Physicists with the ATLAS Collaboration at CERN’s Large Hadron Collider have announced the first observation of VVZ production — a rare combination of three massive vector bosons.
A three-vector-boson event recorded by ATLAS, with one W boson decaying to an electron and a neutrino, one W boson decaying to a muon and a neutrino, and a Z boson decaying to two muons; muons are shown by red lines, the electron by a green line, and ‘missing energy’ from the neutrino by a dashed white line. Image credit: ATLAS / CERN.
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.
“The production of three vector (V) bosons is a very rare process at the LHC,” said Dr. Fabio Cerutti, ATLAS physics coordinator.
“Its measurement provides information on the interactions among multiple bosons, which are linked to underlying symmetries of the Standard Model.”
“This is a powerful tool for uncovering new physics phenomena, such as new, undiscovered particles that are too heavy to be directly produced at the LHC.”
The ATLAS team observed VVZ production with a statistical significance of 6.4 standard deviations, surpassing the five standard deviation threshold needed to claim observation.
The observation extends previous results from the ATLAS and CMS collaborations, including the observation of VVV production by CMS and the observation of WWW production by ATLAS.
As some of the heaviest known particles, the W and Z bosons can decay in a myriad of different ways.
In the new study, the ATLAS physicists focused on seven decay channels with the highest discovery potential.
These channels were further refined using a machine-learning technique called boosted decision trees, with algorithms trained for each channel to identify the sought-after signal.
By combining the decay channels, the researchers were able to observe VVZ production and set limits on contributions from new-physics phenomena to the signal.
“The resulting limits confirm the validity of the Standard Model and are in line with previous results on the production of three vector bosons,” they said.
“As we analyze larger data sets from the LHC’s third run and the upcoming HL-LHC, we will further refine our measurements of the production of three vector bosons — deepening our understanding of these fundamental particles and their role in the Universe.”
The team’s results will be published in the journal Physics Letters B.
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ATLAS Collaboration. 2025. Observation of VVZ production at s√=13 TeV with the ATLAS detector. Phys. Lett. B, in press; arXiv: 2412.15123
