Observation of the decay B¯s0→χc2^{K+ K−} in the ϕ mass region

The B¯s0→χc2K+K− decay mode is observed and its branching fraction relative to the corresponding χ_c1 decay mode, in a ±15 MeV/c² window around the ϕ mass, is found to be ℬ(B¯s0→χc2K+K−)ℬ(B¯s0→χc1K+K−)=(17.1±3.1±0.4±0.9)%, where the first uncertainty is statistical, the second systematic and the third due to the knowledge of the branching fractions of radiative χ_c decays. The decay mode B¯s0→χc1K+K− allows the B_s ⁰ mass to be measured as m(Bs0)=5366.83±0.25±0.27MeV/c2, where the first uncertainty is statistical and the second systematic. A combination of this result with other LHCb determinations of the B_s ⁰ mass is made.[Figure not available: see fulltext.]