Missing monopole strength of the Hoyle state in the alpha inelastic scattering

Cross sections for the alpha inelastic scattering exciting the low-lying monopole states in 12C, 16O, 24Mg, 28Si, and 40Ca were measured and compared with the distorted- wave Born-approximation (DWBA) calculation to examine the puzzle of the missing monopole strength of the Hoyle state. It was found the DWBA calculation using the density-dependent aN interaction systematically overestimates the cross sections for the 0+ transitions, and the puzzle is a universal problem in light nuclei but not special in the Hoyle state. Since the DWBA calculation using the density-independent interaction reasonably well reproduces the experiment, this puzzle might be related to the density dependence of the effective interaction. The coupled-channel effect for the alpha inelastic scattering is also examined. The coupled- channel effect reduces the calculated cross section, and solve the puzzle of the missing monopole strengths in part, but the improvement of the density dependence of the effective interaction is still necessary to solve the puzzle.