Further Improvement of the Upper Limit on the Direct 3 alpha Decay from the Hoyle State in C12

The direct 3 alpha decay branch from the 02+ state at Ex=7.65 MeV in C12, which is known as the Hoyle state, is considered to affect the triple-a reaction rate strongly and to give crucial information on its structure. We have performed a high-precision measurement of the 3a decay from this state using the C12(C12,3 alpha )C12 reaction at EC12=110 MeV. The branching ratio of the direct 3a decay was under the detection limit in the present experiment. By comparing with Monte Carlo simulations for three decay mechanisms as the sequential decay through the ground state of Be8, the direct decay with equal energies of three a particles, and the direct decay to the phase space uniformly, we have obtained the upper limit of 0.2% on the direct 3a decay.