NNLO QCD corrections to $WWγ$ production at the LHC
Triboson production processes are crucial to study quartic gauge-boson couplings. We present the computation of the radiative corrections to $WWγ$ production at the next-to-next-to-leading order (NNLO) in QCD. The leptonic decays of the $W$ bosons and off-shell effects are fully included. The calculation is exact, apart from the finite part of the two-loop amplitudes, which is evaluated in a soft-photon approximation. We validate our approach by using $Zγ$ production as a reference process, where we take advantage of the availability of the exact two-loop amplitudes to derive a conservative error estimate for our approximation, and then apply it to $WWγ$ production. For typical selection cuts, at the centre-of-mass energy $\sqrt{s}=13$ TeV, the NNLO corrections increase the next-to-leading order (NLO) result by about $16\%$, and the perturbative uncertainties are reduced to the $\pm4\%$ level. The uncertainty from the soft approximation turns out to be at the few per mille level, largely subdominant compared to the residual perturbative uncertainties, both for the fiducial cross section and the most relevant differential distributions.