Unambiguous Identification of the Second 2+ State in 12C and the Structure of the Hoyle State

TL;DR

This study unambiguously identified the second 2+ excited state of 12C, providing precise measurements of its properties, which challenge existing nuclear structure models of the Hoyle state and its rotational excitations.

Contribution

The paper reports the first unambiguous experimental identification of the second 2+ state in 12C using gamma-ray induced reactions and advanced detection, offering new data to constrain nuclear models.

Findings

Second 2+ state identified at 10.03 MeV with width 800 keV

Measured B(E2) value for 2+ to ground state transition

Results challenge ab-initio L-EFT predictions about state sizes

Abstract

The second 2+ state of 12C, predicted over fifty years ago as an excitation of the Hoyle state, has been unambiguously identified using the 12C(g,a_0)8Be reaction. The alpha particles produced by the photodisintegration of 12C were detected using an Optical Time Projection Chamber (O-TPC). Data were collected at beam energies between 9.1 and 10.7 MeV using the intense nearly mono-energetic gamma-ray beams at the HIgS facility. The measured angular distributions determine the cross section and the E1-E2 relative phases as a function of energy leading to an unambiguous identification of the second 2+ state in 12C at 10.03(11) MeV, with a total width of 800(130) keV and a ground state gamma-decay width of 60(10) meV; B(E2: 2+ ---> gs) = 0.73(13) e2fm4 [or 0.45(8) W.u.]. The Hoyle state and its rotational 2+ state that are more extended than the ground state of 12C presents a challenge and constraints for models attempting to reveal the nature of three alpha particle states in 12C. Specifically it challenges the ab-initio Lattice Effective Field Theory (L-EFT) calculations that predict similar r.m.s. radii for the ground state and the Hoyle state.