Meson exchange currents in electromagnetic one-nucleon emission

The role of meson exchange currents (MEC) in electron- and photon-induced one-nucleon emission processes is studied in a nonrelativistic model including correlations and final state interactions. The nuclear current is the sum of a one-body and of a two-body part. The two-body current includes pion seagull, pion-in-flight, and the isobar current contributions. Numerical results are presented for the exclusive ¹⁶O(e,e^′p)¹⁵N and ¹⁶O(γ,p)¹⁵N reactions. MEC effects are in general rather small in (e,e^′p), while in (γ,p) they are always large and important to obtain a consistent description of (e,e^′p) and (γ,p) data, with the same spectroscopic factors. The calculated (γ,p) cross sections are sensitive to short-range correlations at high values of the recoil momentum, where MEC effects are larger and overwhelm the contribution of correlations.