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Measurement of the branching ratio $Γ(Λ_b^0 \rightarrow ψ(2S)Λ^0)/Γ(Λ_b^0 \rightarrow J/ψΛ^0)$ with the ATLAS detector

An observation of the $Λ_b^0 \rightarrow ψ(2S) Λ^0$ decay and a comparison of its branching fraction with that of the $Λ_b^0 \rightarrow J/ψΛ^0$ decay has been made with the ATLAS detector in proton--proton collisions at $\sqrt{s}=8\,$TeV at the LHC using an integrated luminosity of $20.6\,$fb$^{-1}$. The $J/ψ$ and $ψ(2S)$ mesons are reconstructed in their decays to a muon pair, while the $Λ^0\rightarrow pπ^-$ decay is exploited for the $Λ^0$ baryon reconstruction. The $Λ_b^0$ baryons are reconstructed with transverse momentum $p_{\rm T}>10\,$GeV and pseudorapidity $|η|<2.1$. The measured branching ratio of the $Λ_b^0 \rightarrow ψ(2S) Λ^0$ and $Λ_b^0 \rightarrow J/ψΛ^0$ decays is $Γ(Λ_b^0 \rightarrow ψ(2S)Λ^0)/Γ(Λ_b^0 \rightarrow J/ψΛ^0) = 0.501\pm 0.033 ({\rm stat})\pm 0.019({\rm syst})$, lower than the expectation from the covariant quark model.