Investigating the predicted breathing-mode excitation of the Hoyle state

Abstract

Knowledge of the low-lying monopole strength in 12 C-the Hoyle state in particular-is crucial for our understanding of both the astrophysically important 3α 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 12 C(α, α ′ ) 12 C and 14 C(p, t) 12 C reactions were employed to populate states in 12 C in order to search for this predicted breathing mode. A selfconsistent, simultaneous analysis of the inclusive spectra with R-matrix lineshapes, together with angular distributions of chargedparticle 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 3h 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 3α rate at T 9 2 and ultimately, the predicted nucleosynthesis in explosive stars.