Fig. 1. Schematic illustration of the quasi-elastic reaction investigated in this work.

Top: quasi-elastic scattering of the ⁴He core from a ⁸He projectile off a proton target in the laboratory frame. The length of the arrows represents the momentum per nucleon (the velocity) of the incoming and outgoing particles. Z_beam is the beam axis. Bottom: the equivalent p–⁴He elastic scattering in their centre-of-mass frame, where we consider reactions at backward angles close to 180°, θ_c.m. ≳ 160°. In this frame, the momentum of the proton balances that of the ⁴He, ${\mathbf{P}}_{\mathrm{p}}=-{\mathbf{P}}_{{}^4\mathrm{He}}$, that is, the proton is four times faster than the ⁴He.