2p2h-effects in the nuclear spin-responder

With the availability of high-resolution (e,e')- and (γ,γ')-reactions as well as intermediate-energy polarized nucleon beams begin to have a detailed experimental picture of the nuclear spin-response. From such experiments the quenching of the spin-strength has emerged as a universal phenomenon. Theoretically this missing strength has been interpreted as either a manifestation of subnucleon degrees of freedom (mostly the Δ isobar) or as 2p2h-effects. This paper attempts to confront both pictures with selected data sets and to reach a conclusion about the relative importance of both mechanisms. The theoretical tool is the Second Random Phase Approximation (SRPA) which includes 1p1h- and 2p2h-excitations consistently