Inclusive breakup measurements detect only part of a projectile while the remaining fragments interact with the target. For a projectile with three constituents, these measurements can probe correlations that are lost when the projectile is treated as two preformed clusters. I derive cross-section sum rules within the distorted-wave Born approximation for detecting either a pair or a single constituent, retaining the full three-body projectile wave function. For pair detection, the detected state is projected from this wave function. Comparing the resulting reaction source with its factorized limit tests, at the amplitude level, what the preformed-cluster approximation omits. For single-particle detection, both unobserved constituents can interact with the target, and their absorption includes a contribution mediated by target excitation that is absent from an independent-fragment description. The optical reduction recovers the Carlson-Frederico-Hussein absorption kernel [Phys. Lett. B 767, 53 (2017)]; explicit coupling of the detected particle to target excitation adds further contributions. The ${}^6\mathrm{Li}=\alpha +n+p$ example shows how neutron-proton states beyond the bound deuteron enter the reaction source. I give a route for calculating the single-particle spectrum using inelastic target transition form factors and three-body propagation.

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• Breakup reactions
• Direct reactions
• Few-body systems
• Nuclear reactions