Evidence for WW production from double-parton interactions in proton-proton collisions at $\sqrt{s} =$ 13 TeV

A search for WW production from double-parton scattering processes using same-charge electron-muon and dimuon events is reported, based on proton-proton collision data collected at a center-of-mass energy of 13 TeV. The analyzed data set corresponds to an integrated luminosity of 77.4 invfb, collected using the CMS detector at the LHC in 2016 and 2017. Multivariate classifiers are used to discriminate between the signal and the dominant background processes. A maximum likelihood fit is performed to extract the signal cross section. This leads to the first evidence for double-parton scattering WW production, with a significance of 3.9 standard deviations. The measured inclusive cross section is 1.41 +/- 0.28 (stat.) +/- 0.28 (syst.) pb.