreases for higher multipoles. Conversely, the nucleon-hole polarization which has a negligible effect on low multipoles plays a predominant role in explaining the quenching of higher multipoles. Meson exchange currents provide a significant enhancement of the orbital operators but have only a slight effect on the spin components. (orig.)

[en] We present an explicit quantum field theoretical calculation of two-photon exchange contribution to elastic electron-proton scattering and to Δ resonance production in electron-proton collisions