A stringent upper limit on the direct 3$\alpha$ decay of the Hoyle State in ${}^{12}$C

Abstract

We investigate an implication of the most recent observation of a second $J^{\pi }=2^{+}$ state in ${}^{12}$C, which was measured using the ${}^{12}$C($\gamma$,$\alpha$)${}^8$Be${}_{\text{(g.s.)}}$ reaction. In addition to the dissociation of ${}^{12}$C to an $\alpha$-particle and ${}^8$Be in its ground state, a small fraction of events (2%) were identified as direct decays and decays to excited states in ${}^8$Be. This allowed a limit on the direct 3$\alpha$ partial decay width to be determined as ${\Gamma }_{3\alpha }<32(4)$ keV. Since this 2${}^{+}$ state is predicted by all theoretical models to be a collective excitation of the Hoyle state, the 3$\alpha$ partial width of the Hoyle state is calculable from the ratio of 3$\alpha$ decay penetrabilities of the Hoyle and 2${}^{+}$ states. This was calculated using the semi-classical WKB approach and we deduce a stringent upper limit for the direct decay branching ratio of the Hoyle state of $\frac{{\Gamma }_{3\alpha }}{\Gamma }<5.7\times 10^{-6}$, over an order of magnitude lower than previously reported. This result places the direct measurement of this rare decay mode beyond current experimental capabilities.

Accepted for publication in Physical Review C (Rapid) on 01/16/2020

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• High precision probe of the fully sequential decay width of the Hoyle state in ${}^{12}$C
• An Optical Readout TPC (O-TPC) for Studies in Nuclear Astrophysics With Gamma-Ray Beams at HIgS
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Cited by in corpus (10)

• Emergent geometry and duality in the carbon nucleus
• Expanding Nuclear Physics Horizons with the Gamma Factory
• The radiative width of the Hoyle state from $\gamma$-ray spectroscopy
• Almost medium-free measurement of the Hoyle state direct-decay component with a TPC
• Wigner SU(4) symmetry, clustering, and the spectrum of ${}^{12}$C
• Investigating the predicted breathing-mode excitation of the Hoyle state
• Correlation in formation of ${}^8$Be nuclei and $\alpha$-particles in fragmentation of relativistic nuclei
• Machine learning method for ${}^{12}$C event classification and reconstruction in the active target time-projection chamber
• Three-$\alpha$ configurations of the second $J^{\pi }=2^{+}$ state in ${}^{12}$C
• Multi-probe study of excited states in ${}^{12}\mathrm{C}$: disentangling the sources of monopole strength between the Hoyle state and $E_x=13$ MeV