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A new ATLAS study reports evidence that Z bosons in a particular electroweak process are often produced in a longitudinal polarization state. The analysis uses proton–proton collision data recorded at the Large Hadron Collider at centre-of-mass energies of 13 TeV and 13.6 TeV. The presence of longitudinally polarised Z bosons in electroweak W±Z production with two jets is observed with a significance of 4.0 standard deviations, and the measured fraction of such events in the chosen region is 0.32 ± 0.09.
The paper studies vector-boson scattering (VBS), a process in which two force-carrying particles (a W and a Z boson) are produced together with two forward jets. Whether a boson is longitudinally or transversely polarised describes the orientation of its field. The longitudinal mode is of special interest because the Standard Model arranges precise cancellations between diagrams involving gauge bosons and the Higgs boson. Those cancellations keep calculated probabilities well-behaved at high energy, so measuring longitudinal production probes this part of the theory and can be sensitive to new effects.
The researchers selected events where both the W and Z decay to electrons or muons, because these leptonic decays are cleaner to reconstruct. They used the full Run 2 data at 13 TeV (140 fb−1) and Run 3 data at 13.6 TeV (164 fb−1) collected by the ATLAS detector. To focus on the electroweak production, the event selection requires at least two jets with transverse momentum above 40 GeV and a large invariant mass of the two leading jets (mjj > 500 GeV). The analysis compared the data to detailed simulations and to a next-to-leading-order electroweak theory calculation.
Quantitatively, the fraction of WZjj–EW events with a longitudinal Z boson (labelled fX0) was measured to be 0.32 ± 0.09 and is compatible with the Standard Model prediction at next-to-leading-order electroweak accuracy. The analysis also sets 95% confidence-level upper limits on related fractions: 0.19 for events with a longitudinal W boson and 0.12 for events where both bosons are longitudinal. In addition, the paper reports the integrated fiducial cross-section per lepton flavour for WZjj–EW production at 13.6 TeV as 0.374 ± 0.077 (stat) ± 0.053 (syst) ± 0.009 (lumi) femtobarns, which agrees with the Standard Model expectation.
Important caveats are noted in the paper. Polarisation fractions are not Lorentz invariant, so they depend on the reference frame chosen for the measurement; the authors fix a specific frame close to the diboson centre-of-mass to define their quantities. The reported fractions are “effective” because they include small non-resonant contributions; theory estimates put off-shell and non-resonant effects at about 1.5% and interference among polarisation states at about 2%. The 4.0 standard-deviation result is evidence for the presence of longitudinal Z bosons in this channel but falls short of the 5 sigma threshold commonly used to claim a discovery. Finally, the measurement is performed inside a restricted fiducial region chosen to match the detector acceptance and so does not directly give a frame-independent or fully inclusive rate.