Direct observation of the decay of first excited Hoyle state in ${}^{12}$C

Abstract

An excited state of ${}^{12}$C having excitation energy E${}_x\sim$ 9.65 $\pm$ 0.02 MeV and width (FWHM) $\sim 607\pm$ 55 keV, which decays to three $\hat{I}\pm$-particles via Hoyle state ($E_x\sim$ 7.65 MeV), has been directly identified for the first time in the exclusive inelastic scattering of 60 MeV ${}^4$He on ${}^{12}$C, measured in coincidence with the recoiling ${}^{12}$C${}^{\ast }$ Hoyle state (decaying mostly as ${}^{12}$C${}^{\ast }$ $\to {}^8$Be + $\hat{I}\pm$ $\to \hat{I}\pm +\hat{I}\pm +\hat{I}\pm$) by event-by-event kinematic reconstruction of the completely detected (4$\hat{I}\pm$) events. This state is likely to be a candidate for 2${}_2^{+}$ first excited Hoyle state, the existence of which has recently been indirectly evidenced from the recent inclusive inelastic scattering studies.

5 pages, 6 figures