Multi-probe study of excited states in ${}^{12}\mathrm{C}$: disentangling the sources of monopole strength between the Hoyle state and $E_x=13$ MeV

Abstract

Knowledge of the low-lying monopole strength in ${}^{12}\mathrm{C}$, the Hoyle state in particular, is crucial for our understanding of both the astrophysically important $3\hat{I}\pm$ reaction and of $\hat{I}\pm$-particle clustering. The ${}^{12}\mathrm{C}(\hat{I}\pm ,\hat{I}{\pm }^{\prime }){}^{12}\mathrm{C}$ and ${}^{14}\mathrm{C}(p,t){}^{12}\mathrm{C}$ reactions were employed to populate states in ${}^{12}$C. A self-consistent, simultaneous analysis of the inclusive spectra with lineshapes was performed, which accounted for distortion due to nuclear dynamics and experimental effects. Clear evidence was found for excess monopole strength at $E_x\sim 9$ MeV, particularly in the ${}^{12}\mathrm{C}(\hat{I}\pm ,\hat{I}{\pm }^{\prime }){}^{12}\mathrm{C}$ reaction at $0^{\circ }$. This additional strength cannot be reproduced by the previously established monopole states between $E_x=7$ and 13 MeV. An additional $0^{+}$ state at $E_x\sim 9$ MeV yielded a significantly improved fit of the data and is the leading candidate for the predicted breathing-mode excitation of the Hoyle state. Alternatively, the results may suggest that a more sophisticated, physically motivated parameterization of the astrophysically important monopole strengths in ${}^{12}\mathrm{C}$ is required.

25 pages (including appendix and bibliography), 16 figures