Study of the B+ c → J/ψ D+ s and B+ c → J/ψ D∗+ s decays with the ATLAS detector

The decays B+ c → J/ψ D+ s and B+ c → J/ψ D∗+ s are studied with the ATLAS detector at the LHC using a dataset corresponding to integrated luminosities of 4.9 and 20.6 fb−1 of pp collisions collected at centre-ofmass energies √s = 7 TeV and 8 TeV, respectively. Signal candidates are identified through J/ψ → μ+μ− and D(∗)+ s → φπ+(γ /π0) decays. With a two-dimensional likelihood fit involving the B+ c reconstructed invariant mass and an angle between the μ+ and D+ s candidate momenta in the muon pair rest frame, the yields of B+ c → J/ψ D+ s and B+ c → J/ψ D∗+ s , and the transverse polarisation fraction in B+ c → J/ψ D∗+ s decay are measured. The transverse polarisation fraction is determined to be Γ(B+ c → J/ψ D∗+ s )/ Γ(B+ c → J/ψ D∗+ s ) = 0.38 ± 0.23 ± 0.07, and the derived ratio of the branching fractions of the two modes is BB+ c →J/ψ D∗+ s /BB+ c →J/ψ D+ s = 2.8 +1.2 −0.8 ± 0.3, where the first error is statistical and the second is systematic. Finally, a sample of B+ c → J/ψπ+ decays is used to derive the ratios of branching fractions BB+ c →J/ψ D+ s /BB+ c →J/ψπ+ = 3.8 ± 1.1 ± 0.4 ± 0.2 and BB+ c →J/ψ D∗+ s /BB+ c →J/ψπ+ = 10.4 ± 3.1 ± 1.5 ± 0.6, where the third error corresponds to the uncertainty of the branching fraction of D+ s → φ(K +K −)π+ decay. The available theoretical predictions are generally consistent with the measurement