The ATLAS collaboration observes the electroweak production of two jets and a Z-boson pair

The ATLAS collaboration, the large research consortium involved in analyzing data collected by the ATLAS particle collider at CERN, recently observed the electroweak production of two Z bosons and two jets. This crucial observation, presented in Nature Physics, could greatly contribute to the understanding of standard model (SM) particle physics.

The SM of particle physics is a well-established theory describing the building blocks and fundamental forces in the universe. This model describes weak bosons (i.e., bosons responsible for the so-called 'weak force') as mediators of the electroweak interaction.

The scattering of massive weak bosons, such as W and Z bosons, is constrained specifically to interactions, where the mediators directly interact and scatter off each other. This scattering, also known as vector-boson scattering (VBS), also involves a type of Feynman diagrams or vertices known (i.e., quartic gauge vertices) that physicists have so far been unable to experimentally probe through other physical processes.

"Quartic gauge vertices are a so far unconfirmed section of the SM, which is, however of central importance to the self-consistency of the model," the authors told Phys.org. "An example for this self-consistency is a delicate cancelation of scattering amplitudes involving Triple Gauge Vertices, Quartic Gauge Vertices and Vertices involving the Higgs Boson. A study of these processes is an independent and crucial test of the Brout-Englert-Higgs (BEH) Mechanism for breaking the electroweak symmetry in the SM (EWSB)."

From the very beginning, one of the crucial objectives of the ATLAS Experiment at CERN has been to search for and measure VBS processes. Run 1 of the Large Hadron Collider (LHC) experiment gathered the first evidence of these processes occurring in two W bosons with the same charge. Subsequently, VBS was also observed during interactions between W and Z bosons.

"The observation of such process in ZZ channel is very challenging due to its small cross section and requires a good modeling and control of the background processes, as well as good reconstruction and calibration of the physics objects by the ATLAS detector," the authors explained.

"Our recent paper completes the observation of all major channels involving massive electroweak gauge bosons and confirms the consistency of the experimental results with the mechanism predicted by the SM. It thus also marks the start of a new era in precision studies of such rare processes in the electroweak sector."

As part of their recent work, the ATLAS collaboration specifically analyzed proton-proton collisions recorded during Run 2 of the LHC particle collider, which spanned from 2015 to 2018. The final state particles in this collider then interact with the ATLAS detector, leaving hits or energy deposits inside it that can be measured and recorded. These recorded energy deposits are then reconstructed into physical objects such as electrons, muons, jets, etc.

"We report the first observation of the electroweak production of two Z bosons and two jets, the rarest process where the scattering of two massive gauge bosons can occur," Navarro said. "The observation as well as the consistency with SM predictions show that the SM of particle physics survives stringent tests down to rarest corner with production cross-section as small as 0.1 fb."

The ATLAS collaboration's observation of the electroweak production of two Z bosons and two jets could have important implications for further research. In addition to providing experimental evidence confirming the robustness of the SM of particle physics, it motivates further investigations of a mechanism called electroweak symmetry breaking (EWSB) that leaves characteristic signatures in VBS processes.

Such investigations will most likely require new data and more advanced experimental techniques. The LHC started collecting new data in 2022, which will soon result in new ATLAS detector measurements. This data could soon lead to new exciting observations and research efforts.

"At a longer time scale, the high-luminosity LHC is expected to deliver 3000/fb of data, which will be 20 times of the dataset analyzed in this paper," the authors added. "After the observation era, we will have more precise measurements of the VBS processes with much larger dataset, which will help to probe the EWSB with higher precision, as well as look for any possible deviation from the SM prediction. Eventually, this will allow us to check if the BEH-Mechanism is the one formulated in the SM or there might be other more structures."

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