Measurement of the ratio of branching fractions $\mathcal{B}(B_c^+ \to J/ψτ^+ ν_τ)/\mathcal{B}(B_c^+ \to J/ψμ^+ ν_μ)$

A measurement of the ratio of semileptonic branching fractions $\mathcal{R}(Jψ)$, defined as $\mathcal{R}(J/ψ) \equiv \mathcal{B}(B_c^+ \to J/ψτ^+ ν_τ)/\mathcal{B}(B_c^+ \to J/ψμ^+ ν_μ)$, is reported using a sample of proton-proton collision data corresponding to an integrated luminosity of 5.4 fb$^