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The second 2+ state of 12C, predicted over fifty years ago as an excitationof the Hoyle state, has been unambiguously identified using the 12C(g,a_0)8Bereaction. The alpha particles produced by the photodisintegration of 12C weredetected using an Optical Time Projection Chamber (O-TPC). Data were collectedat beam energies between 9.1 and 10.7 MeV using the intense nearlymono-energetic gamma-ray beams at the HIgS facility. The measured angulardistributions determine the cross section and the E1-E2 relative phases as afunction 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 groundstate gamma-decay width of 60(10) meV; B(E2: 2+ ---> gs) = 0.73(13) e2fm4 [or0.45(8) W.u.]. The Hoyle state and its rotational 2+ state that are moreextended than the ground state of 12C presents a challenge and constraints formodels 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 theHoyle state.

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