Meson-exchange currents and final-state interactions in quasielastic electron scattering at high momentum transfers

Abstract

The effects of meson-exchange currents (MEC) are computed for the one-particle one-hole transverse response function for finite nuclei at high momentum transfers q in the region of the quasi-elastic peak. A semirelativistic shell model is used for the one-particle-emission (e, e ) reaction. Relativistic effects are included using relativistic kinematics, performing a semirelativistic expansion of the current operators, and using the Dirac-equation-based (DEB) form of the relativistic mean-field potential for the final states. It is found that final-state interactions (FSI) produce an important enhancement of the MEC in the high-energy tail of the response function for q 1 GeV/c. The combined effect of MEC and FSI goes away when other models of the FSI, not based on the DEB potential, are employed. In recent years much of the emphasis in studies of inclusive (e, e ) scattering was placed on investigations of the scaling properties of the cross section and on the possibility of predicting neutrino cross sections assuming the universality of the scaling function for electromagnetic and weak interactions. An exhaustive analysis of (e, e ) world data demonstrated the scaling at energy transfers ω below the quasielastic (QE) peak [1,2], namely the independence of the reduced cross sections on the momentum transfer (first-kind scaling) and on the nuclear target (second-kind scaling) when plotted versus the appropriate scaling variable. It is well known that at energies above the QE peak scaling is violated in the transverse (T ) channel by effects beyond the impulse approximation: inelastic scattering [3,4], correlations, and meson-exchange currents (MEC) in both the one-particle one-hole (1p-1h) and two-particle two-hole (2p-2h) sectors [5][6][7][8].In contrast, the available data for the longitudinal (L) response are compatible with scaling throughout the QE region and permitted [9] the extraction of a phenomenological scaling function f L . In recent work [10][11][12] it was shown that only a few models [the relativistic mean field (RMF), the semirelativistic (SR) approach with Dirac-equation-based (DEB) and a "BCS-like" model] are capable of reproducing the detailed shape of f L , while other models fail to reproduce the long tail appearing at high ω. Theses models effectively account for the major ingredients needed to describe the (e, e ) responses for intermediate-to-high momentum transfers, namely relativistic effects and an appropriate description of the effective final-state interactions (FSI).Approximate treatments of these two ingredients are also possible using SR models, which have the advantage of permitting the use of standard nonrelativistic techniques when correctly extrapolated to high values of q. In this article we use the approach of Refs. [11,13] where a specific SR expansion of the electroweak single-nucleon current was used in a continuum shell-model description of electron and neutrino inclusive QE scattering from closed-shell nuclei. In the model the (nonrelativistic) hole states are taken to...

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