Branching fraction for Z decays to four leptons and constraints on new physics

The LHC experiments have measured the branching fraction for $Z$ decays to four leptons (electrons or muons). We have combined these measurements with the result $\mathcal{B}(Z \to 4\ell) = (4.58 \pm 0.26) \times 10^{-6}$, allowing a precise comparison to the standard model prediction of $(4.70 \pm 0.03) \times 10^{-6}$. We use a minimal extension of the standard model to demonstrate that this combined value may be used to set stringent limits on new physics.