  For the first time, a search for the rare decay of the W boson to three
charged pions has been performed. Proton-proton collision data recorded by the
CMS experiment at a center-of-mass energy of 13 TeV, corresponding to an
integrated luminosity of 77.3 fb$^{-1}$, have been analyzed. No significant
excess is observed above the background expectation. An upper limit of 1.01
$\times$ 10$^{-6}$ is set at 95% confidence level on the branching fraction of
the W boson to three charged pions. This provides a strong motivation for
theoretical calculations of this branching fraction.
