Emission of neutron-proton and proton-proton pairs in electron scattering induced by meson-exchange currents

We use a relativistic model of meson-exchange currents to compute the proton-neutron and proton-proton yields in $(e,e^{\prime })$ scattering from ${}^{12}\mathrm{C}$ in the 2p-2h channel. We compute the response functions and cross section with the relativistic Fermi gas model for a range of kinematics from intermediate- to high-momentum transfers. We find a large contribution of neutron-proton configurations in the initial state, as compared to proton-proton pairs. The different emission probabilities of distinct species of nucleon pairs are produced in our model only by meson-exchange currents, mainly by the $\mathrm{\Delta }$ isobar current. We also analyze the effect of the exchange contribution and show that the direct-exchange interference strongly affects the determination of the $np/pp$ ratio.

Figure 1

Electromagnetic transverse response function for $np$ and $pp$ pair emission off ${}^{12}\mathrm{C}$ as a function of $\omega$ for two values of $q$.

Figure 2

Same as for Fig. 1, but now for the longitudinal response function.

Figure 3

Electromagnetic transverse response function showing the contributions of direct-exchange interference terms to the separate $np$ and $pp$ channels.

Figure 4

Electromagnetic differential cross section separated into the different charge contributions.

Figure 5

Same as for Fig. 4, but now the $pp$ contribution has been scaled by a constant factor, as shown in the two panels.

Figure 6

Same as for Fig. 5, but now for the electron kinematics of the experimental setup of Ref. [1].