Investigating the predicted breathing-mode excitation of the Hoyle state

F. D. Smit*, P. Adsley, R. Neveling, P. Papka, E. Nikolskii, J. W. Brümmer, L. M. Donaldson, M. Freer, M. N. Harakeh, F. Nemulodi, L. Pellegri, V. Pesudo, M. Wiedeking, E. Z. Buthelezi, V. Chudoba, S. V. Förtsch, M. Kamil, J. P. Mira, E. Sideras-Haddad, S. SiemG. F. Steyn, J. A. Swartz, I. T. Usman, J. J. van Zyl

*Corresponding author for this work

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Knowledge of the low-lying monopole strength in 12C—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≈ 9 MeV, corresponding to a radial in-phase oscillation of the underlying α clusters. The 12C(α, α') 12C  and 14C(p, t) 12C 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≈ 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 D3h 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 T9 ≳ 2 and ultimately, the predicted nucleosynthesis in explosive stars.
Original languageEnglish
Article number136928
Number of pages7
JournalPhysics Letters B
Early online date29 Jan 2022
Publication statusPublished - 10 Apr 2022


  • Nuclear structure
  • Nuclear astrophysics
  • Cluster models
  • Nuclear reactions
  • Collective levels
  • Direct reactions


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