Structure and rotations of the Hoyle state

TL;DR

This paper uses ab initio lattice calculations to explore the structure of the Hoyle state in carbon-12, revealing its cluster configurations and rotational excitations, which are crucial for understanding stellar nucleosynthesis.

Contribution

It provides the first ab initio lattice evidence for the structure and rotational excitations of the Hoyle state in 12C.

Findings

The ground and first excited spin-2 states have a compact triangular alpha cluster configuration.

The Hoyle state and second excited spin-2 state have a bent-arm alpha cluster configuration.

Electromagnetic transition rates between low-lying states are calculated.

Abstract

The excited state of the 12C nucleus known as the "Hoyle state" constitutes one of the most interesting, difficult and timely challenges in nuclear physics, as it plays a key role in the production of carbon via fusion of three alpha particles in red giant stars. In this letter, we present ab initio lattice calculations which unravel the structure of the Hoyle state, along with evidence for a low-lying spin-2 rotational excitation. For the 12C ground state and the first excited spin-2 state, we find a compact triangular configuration of alpha clusters. For the Hoyle state and the second excited spin-2 state, we find a "bent-arm" or obtuse triangular configuration of alpha clusters. We also calculate the electromagnetic transition rates between the low-lying states of 12C.