New candidate for an alpha cluster condensed state in O16(α,α′) at 400 MeV

Inelastic α scattering on 16O is studied at 400 MeV using an ice target. Near the 4α   breakup threshold of 14.4 MeV, a broad peak is observed at an excitation energy of 13.6±0.1 MeV13.6±0.1 MeV with a width of 0.6±0.2 MeV0.6±0.2 MeV. The spin-parity is estimated to be 0+0+ from the momentum-transfer dependence. The observed width is significantly larger than that of the 0+0+ state at 12.0 MeV with the α+C12 structure, indicating a well-developed α cluster structure. The magnitude of the cross section is sensitive to the density distribution of the constituent α clusters. The observed cross section is consistent with the theoretical prediction for the α cluster condensed state, characterized by its dilute density distribution with a large root-mean-square radius.