Multiprobe study of excited states in C-12: Disentangling the sources of monopole strength between the energy of the Hoyle state and E-x=13 MeV

Background: The Hoyle state is the archetypal alpha-cluster state which mediates the 3a reaction to produce C-12 and is of great interest for both nuclear structure and astrophysics. Recent theoretical calculations predict a breathing-mode excitation of the Hoyle state at E-x approximate to 9 MeV. Its observation is hindered by the presence of multiple broad states and potential interference effects. An analysis with Gaussian line shapes of measurements at the Research Center for Nuclear Physics (Osaka University) with the Grand Raiden spectrometer suggested that additional strength was needed at E-x approximate to 9 MeV to reproduce the data; this analysis did not account for the well-known threshold effects observed in C-12. Nevertheless, various theoretical studies have since concluded that this additional strength corresponds to the predicted breathing-mode excitation of the Hoyle state. To meaningfully identify a new source of monopole strength in this astrophysically significant region, a more appropriate phenomenological analysis which accounts for penetrability and interference effects must be used to determine whether the data can be explained with previously established states. Purpose: We aim to investigate the monopole strength in the astrophysically important excitation-energy region of C-12 between E-x = 7 and 13 MeV to determine whether the previously established sources of monopole strength are able to reproduce the data. Method: The C-12(alpha, alpha')C-12 and C-1(4)(p, t)C-12 reactions, which are expected to exhibit contrasting selectivity towards different monopole excitations, were employed at various detection angles and beam energies to populate states in C-12. The inclusive excitation-energy spectra were simultaneously analyzed with multilevel, multichannel line shapes. Various scenarios with different sources of monopole strength and interference effects were considered to determine whether the ghost of the Hoyle state and the previously established broad 0(3)(+) state at E-x approximate to 10 MeV are able to reproduce the observed monopole strength. Results: Clear evidence was found for excess monopole strength at E-x approximate to 9 MeV, particularly in the C-12(alpha, alpha')C-12 reaction at 0 degrees. This additional strength cannot be reproduced by the previously established monopole states between E-x = 7 and 13 MeV. Coincident charged-particle decay data suggest that the strength at E-x approximate to 9 MeV is dominantly monopole, with no evidence of a J > 0 contribution. Conclusions: The data support a new source of monopole strength at E-x approximate to 9 MeV, which cannot be described with a phenomenological parametrization of all previously established states. An additional 0(+) state at E-x approximate to 9 MeV yielded a significantly improved fit of the data and is a clear candidate for the predicted breathing-mode excitation of the Hoyle state. Alternatively, the results may suggest that a more sophisticated, physically motivated parametrization of the astrophysically important monopole strengths in C-12 is required.