Pionic fusion study of the ⁶He halo

The halo nucleus ⁶He has been studied in a pionic fusion experiment at the CELSIUS ring in Uppsala. The aim of the experiment was to investigate, in particular, the high-momentum part of the halo wave function by measuring the differential cross section for the ⁴He(d, ⁶He)π⁺ reaction 0.6, 1.2 and 5.0 MeV above threshold in the centre-of-mass frame. The ⁶He ions were detected in a Δ E - E solid-state detector telescope inserted into the ring vacuum. The result for the total cross section is, respectively, 22(1), 38(1) and 57(9) nb with a common systematic uncertainty of ± 35 %. The differential cross section is clearly anisotropic: [frac(d σ, d Ω) (0 °) - frac(d σ, d Ω) (180 °)] / [frac(d σ, d Ω) (0 °) + frac(d σ, d Ω) (180 °)] = - 0.25 (5), - 0.29 (5) and - 0.35 (15) at the same three energies. The preferred direction of emission for the pion is parallel to the momentum of the ⁴He ion in the initial state. In terms of a simple model for the reaction the results depend sensitively on the state of motion of the centre of mass of the halo relative to that of the core.