Probing nuclear forces beyond the nuclear drip line: The cases of ${}^{16}$F and ${}^{15}$F

The unbound proton-rich nuclei ${}^{16}$F and ${}^{15}$F are investigated experimentally and theoretically. Several experiments using the resonant elastic scattering method were performed at GANIL with radioactive beams to determine the properties of the low lying states of these nuclei. Strong asymmetry between ${}^{16}$F-${}^{16}$N and ${}^{15}$F-${}^{15}$C mirror nuclei is observed. The strength of the $nucleon-nucleon$ effective interaction involving the loosely bound proton in the $s_{1\mathrm{/}2}$ orbit is significantly modified with respect to their mirror nuclei ${}^{16}$N and ${}^{15}$C. The reduction of the effective interaction is estimated by calculating the interaction energies with a schematic zero-range force. It is found that, after correcting for the effects due to changes in the radial distribution of the single-particle wave functions, the mirror symmetry of the $n-p$ interaction is preserved between ${}^{16}$F and ${}^{16}$N, while a difference of 63% is measured between the $p-p$ versus $n-n$ interactions in the second excited state of ${}^{15}$F and ${}^{15}$C nuclei. Several explanations are proposed.