First observation of Z-boson production via weak-boson fusion

The fusion of two weak bosons is an important process that can be used to probe the electroweak sector of the Standard Model. Measurements of Higgs production via weak-boson fusion are crucial for precise extraction of the Higgs-boson couplings and have the potential to help pin down the charge conjugation and parity of the Higgs boson. A similar process, weak-boson scattering, is sensitive to alternative electroweak symmetry-breaking models and to anomalous weak-boson gauge couplings. These processes are extremely rare and the experimental observation of the production of heavy bosons via weak-boson fusion has become possible only recently with the large centre-of-mass energy and luminosity provided by the LHC. Extracting the signals from the huge backgrounds in the high pile-up conditions at the LHC is a major challenge.

The production of a Z boson via weak-boson fusion (figure 1) is an excellent benchmark for these rare processes. Weak-boson fusion has the characteristic signature of two low-angle jets, one on each side of the detector. These “tagging” jets typically have transverse momentum of the order of the W mass, because they arise from quarks in each proton recoiling against the two W bosons that fuse to produce the Z boson. Another interesting feature is the lack of colour flow between the tagging jets, which means there is little hadronic activity in that region. These features have been exploited by the ATLAS collaboration to extract the purely electroweak contribution to Z-plus-two-jet production, which includes the weak-boson fusion process.

The analysis was carried out using proton–proton collisions at a centre-of-mass energy of 8 TeV recorded by the ATLAS detector in 2012. Events containing a Z boson candidate in association with two high-transverse-momentum jets were selected in the e⁺e^– and μ⁺μ^– decay channels. The electroweak component was extracted by a fit to the dijet invariant mass spectrum in an electroweak-enhanced region that was defined, in part, by a veto on additional jet activity in the interval between the tagging jets. The background model was constrained using data in a signal-suppressed control region that was defined by reversing the jet-veto requirement. This data-driven constraint reduced the experimental and theoretical modelling uncertainties on the background model, allowing the electroweak signal to be extracted with a significance above the 5σ level. Figure 2 clearly demonstrates that the background-only model is inconsistent with the data in the electroweak-enhanced region. The cross-section measured for electroweak Z-plus-two-jet production, σ = 54.7±4.6 (stat.) ^+9.8_–10.4 (syst.) ±1.5 (lumi.)fb, is in good agreement with the Standard Model prediction of 46.1±1.2 fb.