Investigating the predicted breathing-mode excitation of the Hoyle state.

Knowledge of the low-lying monopole strength in C 12 —the Hoyle state in particular—is crucial for our understanding of both the astrophysically important 3 α reaction and of α -particle clustering. Multiple theoretical models have predicted a breathing mode of the Hoyle State at E x ≈ 9 MeV, corresponding to a radial in-phase oscillation of the underlying α clusters. The C 12 (α , α ′) C 12 and C 14 (p , t) C 12 reactions were employed to populate states in 12C in order to search for this predicted breathing mode. A self-consistent, simultaneous analysis of the inclusive spectra with R -matrix lineshapes, together with angular distributions of charged-particle decay, yielded clear evidence for excess monopole strength at E x ≈ 9 MeV which is highly collective. Reproduction of the experimentally observed inclusive yields using a fit, with consistent population ratios for the various broad states, required an additional source of monopole strength. The interpretation of this additional monopole resonance as the breathing-mode excitation of the Hoyle state would provide evidence supporting a D 3 h symmetry for the Hoyle state itself. The excess monopole strength may complicate analysis of the properties of the Hoyle state, modifying the temperature dependence of the 3 α rate at T 9 ≳ 2 and ultimately, the predicted nucleosynthesis in explosive stars. [ABSTRACT FROM AUTHOR]