Paper detail

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

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.