Improved Limit on DirectαDecay of the Hoyle State

O. S. Kirsebom,* M. Alcorta, M. J. G. Borge, M. Cubero, C. Aa. Diget, L.M. Fraile, B. R. Fulton, H.O.U. Fynbo, D. Galaviz, B. Jonson, M. Madurga, T. Nilsson, G. Nyman, K. Riisager, O. Tengblad, and M. Turrion2,k Department of Physics and Astronomy, Aarhus University, DK-8000 Arhus C, Denmark Instituto de Estructura de la Materia, CSIC, Serrano 113 bis, E-28006 Madrid, Spain Department of Physics, University of York, York YO10 5DD, United Kingdom Grupo de Fisica Nuclear, Universidad Complutense, E-28040 Madrid, Spain Fundamental Physics, Chalmers University of Technology, S-41296 Goteborg, Sweden (Received 27 February 2012; published 14 May 2012) The current evaluation of the triplereaction rate assumes that the decay of the 7.65 MeV, 0þ state in C, commonly known as the Hoyle state, proceeds sequentially via the ground state of Be. This assumption is challenged by the recent identification of two direct -decay branches with a combined branching ratio of 17ð5Þ%. If correct, this would imply a corresponding reduction in the triplereaction rate with important astrophysical consequences. We have used the 11Bð3He; dÞ reaction to populate the Hoyle state and measured the decay to three particles in complete kinematics. We find no evidence for direct -decay branches, and hence our data do not support a revision of the triplereaction rate. We obtain an upper limit of 5 10 3 on the direct decay of the Hoyle state at 95% C.L., which is 1 order of magnitude better than a previous upper limit.