Observation of B_s ⁰ mesons and measurement of the B_s ⁰/B⁺ yield ratio in PbPb collisions at [Formula presented] TeV

The B_s ⁰ and B⁺ production yields are measured in PbPb collisions at a center-of-mass energy per nucleon pair of 5.02 TeV. The data sample, collected with the CMS detector at the LHC, corresponds to an integrated luminosity of 1.7 nb⁻¹. The mesons are reconstructed in the exclusive decay channels B→s0J/ψ(μ⁺μ⁻)ϕ(K⁺K⁻) and B→+J/ψ(μ⁺μ⁻)K⁺, in the transverse momentum range 7–50 GeV/c and absolute rapidity 0–2.4. The B_s ⁰ meson is observed with a statistical significance in excess of five standard deviations for the first time in nucleus-nucleus collisions. The measurements are performed as functions of the transverse momentum of the B mesons and of the PbPb collision centrality. The ratio of production yields of B_s ⁰ and B⁺ is measured and compared to theoretical models that include quark recombination effects.