Measurement of $\mathrm{W}^{\pm}\mathrm{W}^{\pm}$ scattering in proton-proton collisions at $\sqrt{s} =13\text{ TeV}$ in final states with one tau lepton

A measurement of the cross section for the scattering of same-sign W bosons is presented. This is the first study of a vector boson scattering process in the decay channel with a tau lepton ($\tau$). The analysis is performed with proton-proton collision data collected by the CMS detector at the LHC at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$. Events are selected with the requirement of one $\tau$, one light lepton ($e$ or $\mu$), missing transverse momentum, and two jets with large pseudorapidity separation and large dijet invariant mass. The measured electroweak same-sign WW scattering cross section, extracted with the amplitude for the QCD-associated diboson production fixed to the standard model value, is $1.44^{+0.63}_{-0.56}$ times the standard model prediction. The observed (expected) signal significance is 2.7 (1.9) standard deviations. A measurement of the combined electroweak and QCD-associated same-sign diboson production yields an observed (expected) significance of 2.9 (2.0) standard deviations.