Observation of the doubly charmed baryon decay Ξ ++cc → Ξ ′+c π+

The Ξ++cc → Ξ′+c π+ decay is observed using proton-proton collisions collected by the LHCb experiment at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.4 fb−1. The Ξ++cc → Ξ′+
c π+ decay is reconstructed partially, where the photon from the Ξ′+c → Ξ+
c γ decay is not reconstructed and the pK−π+ final state of the Ξ+c baryon is employed. The Ξ++cc → Ξ′+c π+ branching fraction relative to that of the
Ξ++cc → Ξ+c π+ decay is measured to be 1.41 ± 0.17 ± 0.10, where the first uncertainty is statistical and the second systematic.