title: Improved limit on direct α decay of the Hoyle state.
abstract: The current evaluation of the triple-α reaction rate assumes that the α decay of the 7.65 MeV, 0+ state in 12C, commonly known as the Hoyle state, proceeds sequentially via the ground state of 8Be. This assumption is challenged by the recent identification of two direct α-decay branches with a combined branching ratio of 17(5)%. If correct, this would imply a corresponding reduction in the triple-α reaction rate with important astrophysical consequences. We have used the 11B(3He,d) reaction to populate the Hoyle state and measured the decay to three α particles in complete kinematics. We find no evidence for direct α-decay branches, and hence our data do not support a revision of the triple-α reaction rate. We obtain an upper limit of 5×10(-3) on the direct α decay of the Hoyle state at 95% C.L., which is 1 order of magnitude better than a previous upper limit.
authors.affiliation: Department of Physics and Astronomy, Aarhus University, DK-8000 Århus C, Denmark. oliskir@phys.au.dk
authors.forename: O S
authors.initials: OS
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authors.lastname: Fraile
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authors.lastname: Tengblad
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authors.lastname: Turrión