Signature of a possible $\hat{I}\pm$-cluster state in $N=Z$ doubly-magic ${}^{56}$Ni

Abstract

An inelastic $\hat{I}\pm$-scattering experiment on the unstable $N=Z$, doubly-magic ${}^{56}$Ni nucleus was performed in inverse kinematics at an incident energy of 50 A.MeV at GANIL. High multiplicity for $\hat{I}\pm$-particle emission was observed within the limited phase-space of the experimental setup. This observation cannot be explained by means of the statistical-decay model. The ideal classical gas model at $kT$ = 0.4 MeV reproduces fairly well the experimental momentum distribution and the observed multiplicity of $\hat{I}\pm$ particles corresponds to an excitation energy around 96 MeV. The method of distributed $m\hat{I}\pm$-decay ensembles is in agreement with the experimental results if we assume that the $\hat{I}\pm$-gas state in ${}^{56}$Ni exists at around $113_{-17}^{+15}$ MeV. These results suggest that there may exist an exotic state consisting of many $\hat{I}\pm$ particles at the excitation energy of $113_{-17}^{+15}$ MeV.

Accepted for publication in the European Physical Journal A