Anharmonicity of multi-octupole-phonon excitations in Pb 208: Analysis with multireference covariant density functional theory and subbarrier fusion of O 16 + Pb 208

We discuss anharmonicity of the multi-octupole-phonon states in Pb208 based on a covariant density functional theory by fully taking into account the interplay between the quadrupole and the octupole degrees of freedom. Our results indicate the existence of a large anharmonicity in the transition strengths, even though the excitation energies are similar to those in the harmonic limit. We also show that the quadrupole-shape fluctuation significantly enhances the fragmentation of the two-octupole-phonon states in Pb208. Using those transition strengths as inputs to coupled-channels calculations, we then discuss the fusion reaction of O16+Pb208 at energies around the Coulomb barrier. We show that the anharmonicity of the octupole vibrational excitation considerably improves previous coupled-channels calculations in the harmonic-oscillator limit, significantly reducing the height of the main peak in the fusion barrier distribution.