The Spherical Nucleus Puzzle and the Discovery of the New Spherical-like γ-Soft Spectra

Since the 1950s, nuclear physicists have believed that we have a complete conceptual framework for understanding the low(est)-energy excitations of atomic nuclei. This perspective has persisted in contemporary nuclear structure researches, but it now appears overly optimistic. In this Review, I present two previously unexpected discoveries, one experimental and one theoretical. Although the spherical phonon excitation spectrum has been considered as a typical paradigm of collective excitations in nuclear structure theories, it has not been supported by recent experiments. The result of the experimental discovery reveals a \textit{new} $\gamma$-soft rotational mode which has never been predicted by the previous theories. This mode differs from previous $\gamma$-soft ones and can be described by the newly proposed SU3-IBM theory, a new spherical-like $\gamma$-soft mode representing a specific shape phase.