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 C12, commonly known as the Hoyle state, proceeds sequentially via the ground state of Be8. 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 B11(He3,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. © 2012 American Physical Society. ; Support of the Spanish CICYT Research Grants No. FPA2009-07387 and No. FPA2010-17142 and the MICINN Consolider Project No. CSD 2007-00042 as well as the support of the European Union VI Framework through RII3-EURONS/JRA4-DLEP(Contract No. 506065). O. S. K. acknowledges support from the Villum Kann Rasmussen Foundation. ; Peer Reviewed