Observation of WW from double-parton scattering in proton-proton collisions at $\sqrt{s} = 13$ TeV

The first observation of WW production from double-parton scattering processes using same-charge electron-muon and dimuon events in proton-proton collisions is reported. The data sample corresponds to an integrated luminosity of $138~\mathrm{fb}^{-1}$ recorded at a center-of-mass energy of 13 TeV using the CMS detector at the CERN LHC during 2016--2018. Multivariate classifiers are used to distinguish the signal process from the main backgrounds. A maximum likelihood fit is performed to extract the signal cross section. The observed significance of the signal is 6.2 standard deviations, where a significance of 6.7 standard deviations is expected based on the model of mutli-parton interactions as implemented in PYTHIA. Cross section measurements for leptocially decaying W bosons in an charge-inclusive phase space and in a $\mathrm{W}^{\pm}\mathrm{W}^{\pm}$ fiducial region are reported. The measured inclusive cross section is $0.16\pm 0.02\,\mathrm{(stat)}\pm 0.02\,\mathrm{(syst)}\pm0.02\,\mathrm{(model)}$ pb, while the measured fiducial cross section is $6.27\pm 0.69\,\mathrm{(stat)}\pm 0.78\,\mathrm{(syst)}$ fb. Sensitivity towards parton correlations is probed using the pseudorapidity-based lepton asymmetry variable.