Speaker
Description
The Hoyle state (Ex = 7.654 MeV, Jπ = 0+) in 12C plays a central role in
stellar nucleosynthesis, serving as the resonant gateway for carbon formation
through the triple-alpha process. Despite its well-established sequential decay via
the ground state of 8Be, the rare possibility of direct three-alpha (DD) decay
remains a key probe of its underlying alpha-cluster. In this work, we explore both
the structural and decay aspects of the Hoyle state and its excitations through
combined experimental and theoretical approaches. A detailed 3-body penetrability
calculation based on semiclassical Wenzel–Kramers–Brillouin theory utilizing
hyperspherical coordinates was performed to estimate upper limits on the
direct-decay branching ratios for various three-alpha configurations. Assuming the
observed 2+ state at ~10 MeV to be a collective excitation of the Hoyle state, the
calculated upper limits for the DDφ, DDL, and DDE decay modes are 3.5×10-6,
2×10-7, and 6.7×10-6, respectively, underscoring the strong configuration
dependence of the decay dynamics. A complementary high-precision
measurement employing an array of eight double-sided silicon strip detectors and
advanced kinematic reconstruction techniques yielded over 2.2×10^5 fully
reconstructed Hoyle events. Likelihood analysis using folded Dalitz-plot projection
and fractional energy difference methods established the most stringent
experimental limits to date, with an upper limit of 0.018% for the DDφ mode and
0.002% for DDE mode. Further refinement using a Bayesian soft-assignment
scheme offered a realistic branching ratio of about 0.0018% for DDφ decay, the
lowest achievable through an experiment and the closest so far to the theoretical
predictions. Further, a dedicated search for the predicted Efimov-like 0+ state at
7.458 MeV in 12C revealed an upper limit of 0.014% for its alpha-decay width
relative to the Hoyle state. Penetrability calculations indicate a more extended
spatial configuration for this state, and astrophysical modeling suggests a modest
enhancement in the triple-alpha reaction rate compatible with stellar helium-flash
conditions. Together, these findings provide refined constraints on the cluster
structure and decay dynamics of the Hoyle state, with significant implications for
nuclear structure models and carbon synthesis in stars.
