Measurement of electroweak Z(nu nubar) gamma jj production and limits on anomalous quartic gauge couplings in pp collisions at sqrt(s) = 13 TeV with the ATLAS detector

The electroweak production of Z(nu¯nu) in association with two jets is studied in a regime with a photon of high transverse momentum above 150 GeV using proton–proton collisions at a centre-of-mass energy of 13 TeV at the Large Hadron Collider. The analysis uses a data sample with an integrated luminosity of 139 fb−1 collected by the ATLAS detector during the 2015–2018 LHC data-taking period. This process is an important probe of the electroweak symmetry breaking mechanism in the Standard Model and is sensitive to quartic gauge boson couplings via vector-boson scattering. The fiducial Z(nu¯nu)jj cross section for electroweak production is measured to be 0.77+0.34 −0.30 fb and is consistent with the Standard Model prediction. Evidence of electroweak Z(nu¯nu) jj production is found with an observed significance of 3.2 sigma for the background-only hypothesis, compared with an expected significance of 3.7 sigma. The combination of this result with the previously published ATLAS observation of electroweak Z(nu¯nu) jj production yields an observed (expected) signal significance of 6.3 sigma (6.6 sigma). Limits on anomalous quartic gauge boson couplings are obtained in the framework of effective field theory with dimension-8 operators.