Electric polarizability of a nucleon in a free space and in nuclear medium

In the framework of the cloudy bag model, CBM, a polarizability of a free nucleon and of a nucleon embedded in a nuclear matter is calculated. The contributions to the polarizability of the various physical processes caused by an external electric field action on the quarks constituting the bagnucleon and on 'the virtual pion cloud' are obtained and discussed. It emerges in our investigation that the whole polarizability value essentially is due to 'the pion cloud' distortion. The calculated free nucleon polarizability value is in a good agreement with experimental one. The nucleon polarizability in a nuclear matter differs from one in a free space because of the Pauli-blicking which brings it down and owing to the pion mode softening which enhances it significantly. Eventually the nuclear medium effect results in the appreciable increase of the nucleon polarizability that is in the satisfactory accord with the present experimental observations. Thus we have got the unambiguous evidence of the pion mode softening in a nuclear matter