The contribution to the nuclear transverse response function R_T arising fromtwo particle-two hole (2p-2h) states excited through the action ofelectromagnetic meson exchange currents (MEC) is computed in a fullyrelativistic framework. The MEC considered are those carried by the pion and byDelta degrees of freedom, the latter being viewed as a virtual nucleonicresonance. The calculation is performed in the relativistic Fermi gas model inwhich Lorentz covariance can be maintained. All 2p-2h many-body diagramscontaining two pionic lines that contribute to R_T are taken into account andthe relative impact of the various components of the MEC on R_T is addressed.The non-relativistic limit of the MEC contributions is also discussed andcompared with the relativistic results to explore the role played by relativityin obtaining the 2p-2h nuclear response.
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