Stellar carbon synthesis occurs exclusively via the 3α process, in which three α particles fuse to
form 12C in the excited Hoyle state, followed by electromagnetic decay to the ground state. The Hoyle state is above the α threshold, and the rate of stellar carbon production depends on the radiative width of this state.
The radiative width cannot be measured directly, and must instead be deduced by combining three separately measured quantities. One of these quantities is the E0 decay branching ratio of the Hoyle state, and the current
10% uncertainty on the radiative width stems mainly from the uncertainty on this ratio. The rate of the 3α process is an important input parameter in astrophysical calculations on stellar evolution, and a high precision is imperative to constrain the possible outcomes of astrophysical models.