Knowledge of the low-lying monopole strength in $\mathrm{ ^{ 12}C}$, the Hoyle state in particular, is crucial for our understanding of both the astrophysically important $3\alpha$ reaction and of $\alpha$-particle clustering. The $\mathrm{ ^{ 12}C}(\alpha, \alpha^{ \prime})\mathrm{ ^{ 12}C}$ and $\mathrm{ ^{ 14}C}(p, t)\mathrm{ ^{ 12}C}$ 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 $\mathrm{ ^{ 12}C}(\alpha, \alpha^{ \prime})\mathrm{ ^{ 12}C}$ 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 $\mathrm{ ^{ 12}C}$ is required.
Categories: nucl-ex