Observation of the doubly charmed baryon decay Ξ cc + + → Ξ c ′ + π + $$ {\varXi}_{cc}^{++}\to {\varXi}_c^{\prime +}{\pi}^{+} $$

The $$ {\varXi}_{cc}^{++}\to {\varXi}_c^{\prime +}{\pi}^{+} $$ 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 $$ {\varXi}_{cc}^{++}\to {\varXi}_c^{\prime +}{\pi}^{+} $$ decay is reconstructed partially, where the photon from the $$ {\varXi}_c^{\prime +}\to {\varXi}_c^{+}\gamma $$ decay is not reconstructed and the pK−π+ final state of the $$ {\varXi}_c^{+} $$ baryon is employed. The $$ {\varXi}_{cc}^{++}\to {\varXi}_c^{\prime +}{\pi}^{+} $$ branching fraction relative to that of the $$ {\varXi}_{cc}^{++}\to {\varXi}_c^{+}{\pi}^{+} $$ decay is measured to be 1.41 ± 0.17 ± 0.10, where the first uncertainty is statistical and the second systematic.