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Lambda_b -> Lambda l+ l- form factors and differential branching fraction from lattice QCD

We present the first lattice QCD determination of the $Λ_b \to Λ$ transition form factors that govern the rare baryonic decays $Λ_b \to Λl^+ l^-$ at leading order in heavy-quark effective theory. Our calculations are performed with 2+1 flavors of domain-wall fermions, at two lattice spacings and with pion masses down to 227 MeV. Three-point functions with a wide range of source-sink separations are used to extract the ground-state contributions. The form factors are extrapolated to the physical values of the light-quark masses and to the continuum limit. We use our results to calculate the differential branching fractions for $Λ_b \to Λl^+ l^-$ with $l=e,μ,τ$ within the standard model. We find agreement with a recent CDF measurement of the $Λ_b \to Λμ^+ μ^-$ differential branching fraction.