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Study of the $B_c^+ \to J/ψD_s^+$ and $B_c^+ \to J/ψD_s^{*+}$ decays with the ATLAS detector

The decays $B_c^+ \to J/ψD_s^+$ and $B_c^+ \to J/ψD_s^{*+}$ are studied with the ATLAS detector at the LHC using a dataset corresponding to integrated luminosities of 4.9 fb$^{-1}$ and 20.6 fb$^{-1}$ of $pp$ collisions collected at centre-of-mass energies $\sqrt{s} = 7$ TeV and 8 TeV, respectively. Signal candidates are identified through $J/ψ\toμ^+μ^-$ and $D_s^{(*)+}\toϕπ^+(γ/π^0)$ 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 $B_c^+ \to J/ψD_s^+$ and $B_c^+ \to J/ψD_s^{*+}$, and the transverse polarisation fraction in $B_c^+ \to J/ψD_s^{*+}$ decay are measured. The transverse polarisation fraction is determined to be $Γ_{\pm\pm}(B_c^+ \to J/ψD_s^{*+})/Γ(B_c^+ \to J/ψD_s^{*+}) = 0.38 \pm 0.23 \pm 0.07$, and the derived ratio of the branching fractions of the two modes is $\mathcal{B}_{B_c^+ \to J/ψD_s^{*+}}/\mathcal{B}_{B_c^+ \to J/ψD_s^+} = 2.8 \,^{+1.2}_{-0.8} \pm 0.3$, where the first error is statistical and the second is systematic. Finally, a sample of $B_c^+\to J/ψπ^+$ decays is used to derive the ratios of branching fractions $\mathcal{B}_{B_c^+ \to J/ψD_s^+}/\mathcal{B}_{B_c^+ \to J/ψπ^+} = 3.8 \pm 1.1 \pm 0.4 \pm 0.2$ and $\mathcal{B}_{B_c^+ \to J/ψD_s^{*+}}/\mathcal{B}_{B_c^+ \to J/ψπ^+} = 10.4 \pm 3.1 \pm 1.5 \pm 0.6$, where the third error corresponds to the uncertainty of the branching fraction of $D_s^+\toϕ(K^+K^-)π^+$ decay. The available theoretical predictions are generally consistent with the measurement.