Search for vector boson scattering at the LHC Run 2 with CMS data in the semi-leptonic $l\nu qq$ final state

A search for electroweak (EW) vector boson scattering in the semi-leptonic decay $l\nu qq$ channel is reported. The search uses the full Run-II CMS dataset of proton-proton collisions at 13 TeV corresponding to an integrated luminosity of 137 fb$^{-1}$. Events are selected requiring one lepton (electron or muon), moderate missing transverse momentum, two jets with large pseudorapidity separation and dijet mass, and an additional hadronic signature consistent with the decay of a W/Z boson. Events are separated in two categories: either the hadronically decaying W/Z boson is reconstructed as one large-radius jet, or it is identified as a pair of jets with dijet mass close to the boson mass. The signal strength is measured fitting the shape of multivariate machine learning discriminators, implemented to separate the signal from the backgrounds in each category. The observed EW signal strength is $\mu_{EW} = 0.85 ^{+0.24}_{-0.20} = 0.85^{+0.21}_{-0.17}(\text{syst}) \; ^{+0.12}_{-0.12}(\text{stat})$, corresponding to a signal significance of 4.4 standard deviations (5.1 expected): the first evidence of vector boson scattering in the semileptonic channel at LHC. A simultaneous measurement of the EW and QCD WW and WZ production is also performed in the same phase space and found to be in agreement within uncertainty with the standard model prediction.