Pygmy resonances probed with electron scattering

Reactions with radioactive beams have attracted great experimental and theoretical interest during the last two decades [ 1]. Progresses of this scientific endeavor were reported on measurements of nuclear sizes [ 2], use of secondary radioactive beams to elucidate reactions of astrophysical interest [ 3, 4], fusion reactions with neutron-rich nuclei [ 5, 6], tests of fundamental interactions [ 7], dependence of the equation of state of nuclear matter upon the asymmetry energy [ 8], and many other research topics. New research areas with nuclei far from the stability line will become possible with newly proposed experimental facilities. One of the projects at the future FAIR facility of the GSI laboratory/Germany is the study of electron scattering off unstable nuclei in an electron-ion collider mode [ 9]. A similar proposal exists for the RIKEN laboratory facility in Japan [ 10]. By means of elastic electron scattering, these facilities will become the main probe of charge distribution in unstable nuclei [ 11, 12]. Coulomb excitation has been very useful in determining the electromagnetic response in light nuclei [ 13]. For neutron-rich isotopes [ 14] the resulting photo-neutron cross sections are characterized by a pronounced concentration of low-lying E1 strength. But it is well known that non-perturbative effects leading to distortion of the energy spectrum of the fragments interacting with the target (see, e.g. ref. [ 15]) is a problem of difficult nature. The nuclear response probed with electrons is free from such effects. The interpretation of the low-lying E1 strength in neutron-rich nuclei engendered a debate: are these “soft dipole modes” just a manifestation of the loosely-bound character of light neutron-rich nuclei, or are they the result of the excitation of a resonance? [ 16, 17, 18, 19]. The electromagnetic response in light nuclei, leading to their dissociation, is related to the nuclear physics needed in several astrophysical sites [ 3, 4, 15]. The existence of pygmy resonances have important implications on theoretical predictions of