Cluster structure and excitation strengths in $^{11}$B

The $^{11}$B($^3$He$,t$), $^{11}$B($d,d'$), and $^{11}$B($p,p'$) reactions were measured at forward scattering including zero degrees. Combining the experimental results from the three reactions, the isoscalar and isovector spin-flip M1 strengths were successfully determined for the low-lying states in $^{11}$B. The proton and neutron quadrupole excitation strengths were also deduced from the $^{11}$B($d,d'$) results and previously measured $\gamma$- decay widths. The obtained excitation strengths were compared with the shell model and antisymmetrized molecular dynamics calculations. It was found that the $3/2^-_3$ state in $^{11}$B has a cluster structure. This state is considered to be an analog state of the $0^+_2$ state in $^{12}$C which is a well-known $3\alpha$ cluster state. For the $5/2^-_3$ state, both the shell-model-like and cluster-like structures were observed.