Weak Response of Nuclei

We discuss some differences and similarities between electron and neutrino scattering off atomic nuclei. We find that, in the giant resonance region, the two processes excite different nuclear modes, therefore the weak and the electromagnetic nuclear responses are rather different. In any case, the scattering of electrons and photons is the best guide we have to test the validity of our nuclear models and their prediction power. The experience in describing electromagnetic excitations of the nucleus, suggests that, when the nucleus is excited in the continuum, the re-interaction between the emitted nucleon and the remaining nucleus should not be neglected. A simple model taking into account this final state interaction is proposed, and applied to the neutrino scattering off 16 O nucleus. The great activity of the last fifteen years in neutrino physics has attracted great attention to the interaction between neutrinos and atomic nuclei. At present, the main interest in the neutrino-nucleus interaction is related to the goal of investigating the properties of the neu-trinos, or of the neutrino sources such as stars, supernovae, earth etc.. In this perspective the nucleus is considered as detector, and therefore the nuclear response to weakly interacting probes should be well controlled. Our knowledge about the interaction of electrons and photons with the nucleus can be used as a guide to make prediction about neutrino-nucleus processes. Both electromagnetic and weak interactions can be well described within a perturbation expansion of the scattering amplitudes. Furthermore, the tensor structure of the electromagnetic current is identical to that of the vector part of the weak current. In fig.1 we present a direct comparison between electron and neutrino double differential cross sections off 16 O target nucleus. The results , shown in the figure as a function of the nuclear excitation energy, ω, have been obtained for the same values of the projectile energy ǫ i = 15 20 25 30 35 40 0.0 0.1