A meson effect in the dipole selection-rule in selfconjugate nuclei

Abstract

We point out an effect by which a well known selection rule for emission of dipole radiation in a transition between two levels having the same charge-parity (or isotopic spin) in selfconjugate nuclei may be weakened. The effect depends on the fact that the total momentum of the nucleons plus the momentum associated with the fluctuations of the mesonic field in a nucleus is a constant of the motion but not the total momentum of the nucleons only; while this last circumstance is assumed in the usual derivation of the above selection rule. The factor of weakening has been calculated and turns out to be of the order (in amplitude) (g2/4π)(μ/Mc2)≊U/Mc2 whereU is some average potential energy in the nucleus,M the mass of a nucleon and μ the inverse Compton wave length of a meson. This is the same order of magnitude as the one due to Coulomb impurity. If it will be possible with an increasing experimental accuracy to separate at least for some transition the mesonic effect from the Coulomb one, this would furnish a direct experimental proof for the existence of virtual mesons in a nucleus or, in other words for a relativistic (meson) theory of nuclear forces.