Measurement of the differential branching fraction of the decay Λ → Λ μ μ

The differential branching fraction of the decay Λ → Λ μ μ is measured as a function of the square of the dimuon invariant mass, q. A yield of 78 ± 12Λ → Λ μ μ decays is observed using data, corresponding to an integrated luminosity of 1.0 fb, collected by the LHCb experiment at a centre-of-mass energy of 7 TeV. A significant signal is found in the q region above the square of the J / ψ mass, while at lower-q values upper limits are set on the differential branching fraction. Integrating the differential branching fraction over q, while excluding the J / ψ and ψ (2 S) regions, gives a branching fraction of B (Λ → Λ μ μ) = (0.96 ± 0.16 (stat) ± 0.13 (syst) ± 0.21 (norm)) × 10, where the uncertainties are statistical, systematic and due to the normalisation mode, Λ → J / ψ Λ, respectively.