the paper.

In classical electrodynamics, it is the dipole excitation and polarization of atoms by the electromagnetic field of charged beams moving in the medium which results in the Breit-Wigner shape of the amplitude $F(E, 0^o)$. The relation (\[delt\]) demonstrates that the forward scattering amplitudes account in some way for properties of excited states as well. Atoms behaving as oscillators provide the refractive indices larger than 1 within their low-energy wings (see, e.g., Fig. 31-5 in vol. 1 of Feynman lectures [@9]). The target rest system is precisely defined. In distinction to Bremsstrahlung where transverse fields are important, the Cherenkov radiation is induced by the longitudinal component of the polarization vector.

The forward scattering amplitude for gluons is infinite. To define the refractive index in the absence of the theory of nuclear media (for a simplified approach see [@a]) I prefer to rely on our knowledge about hadronic reactions translated in partonic language to develop phenomenological models.

There are two common features for all hadron-hadron collisions. First, the prominent resonances are formed at rather low energies. They are described by the Breit-Wigner amplitudes which have the positive real part in their low-mass wings. In hadronic medium, there should be some 