Study of Bc+→χcπ+ decays

A study of B _c ⁺→χ _cπ ⁺ decays is reported using proton-proton collision data, collected with the LHCb detector at centre-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of 9 fb ⁻¹. The decay B _c ⁺→χ _c2π ⁺ is observed for the first time, with a significance exceeding seven standard deviations. The relative branching fraction with respect to the B _c ⁺→J/ψπ ⁺ decay is measured to be (Formula presented.) where the first uncertainty is statistical, the second is systematic, and the third is due to the knowledge of the χ _c2→ J/ψγ branching fraction. No significant B _c ⁺→χ _c1π ⁺ signal is observed and an upper limit for the relative branching fraction for the B _c ⁺→χ _c1π ⁺ and B _c ⁺→χ _c2π ⁺ decays of (Formula presented.) is set at the 90% confidence level.