Extremely large oblate deformation of the first excited state in $^{12}$C: a new challenge to modern nuclear theory

TL;DR

This study measures the large oblate deformation of the first excited state in carbon-12, revealing a challenge for current nuclear theories and highlighting the importance of alpha clustering and triaxiality effects.

Contribution

The paper provides a precise measurement of the quadrupole moment in $^{12}$C's first excited state, demonstrating a deformation that challenges existing nuclear models.

Findings

Measured quadrupole moment: +0.090(14) eb.

Large oblate deformation observed in $^{12}$C.

Highlights the need for alpha clustering in nuclear theory.

Abstract

A Coulomb-excitation study of the high-lying first excited state at 4.439 MeV in the nucleus $^{12}$C has been carried out using the $^{208}$Pb($^{12}$C,$^{12}$C$^*$)$^{208}$Pb$^*$ reaction at 56 MeV and the {\sc Q3D} magnetic spectrograph at the Maier-Leibnitz Laboratorium in Munich. High-statistics achieved with an average beam intensity of approximately 10$^{11}$ ions/s together with state-of-the-art {\it ab initio} calculations of the nuclear dipole polarizability permitted the accurate determination of the spectroscopic quadrupole moment, $Q_{_S}(2_{_1}^+) = +0.076(30)$~eb, in agreement with previous measurements. Combined with previous work, a weighted average of $Q_{_S}(2_{_1}^+) = +0.090(14)$ eb is determined, which includes the re-analysis of a similar experiment by Vermeer and collaborators, $Q_{_S}(2_{_1}^+) = +0.103(20)$~eb. Such a large oblate deformation challenges modern nuclear theory and emphasizes the need of $\alpha$ clustering and associated triaxiality effects for full convergence of $E2$ collective properties.