Fine structure of Vavilov-Cherenkov radiation near the Cherenkov threshold

We analyze the Vavilov-Cherenkov radiation (VCR) in a dispersive nontransparent dielectric air-like medium both below and above the Cherenkov threshold, in the framework of classical electrodynamics. It is shown that the transition to the subthreshold energies leads to the destruction of electromagnetic shock waves and to the sharp reduction of the frequency domain where VCR is emitted. The fine wake-like structure of the Vavilov-Cherenkov radiation survives and manifests the existence of the subthreshold radiation in the domain of anomalous dispersion. These domains can approximately be defined by the two phenomenological parameters of the medium, namely, the effective frequency of oscillators and the damping describing an interaction with the other degrees of freedom.Comment: 9 pages, 6 figure

Λc+\Lambda^+_c- and Λb\Lambda_b-hypernuclei

$\Lambda^+_c$- and $\Lambda_b$-hypernuclei are studied in the quark-meson coupling (QMC) model. Comparisons are made with the results for $\Lambda$-hypernuclei studied in the same model previously. Although the scalar and vector potentials felt by the $\Lambda$, $\Lambda_c^+$ and $\Lambda_b$ in the corresponding hypernuclei multiplet which has the same baryon numbers are quite similar, the wave functions obtained, e.g., for $1s_{1/2}$ state, are very different. The $\Lambda^+_c$ baryon density distribution in $^{209}_{\Lambda^+_c}$Pb is much more pushed away from the center than that for the $\Lambda$ in $^{209}_\Lambda$Pb due to the Coulomb force. On the contrary, the $\Lambda_b$ baryon density distributions in $\Lambda_b$-hypernuclei are much larger near the origin than those for the $\Lambda$ in the corresponding $\Lambda$-hypernuclei due to its heavy mass. It is also found that level spacing for the $\Lambda_b$ single-particle energies is much smaller than that for the $\Lambda$ and $\Lambda^+_c$.Comment: Latex, 14 pages, 4 figures, text was extended, version to appear in Phys. Rev.