Electroweak symmetry breaking is a key question of particle physics. In the Standard Model, the electroweak symmetry breaking is described by the Higgs mechanism. In 2012, the Higgs boson was simultaneously discovered by two great collaborations, ATLAS and CMS. It is a breakthrough in electroweak symmetry breaking. But this is not the end, many properties of the Higgs boson remain mystery. The nature of electroweak symmetry breaking still need to be understood. Besides directly measure the properties of the Higgs boson, vector boson scattering provides another way to understand the nature of the electroweak symmetry breaking. The presence of the Higgs boson prevents the vector boson scattering amplitudes from violating unitarity at the TeV scale. Therefore, vector boson scattering processes are important to probe the Standard Model and beyond the Standard Model. From 2015 to 2018, more and more data collected by the ATLAS detector. Its high collision energies and luminosities provide an unprecedented opportunity to study vector boson scattering. At the LHC, the scattering of massive electroweak gauge bosons is accessible through the measurement of purely electroweak production of two jets and two gauge bosons. Among all the electroweak VVjj processes, ZZjj offers a clean and competitive channel to study electroweak symmetry breaking physics. Based on the full run-2 datasets collected by the ATLAS detector, the ZZjj process is first observed with an observed (expected) significance of 5.5σ (4.3 σ).
