The ATLAS Collaboration announces the first observation of “VVZ production” – a rare combination of three massive vector bosons produced simultaneously at the LHC.

As the carriers of the weak force, the W and Z bosons are central to the Standard Model of particle physics. Precise measurements of multi-boson production processes provide an excellent test of the Standard Model and could shed light on new physics phenomena.

In a new study by the ATLAS Collaboration, researchers analysed the full LHC Run-2 dataset (recorded 2015-2018) in search of a rare simultaneous production of three vector bosons. They observed the production of VVZ (where V is a W or Z boson) with a statistical significance of 6.4 standard deviations. This result builds on previous studies of tri-boson production processes, including the observation of inclusive three weak boson production by the CMS Collaboration and the observation of WWW production by the ATLAS Collaboration.

Achieving this result showcases the meticulous work required to analyse such rare processes, given the low production rate (small cross section) and the complexity of boson decay combinations. ATLAS physicists focused on seven decay channels with the highest discovery potential and, in all, one Z boson always decays into a pair of charged leptons. The types and decay modes of the other two bosons define the channels. In the “3l+jets” channel, one W boson decays into a lepton and neutrino, while the other boson (W or Z) decays into two light quarks. In the “4l” channel, two W bosons each decay into a charged lepton and neutrino. In the “5l” channel, one W boson decays into a charged lepton and neutrino, and another Z boson decays into a pair of leptons.

These decay channels could be further divided into signal regions to enhance the separation of signal and background. For instance, in the most sensitive sub-channel, “4l-DF”, the two W bosons decay into different flavour (DF) leptons (into an electron and a muon, see event display). As Standard-Model Z-boson-pair production is expected to produce same-flavour leptons, it is suppressed in the “4l-DF” channel. Researchers also defined five control regions – similar enough to the signal region but with minimal signal contributions – in order to constrain the background processes. Figure 1 shows the distribution of events over all the regions.

To further extract the VVZ signal, ATLAS physicists used a machine-learning classifier called a Boosted Decision Tree (BDT). BDTs are capable of learning the subtle correlations between input features and provide excellent classification accuracy. The BDTs are trained using well-modelled kinematic variables in each signal region. Figure 2 shows the BDT distribution for the 4l-DF signal region for signal and background samples.

By combining these search channels, the ATLAS Collaboration determined the VVZ production signal strength with an observed significance of 6.4 standard deviations. Their measurement was also interpreted in the framework of Effective Field Theory (EFT) to set limits on contributions from new-physics phenomena. The resulting limits confirm the validity of the Standard Model and match the sensitivity of limits obtained in other sensitive channels.

As ATLAS researchers analyse larger datasets from LHC Run 3 and the upcoming HL-LHC, they will further refine their measurements of tri-boson production across all production modes, enabling even greater precision and understanding of these fundamental particles.

Learn more

• Observation of VVZ production at 13 TeV with the ATLAS detector (arXiv:2412.15123, see figures)
• Observation of WWW Production in proton-proton collisions at 13 TeV with the ATLAS Detector (Phys. Rev. Lett. 129 (2022) 061803, arXiv:2201.13045, see figures)
• ATLAS reports first observation of WWW production, Physics Briefing, July 2021
• CMS Collaboration: Observation of the production of three massive gauge bosons at 13 TeV (arXiv:2006.11191)