Direct Observation of a New $J^\pi = 2^+$ State in ${}^{12}\mathrm{C}$ through the ${}^{12}\mathrm{C}+\gamma \rightarrow 3\alpha$ Reaction

The second $J^\pi = 2^+$ state in ${}^{12}\mathrm{C}$, predicted over fifty years ago to exist as an excitation of the Hoyle state, has been unambiguously identified in the ${}^{12}\mathrm{C}$($\gamma$,$\alpha$)${}^{8}\mathrm{Be}$ reaction. The $\alpha$ particles produced by the photo\-disintegration of ${}^{12}\mathrm{C}$ were detected using an optical time projection chamber. Initial data were collected at beam energies between 9.1 and 10.7~MeV using intense, nearly mono\-energetic $\gamma$-ray beams available at the HI$\gamma$S facility. The measured cross sections and angular distributions unambiguously establish the existence of a broad $2^+$ state near 10~MeV in ${}^{12}\mathrm{C}$\@. Additional data were recently taken at beam energies of up to 11.2~MeV and show no evidence for an additional $2^+$ state previously reported to exist near 11 MeV [1]. \\[4pt] [1] F.~Ajzenberg-Selove, Nucl.\ Phys.\ {\bf A506}, 1 (1990).