Two-Neutron Halo is Unveiled in 29F

We report the measurement of reaction cross sections (σexR) of 27, 29F with a carbon target at RIKEN. The unexpectedly large σexR and derived matter radius identify 29F as the heaviest two-neutron Borromean halo to date. The halo is attributed to neutrons occupying the 2p3/2 orbital, thereby vanishing the shell closure associated with the neutron number N = 20. The results are explained by state-of-the-art shell model calculations. Coupled-cluster computations based on effective field theories of the strong nuclear force describe the matter radius of 27 F but are challenged for 29F.