Scientists working on the ATLAS experiment at the Large Hadron Collider report the first observation of two Z bosons, both in their longitudinal polarization state, produced at the same time. Longitudinal polarization means the internal field of each Z boson oscillates along its direction of motion. This measurement tests a subtle prediction of the Standard Model of particle physics about how W and Z bosons behave when they are produced in pairs.

The team analyzed proton–proton collision data collected at a center-of-mass energy of 13.6 teraelectronvolts and corresponding to an integrated luminosity of 164 inverse femtobarns — a measure of the total amount of collision data. They looked specifically for events where each Z boson decays into a pair of electrons or muons, giving a final signature of four charged leptons. Events were selected in a defined detector region and kinematic range (the “fiducial phase space”), and the polarization fractions were measured both across that whole region and in two subregions defined by the combined mass of the four leptons.

The measured fractions of Z boson pairs in the different polarization states were found to agree with Standard Model predictions. In other words, the data do not show unexpected behavior in how often the bosons are produced with longitudinal versus other polarization states. The analysis used detailed reconstruction of the electron and muon pairs and statistical techniques to separate the different polarization contributions.

To reach the level of certainty conventionally called an “observation,” the ATLAS result was combined with an earlier measurement made at 13 TeV. After this statistical combination, the production of two simultaneously longitudinally polarized Z bosons was observed with a significance of 6.5 standard deviations. The measurement had an expected significance of 5.6 standard deviations under the Standard Model. A significance above about 5 standard deviations is the standard threshold in particle physics for claiming an observation.