Extended Data Fig. 8. Reaction vertex of the selected events.

Reaction vertex for the selected quasi-elastic ${}^8\mathrm{H}\mathrm{e}(\mathrm{p},\mathrm{p}{}^4\mathrm{H}\mathrm{e})$ events. Data sample is shown by the full symbols, and the low-energy peak region is shown by the open symbols and normalized according to the number of events for the full data set, together with a simulation of one-step process (red curve), and two-step process (blue curve): break-up of ⁸He into ⁶He followed by quasi-elastic $\mathrm{p}-{}^4\mathrm{H}\mathrm{e}$ scattering. As can be seen, an exponential increase is expected towards the exit of the target, which is not observed in the data, confirming that the contribution from this background is indeed as small as expected from the evaluation based on measured cross-sections. Simulated distributions for other two-step processes show the same exponential behaviour

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